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New loads on old switches, relays and contractors

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While power electronics is continuously superseding the traditional “hardware” solutions in electrical engineering, common mechanical contacts still remain important constituents of any electrical power distribution system. A survey of the types of switches, relays and contactors, their properties and areas of application will be given. Further, a detailed overview shall draw a picture of the impact caused by new electronic types of loads upon these conventional mechanical contacts. More in particular, the inrush currents resulting from this situation are far from negligible. This paper highlights the causes, types and effects of inrush currents. It discusses how these problems are dealt with by different suppliers and what information these provide their customers with in order to deal with these impacts.

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New loads on old switches, relays and contractors

  1. 1. APPLICATION NOTE NEW LOADS ON OLD SWITCHES, RELAYS AND CONTACTORS Stefan Fassbinder, Guy Kasier October 2014 ECI Publication No Cu0204 Available from www.leonardo-energy.org
  2. 2. Publication No Cu0204 Issue Date: October 2014 Page Document Issue Control Sheet Document Title: Application Note – New loads on old switches, relays and contactors Publication No: Cu0204 Issue: 01 Release: October 2014 Author(s): Stefan Fassbinder, Guy Kasier Reviewer(s): Bruno De Wachter Document History Issue Date Purpose 1 October 2014 First publication in the framework of the Good Practice Guide 2 3 Disclaimer While this publication has been prepared with care, European Copper Institute and other contributors provide no warranty with regards to the content and shall not be liable for any direct, incidental or consequential damages that may result from the use of the information or the data contained. Copyright© European Copper Institute. Reproduction is authorized providing the material is unabridged and the source is acknowledged.
  3. 3. Publication No Cu0204 Issue Date: October 2014 Page i CONTENTS Summary.........................................................................................................................................................1 Inrush currents in general ...............................................................................................................................2 Causes of inrush currents....................................................................................................................................... 2 Inrush currents in theory ....................................................................................................................................... 2 Double trouble: inrush currents after a short interruption ................................................................................... 5 Inrush currents in practice ..................................................................................................................................... 6 Blank capacitors........................................................................................................................................ 6 The major difference: lamps with and without electronic gear ............................................................... 7 PC Power supplies..................................................................................................................................... 9 Transformers .......................................................................................................................................... 11 Inrush currents in the suppliers’ specifications ................................................................................................... 13 Contact bouncing................................................................................................................................................. 14 Remedial measures.............................................................................................................................................. 14 Limitation resistors ................................................................................................................................. 15 Tungsten pre-contact.............................................................................................................................. 15 NTCS........................................................................................................................................................ 17 Electronic solutions................................................................................................................................. 18 Types and properties of different switches ...................................................................................................19 Types and properties of different relays and contactors ...............................................................................20 NO, NC or both..................................................................................................................................................... 20 What is the difference?........................................................................................................................................ 21 Special types of relays.......................................................................................................................................... 22 Reed relays ............................................................................................................................................. 22 Remanence relays................................................................................................................................... 23 Latching relays ........................................................................................................................................ 24 Multiple relays ........................................................................................................................................ 25 Zero-crossing switching of relays............................................................................................................ 26 Electronic power relays (solid state)....................................................................................................... 26 “Energy efficiency” of relays and contactors ....................................................................................................... 26 Inrush currents in Switches ...........................................................................................................................28 Example 1............................................................................................................................................................. 28
  4. 4. Publication No Cu0204 Issue Date: October 2014 Page ii Example 2............................................................................................................................................................. 29 How many on one circuit? ................................................................................................................................... 30 The impact appears to be language dependent .................................................................................................. 31 Inrush currents in relays and contactors .......................................................................................................33 Niko ............................................................................................................................................................... 34 MK Electric Limited .............................................................................................................................................. 34 Eltako ............................................................................................................................................................... 35 Doepke ............................................................................................................................................................... 36 Finder ............................................................................................................................................................... 37 Hager ............................................................................................................................................................... 37 Legrand ............................................................................................................................................................... 37 Schneider Electric................................................................................................................................................. 39 Summary of results .......................................................................................................................................40 Findings and conclusions...............................................................................................................................41
  5. 5. Publication No Cu0204 Issue Date: October 2014 Page 1 SUMMARY While power electronics is continuously superseding the traditional “hardware” solutions in electrical engineering, common mechanical contacts still remain important constituents of any electrical power distribution system. A survey of the types of switches, relays and contactors, their properties and areas of application will be given. Further, a detailed overview shall draw a picture of the impact caused by new electronic types of loads upon these conventional mechanical contacts. More in particular, the inrush currents resulting from this situation are far from negligible. This paper highlights the causes, types and effects of inrush currents. It discusses how these problems are dealt with by different suppliers and what information these provide their customers with in order to deal with these impacts.
  6. 6. Publication No Cu0204 Issue Date: October 2014 Page 2 INRUSH CURRENTS IN GENERAL When procuring a switch, a relay or a contactor, the user is normally confronted with a voltage and a current rating. At first sight, it appears logical and sufficient to indicate the maximum permissible voltage across the open contacts and their maximum current carrying capability when closed. In some cases a minimum power factor is added, with any luck including an indication of whether this is applicable to an inductive or a capacitive power factor, or both. But this is not really sufficient as criteria for the appropriate selection of any switching devices, since:  In most loads – particularly those of capacitive character– the current at the first instance of contact closing, the so-called inrush, is many times higher than the continuous or rated current, respectively.  In all loads with a greater or lesser inductive component, whereas any current circuit has its share of inductance, the voltage across the contacts at the instance of opening is several times greater than the regular voltage of the circuit under steady-state conditions. Moreover, a distinction is often made between the “make” and the “break” capability ratings of relay contacts. For inductive or capacitive loads either the break rating or the make rating (contact closing), is considerably smaller than the continuous rating. This is logical with respect to the corresponding inrush currents of capacitive loads and the switch-off voltage peaks of inductive loads. However, it also shows that care has to be taken with the nameplate values. Therefore many catalogues indicate the maximum number of power consumers of a particular type that can be connected to a particular switch, relay or contactor. Some manufacturers also do this for other devices, such as time relays, time switches, dimmer switches, motion detectors, etc. This number, multiplied by the current rating of the particular type of power consumer, is often considerably lower than the current rating of the respective switching device. CAUSES OF INRUSH CURRENTS Practically all sorts of lamps cause inrush currents – although for very different reasons:  In incandescent lamps it is the resistance of the filament which, when cold, is only 1/10 the value of the normal operating temperature.  In fluorescent lamps with magnetic ballasts it is due to the saturation effect in the core material.  A parallel compensation capacitor adds much more to this and has to be considered as an additional impact in its own right.  In fluorescent lamps with electronic ballasts > 25 W and active power factor correction (PFC) it is due to the AC filtering capacitor across the input terminals, meant to reduce radio frequency disturbances.  In compact fluorescent lamps ≤ 25 W without active PFC it is due to the DC smoothing capacitor across the rectifier output terminals.  To LED lights the same applies as has been said above about electronic ballasts, depending on the wattage and hence whether there is PFC or not.  Halogen lamps are incandescent lamps. If designed for line voltage they need to be treated like these. SELV halogen lamps (12 V, 24 V) could be treated like magnetic ballasts if operated with conventional (“magnetic”) transformers or like electronic ballasts if operated with electronic gear. INRUSH CURRENTS IN THEORY With inductances nothing happens at all if switched on at the peak of the line voltage, since this is the point of the phase where the current would be zero anyway. The current then starts from zero at a point in time where it would also start if it had already been in a steady state of operation before.
  7. 7. Publication No Cu0204 Issue Date: October 2014 Page 3 With capacitances, representing the perfect complement to inductances, one would expect precisely the complementary behaviour, i.e. no inrush current at all when switched on at the zero-crossing of the line voltage. A limitation has to be made here since this only applies when the capacitor has been switched off at a corresponding point of the phase and is hence still pre-charged up to the respective peak voltage (with correct polarity), which is actually sometimes done. Figure 1 – Calculated inrush current of a 1 MF capacitor in a mains with 1 MH of intrinsic inductance and an inner resistance of 316 MΩ when switched on at line voltage zero-crossing. Figure 2 – Calculated inrush current of a 1 MF capacitor on a mains with 1 MH of intrinsic inductance and an inner resistance of 316 MΩ when switched on at line voltage peak. Otherwise a certain current peak still needs to be expected even when applying mains voltage to a capacitor at zero-crossing (Figure 1 – the no-load voltage u0 of the mains, that which would be there without the capacitor, is included as a dotted line). -300A -250A -200A -150A -100A -50A 0A 50A 100A 150A 200A 250A 300A -460V -368V -276V -184V -92V 0V 92V 184V 276V 368V 460V 0ms 2ms 4ms 6ms 8ms i→ u→ t → uc u0 i -300A -250A -200A -150A -100A -50A 0A 50A 100A 150A 200A 250A 300A -460V -368V -276V -184V -92V 0V 92V 184V 276V 368V 460V 0ms 2ms 4ms 6ms 8ms i→ u→ t → uc u0 i
  8. 8. Publication No Cu0204 Issue Date: October 2014 Page 4 Figure 3 – Calculated inrush current of a 1µF capacitor on a mains with 316 MΩ and 316 µH in the worst case (line voltage peak). Figure 4 – Calculated inrush current of a 10 µF capacitor on a mains with 316 MΩ and 316 µH in the worst case (line voltage peak). Figure 5 -- Calculated inrush current of a 10 µF capacitor on a mains with 316 MΩ and 316 µH in the worst case (line voltage peak). But normally, connecting an empty capacitance directly to line voltage and accidentally hitting the voltage peak will cause the worst inrush currents of all (Figure 2). The capacitive load is in principle a short-circuit at the very first instance after switching it on. Still, also the inrush current of a capacitor is attenuated by the intrinsic inductance and the resistance of the mains itself. The advantage of this is that the current increases with a -20A -15A -10A -5A 0A 5A 10A 15A 20A -650V -550V -450V -350V -250V -150V -50V 50V 150V 250V 350V 450V 550V 650V 0ms 2ms 4ms 6ms 8ms i→ u→ t → uc u0 i -60A -50A -40A -30A -20A -10A 0A 10A 20A 30A 40A 50A 60A -650V -550V -450V -350V -250V -150V -50V 50V 150V 250V 350V 450V 550V 650V 0ms 2ms 4ms 6ms 8ms i→ u→ t → uc u0 i -160A -120A -80A -40A 0A 40A 80A 120A 160A -650V -550V -450V -350V -250V -150V -50V 50V 150V 250V 350V 450V 550V 650V 0ms 2ms 4ms 6ms 8ms i→ u→ t → uc u0 i
  9. 9. Publication No Cu0204 Issue Date: October 2014 Page 5 finite rise time edge and a finite peak value. The disadvantage is that a resonant oscillation occurs between the switched capacitance and the inductance of the line, the resonant frequency being calculated according to: LC f 2 1 0  Therefore, a small capacitance of, say, 1 µF will produce a relatively high oscillation frequency (Figure 3). Going to 100 µF yields a reduction down to 1/10 of that value (Figure 5). With 10 µF you land in the middle between these two points (Figure 4). At the same time it becomes evident that an increase in magnitude of the capacitance by e. g. a factor of 100 does increase the magnitude of the inrush current – though not by a factor of 100 but less than 4. Upsizing it from 1 µF (Figure 3) to 10 µF (Figure 4) yields a factor of 2.4 in inrush current, a further increase from 10 µF to 100 µF (Figure 5) yields a factor of only 1.6, because then the intrinsic impedance of the mains becomes the greater one of the two inrush current limiting factors. Vice versa, the voltage uC across the capacitor (also included in the plots) increases as its capacitance is reduced, from barely 600 V across a 100 µF capacitor (Figure 5) up to well over 1000 V across a 1 µF capacitor (Figure 3). Figure 6 – Inductance always attenuates the rise time edge at the beginning, even if later on saturation occurs. Contrary to a capacitance, an inductance may cause a huge voltage peak when interrupting the current, but, if it is of a linear nature, the inrush current peak cannot become any higher than 2√2 times the RMS current, i. e. 2 times the peak of the RMS current. If such a peak suffices to exceed the linear range due to core saturation effects, things may become many times worse. However, such peak is still not as bad as that of a capacitor because it starts with a moderate rise time edge and becomes steeper only some time later. DOUBLE TROUBLE: INRUSH CURRENTS AFTER A SHORT INTERRUPTION Now in the case of a short interruption, which is often carried out automatically by the utility in order to clear an arc fault and which lasts between some 0.2 s and 2.2 s, it may happen that a compensation capacitor is still charged with approximately the peak of the line voltage when being reconnected, while the line voltage is going through its inverse peak at this moment. So at the instance of re-closing, the voltage across the open contacts is nearly double the peak value of the line voltage, and the inrush current will subsequently also rise to nearly double the values calculated before. One of the remedial measures listed at the end of this chapter will need to be applied. 0,00A 0,05A 0,10A 0,15A 0,20A 0,25A 0,30A 0,35A 0,40A 60° 90° 120° 150° 180° i φ 
  10. 10. Publication No Cu0204 Issue Date: October 2014 Page 6 INRUSH CURRENTS IN PRACTICE Following, a selection of inrush current measurements shall be given for a number of devices and components. They were carried out using an Eichhoff type E3206S relay (Figure 14) with the wiring shown in Figure 50. The short-circuit current in the location of test was 500 A at 230 V single-phase. BLANK CAPACITORS Figure 7 – Measured inrush current of a 1 µF capacitor. Figure 8 – Measured inrush current of a 10 µF capacitor. Figure 9 – Measured inrush current of a 100 µF capacitor. -6A -4A -2A 0A 2A 4A 6A 8A 10A 12A -160V -80V 0V 80V 160V 240V 320V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u i -20A -10A 0A 10A 20A 30A 40A 50A 60A 70A 80A 90A -80V -40V 0V 40V 80V 120V 160V 200V 240V 280V 320V 360V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u i -15A 0A 15A 30A 45A 60A 75A 90A 105A 120A 135A 150A 165A -30V 0V 30V 60V 90V 120V 150V 180V 210V 240V 270V 300V 330V 360V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u i
  11. 11. Publication No Cu0204 Issue Date: October 2014 Page 7 THE MAJOR DIFFERENCE: LAMPS WITH AND WITHOUT ELECTRONIC GEAR Figure 10 – Inrush current of a 100 W incandescent lamp, both actual found mains voltage and ideal line voltage plotted additionally. Figure 11 – Inrush current of a 60 W incandescent lamp with a bridge rectifier and smoothing capacitance of 500 µF. Figure 12 – Inrush current of a 21 W compact fluorescent lamp without PFC. -9A -6A -3A 0A 3A 6A 9A -350V -250V -150V -50V 50V 150V 250V 350V 0ms 5ms 10ms 15ms 20ms 25ms i→ u→ t → u u (ideal) i -180A -140A -100A -60A -20A 20A 60A 100A 140A 180A 220A -270V -210V -150V -90V -30V 30V 90V 150V 210V 270V 330V 0ms 5ms 10ms 15ms 20ms i→ u→ t → u i -1A 0A 1A 2A 3A 4A 5A 6A 7A 8A 9A -36V 0V 36V 72V 108V 144V 180V 216V 252V 288V 324V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u i
  12. 12. Publication No Cu0204 Issue Date: October 2014 Page 8 Figure 13 – Inrush current of a 30 W compact fluorescent lamp with PFC. Figure 14 – Relay used for the tests, usually bounce-free. Figure 15 – Inrush current of a 3 W LED bulb with a current rating of just 28 mA. Due to the line inductance, an initial over-rush occurs so that the periodic recharge of the smoothing capacitor starts only 7 periods later; ideal voltage curve added. Figure 7 to Figure 9 show what has been calculated before in Figure 3 to Figure 5. Only the relatively long resonant oscillation is missing. This may be partly due to a higher resistance in the system (≈ 500 mΩ) and partly to energy absorption in the spark when switching on (Figure 14). The slight oscillation before the -1A 1A 3A 5A 7A 9A 11A 13A 15A -20V 20V 60V 100V 140V 180V 220V 260V 300V 4,5ms 5,0ms 5,5ms 6,0ms 6,5ms i→ u→ t → u i -1A 0A 1A 2A 3A 4A 5A 6A 7A 8A 9A 10A -33V 0V 33V 66V 99V 132V 165V 198V 231V 264V 297V 330V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u u (ideal) i
  13. 13. Publication No Cu0204 Issue Date: October 2014 Page 9 instance of contact-closing does not really exist but is caused by the evaluation algorithm. This phenomenon is known as “Pre-ringing impulse” and can only be offset compromising the accuracy of the transients’ resolution. Figure 10 shows that even an incandescent lamp is not a totally linear load. The variance of resistivity with temperature, here e. g. for tungsten, goes far enough to allow the current to rise some 20 times higher at room temperature than at its regular working temperature. The electronic control gear of CFLs (Figure 12) and LED lamps in the range below 25 W are basically the same when viewed from the input side. The first thing seen from there downstream is a bridge rectifier with a smoothing capacitor. Hence, the effects upon the line voltage and input current are also the same: extremely high inrush currents. They reach a peak of approximately 9 A for a consumer with a power rating of only 3 W (≈ 6 VA – Figure 15), or a ratio of nearly 400 against the TRMS current intake. It could be considerably less in a lamp rated > 25 W, hence a consumer with an active front end, but not necessarily (Figure 13). PC POWER SUPPLIES Figure 16 – Inrush current of a 300 W desktop PC power supply unit with passive filtering – a smoothing reactor at the AC input side. Figure 17 – Inrush current of a 380 W desktop PC power supply unit with active power factor corrector (PFC). -1A 2A 5A 8A 11A 14A 17A 20A -15V 15V 45V 75V 105V 135V 165V 195V 225V 255V 285V 315V 0ms 3ms 5ms 8ms i→ u→ t → u i -42A -35A -28A -21A -14A -7A 0A 7A 14A 21A 28A 35A 42A -320V -240V -160V -80V 0V 80V 160V 240V 320V 0ms 10ms 20ms 30ms 40ms i→ u→ t → u i
  14. 14. Publication No Cu0204 Issue Date: October 2014 Page 10 Figure 18 – Desktop PC power unit with an AC reactor at the input side to attenuate harmonics. Figure 19 – Laptop PC power supplies 90 W with PFC (top) and 65 W (bottom); however, the difference not being visible from outside. There are different approaches of designing PC power supplies. Their inrush behaviours may differ accordingly. In the small external units for laptop PCs we find the same differentiation again as with lamps, the difference being that the applicability of any limits at all is here at 75 W input rating. In big PC power units for desktop PCs a reactor can sometimes be found connected in series with the input circuit (Figure 18), meant to attenuate the harmonic currents to a level that suffices to comply with the – again – quite lenient standard values. Such a reactor, if present, also dampens the inrush current (Figure 16). Figure 20 – Inrush current of a 65 W laptop PC power supply unit without PFC. -5A 5A 15A 25A 35A 45A 55A 65A 75A 85A 95A 105A 115A -15V 15V 45V 75V 105V 135V 165V 195V 225V 255V 285V 315V 345V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u i
  15. 15. Publication No Cu0204 Issue Date: October 2014 Page 11 Figure 21 – Inrush current of 90 W laptop PC power supply unit with PFC. The alternative is, again, to use an active front end, and again, while this would provide the opportunity to integrate also some circuitry to reduce inrush, such circuitry is not necessarily found in said devices (Figure 17). TRANSFORMERS Inrush currents are not an invention that came along with the introduction of power electronics. They have been around ever since electrical energy has been in use. Often they form a transient preceding – and by far exceeding – a start-up current that, in its own right, exceeds the regular operating current. All recordings were taken with no load connected to the secondary windings of the transformers under test. Figure 22 – Inrush current of a 400 VA standard transformer with a laminated core (steady-state no-load current = 250 mA). Figure 23 – Laminated halogen lamp transformer 400 VA. 0A 15A 30A 45A 60A 75A 90A 105A 0V 40V 80V 120V 160V 200V 240V 280V 320V 0ms 2ms 4ms 6ms 8ms i→ u→ t → u i -2A 2A 6A 10A 14A 18A 22A 26A 30A 34A 38A 42A 46A 50A -14V 14V 42V 70V 98V 126V 154V 182V 210V 238V 266V 294V 322V 350V 0ms 20ms 40ms 60ms 80ms i→ u→ t → u i
  16. 16. Publication No Cu0204 Issue Date: October 2014 Page 12 The cause for the inrushes is mostly magnetic saturation, and the magnetisation curves of different magnetic materials differ remarkably. A laminated core stacked from conventional warm-rolled magnetic steel (Figure 22; Figure 23) exhibits quite a different behaviour than a toroidal core wound from grain-oriented magnetic steel (Figure 23; Figure 25). A transformer of 2.9 kVA can be switched on only via an inrush current limiter. This can be a conventional one with a relay in this case (Figure 26) that shorts out a serial resistor a few milliseconds after connecting to the mains voltage. Figure 24 – Inrush current of a 400 VA toroidal core transformer (steady-state no-load current = only 20 mA). Figure 25 – Toroidal halogen lamp transformer 400 VA. Figure 26 – Inrush current of a 2.9 kVA isolation transformer with laminated core and relay-controlled inrush current limiter (steady-state no-load current ≈1 A). -7A 0A 7A 14A 21A 28A 35A 42A 49A 56A 63A 70A -32V 0V 32V 64V 96V 128V 160V 192V 224V 256V 288V 320V 0ms 20ms 40ms 60ms 80ms i→ u→ t → u i -22A -18A -14A -10A -6A -2A 2A 6A 10A 14A 18A 22A -320V -240V -160V -80V 0V 80V 160V 240V 320V 0ms 20ms 40ms 60ms 80ms i→ u→ t → u i
  17. 17. Publication No Cu0204 Issue Date: October 2014 Page 13 Figure 27 – Inrush current of a 2kVA toroidal core transformer without any provisions: circuit breaker B 16A trips (steady-state no-load current, however = only 22.5 mA). Toroidal core transformers are infamous for their extremely high inrush peaks, followed by a start-up current that only very gradually dwindles down to the regular no-load current (which, on the other hand, is extremely low). Both together will easily throw the circuit breaker about every second attempt at connecting, as happened here with a 2 kVA type (Figure 27). INRUSH CURRENTS IN THE SUPPLIERS’ SPECIFICATIONS Depending on the type of load, the duration of an inrush current varies from a few microseconds to several periods of the line frequency. Some manufacturers also indicate durations of minutes, but these are not really inrushes but rather start-up and warm-up processes. When choosing the right switch, relay or contactor, the inrush current will therefore have to be taken into consideration as well as the nominal current. Following, some inrush current information from a selection of manufacturers was compiled. In Table 2 we can see, for example, that the inrush current of a fluorescent lamp is 80 times greater than the steady-state current. This factor, however, is valid for a combination with an electronic ballast. If the same lamp is fed from a magnetic ballast without compensation the current will be 0.67 A, and with serial (lead-lag) compensation only the current for a pair of lamps could be given (which would also be around 0.7 A). In either case, the inrush factor hardly exceeds 2. In the case of parallel compensation the capacitor represents a load in its own right. The values given by that manufacturer are hence not sufficiently differentiated. Beyond this, we need to keep on mind what we are talking about when using relative figures: In the case of an energy-saving lamp, for instance, the ratio already increases to 180. If we connect a PC monitor with LCD screen, the inrush current is even 1600 times greater than the operating current, but the basis this relates to is only 40 mA. The figure, however, appears to be slightly outdated. Today’s screens are larger and have a higher current intake. On a contemporary model 230 mA (170 mA at reduced brightness) has been measured. -150A -120A -90A -60A -30A 0A 30A 60A 90A 120A 150A -820V -656V -492V -328V -164V 0V 164V 328V 492V 656V 820V 0ms 5ms 10ms 15ms 20ms 25ms i→ u→ t → u u (ideal) i
  18. 18. Publication No Cu0204 Issue Date: October 2014 Page 14 Table 1 – Inrush current amplitudes as multiples of the current ratings and their durations according to teletask. Table 2 – Inrush currents and their duration according to zettler electronics. Incandescent lamps including halogen lamps also exhibit an inrush current. This is because the resistance when cold is much lower than when the lamp is at operating temperature. The ratio of inrush current by nominal current is as high as 10 to 20. As with capacitors, if n lamps are switched on simultaneously, the inrush current will be approximately n times greater – as long as it does not converge towards the respective short-circuit current at the point of installation. So it should be clear that the contacts of switches, relays or contactors need to be dimensioned to match these high inrush currents. It is important not to forget including any automatic switches into these considerations. CONTACT BOUNCING What may make things worse is the fact that – due to the elasticity of the contact materials and the contact springs – the contacts may bounce back immediately after getting into touch with each other, still right in midst of the excessive inrush current, giving rise to excessive arcing. Since contacts must close as quickly as possible, they need to move fast, which, on the other hand, gives rise to bouncing. This multiplies the wear of the contacts accordingly, bears the risk of contacts welding together and also enhances possible EMC problems due to sparking. REMEDIAL MEASURES Now what to do? There are several long proven and fairly simple approaches available to reduce the listed drawbacks of and detrimental impacts upon the contacts of switches, relays and contactors. They may come as Inrush currents according to Teletask Typical inrush current factor Time to reach I n Resistive load 1 0 Tungsten incandescent lamp 10 ... 15 300 ms Halogen lamp 20 600 µs Gas-discharge lamp 5 ... 10 10 s Energy-saving lamp with built-in capacitor 20 ... 40 5 ... 20 ms. Mercury or sodium vapour lamp & choke 1 ... 3 2 min Magnetising coil AC 3 ... 20 100 ms Motor AC 5 ... 10 200 ... 500 ms Transformer 5 ... 15 100 ms Capacitor 20 ... 40 5 ... 20 ms Some measured inrush currents for typical mains switching loads according to Zettler Ratings Type of load S I Resistive load 100VA 0.43A 0.61A 1.41 6.50ms Incandescent lamp, incl. halogen 100VA 0.43A 8.50A 20 0.60ms Fluorescent lamp 58VA 0.25A 20.00A 80 0.18ms Energy-saving lamp 17VA 0.07A 13.50A 180 0.07ms Transformer 85VA 0.37A 13.40A 36 4.50ms Contactor 115VA 0.50A 15.00A 30 5.00ms Roll-down shutters 110VA 0.48A 1.10A 2.3 5.00ms Fan motor 1200VA 5.30A 44.00A 8.3 5.00ms PC monitor with CRT screen 60VA 0.26A 76.00A 290 1.00ms PC monitor with LCD screen 10VA 0.04A 70.00A 1600 0.25ms Max. switching current Ratio î /I N Pulse widthat halfheight
  19. 19. Publication No Cu0204 Issue Date: October 2014 Page 15 supplementary external circuitry or be implemented as an integral component of the switching device. The external means are the following. LIMITATION RESISTORS A resistor can be connected in series with a load or group of loads which cause excessive inrush currents. For instance, the »Power Manager« by Conrad (Figure 29) got such a sort of protection built in. In a previous model (Figure 28) the eight individual switches used to fail one by one after only short periods of operation. By means of a relay the resistor is shunted a few milliseconds after the main contact has been closed. Unfortunately the effect was limited to the main switch only, while individual loads also used to be switched more or less frequently via the individual switches. The switching activity still destroyed the laptop PC switch after about three years. TUNGSTEN PRE-CONTACT Different contacting systems may be combined, such as a plain manual main switch with a supplementary inrush current limiting relay that reacts to the line voltage being switched on by the main switch. This latter switch closes the circuit only via the limiting resistor, and as soon as the circuit is closed, the relay reacts and shorts out the resistor (Figure 30). The relay’s natural time delay will usually be just about long enough to limit the inrush current but also short enough to limit the heat generation in the resistor. Figure 28 – A small 65 W power supply for a laptop PC was enough to ruin the respective individual switch after about 2 years of use. Figure 29 – The successor model has an inrush current limiter built in – only one, though, for seven switches, hence same result as before.
  20. 20. Publication No Cu0204 Issue Date: October 2014 Page 16 Such resistor is usually dimensioned to bear the great power for a very short moment only, less than 1 s. The relay’s time delay can be influenced via the ratio of the capacitance ratings C~/C= of the AC limiting capacitor and the DC smoothing capacitor, whereas this principle of wiring is only an example. The relay may as well be supplied via a miniature transformer (≈1 VA), or a line voltage relay may be selected. Care has to be taken, though, not to fight fire with fire and to select a sort of supply that itself creates its own – though smaller – portion of inrush currents. In the version presented here this is indeed the case. Another resistor connected in series with the »C~« AC limiting capacitor, but which remains in the loop, would do the job here. Due to the low current a value of several hundred ohms could still be selected here. A signalling lamp, indicating power, would also do. Figure 30 – Inrush current limitation added to a conventional switch, time sensitive to the amplitude and duration of the inrush current. Figure 31 – Inrush current limitation added to a conventional switch – with fixed tripping time being inrush current insensitive. The main resistor should have a minimum rating of approximately 6 Ω. In this case, a B 16 A circuit breaker will trip under no circumstances. However, the resistance value should neither be selected much higher than this; otherwise, when operating a really strong load, the resistor might not only be overloaded but also the voltage across the load might drop to such a low value that, next to the well attenuated inrush, a second inrush occurs when the relay contact closes. A further risk is that the relay may never trip at all, and the resistor will blow like a fuse very soon. A slight variation of the wiring overcomes this drawback, although at the cost of a lost advantage (Figure 31): now the response time will no longer adapt automatically to the height and length of the inrush current. If the inrush current has not yet been appropriately attenuated after the expected time, the contact will still close and might throw the circuit breaker in the extreme case – rescuing the resistor, though. 230 V 50 Hz C~ C= 230 V 50 Hz C~ C=
  21. 21. Publication No Cu0204 Issue Date: October 2014 Page 17 NTCS Another option is the use of NTCs. These are resistors with a negative temperature coefficient (Figure 34). Their rated resistance values are referred to a working temperature of 20°C, but at elevated temperatures the resistance values drop drastically. Consequently, they first limit the inrush currents but then quickly heat up due to the current flow until they no longer cause a substantial voltage drop. The advantages are obvious:  No mechanical parts, no wear  Easy to handle and mantle  Small, plain, simple  Cheap But the disadvantages are just as obvious:  Some heat loss and hence the correlated heat dissipation remains. The resistance must not and will not drop to zero because the device needs to be kept hot as long as the load is drawing current.  When the load is switched on again after just a short interruption, the NTC is still hot, which hampers the desired effect.  The resistance value must match the load current. Otherwise the NTC will either be ineffective or blow. Figure 32 – NTC resistors are readily available with a wide variety of resistance ratings. Figure 33 – An NTC on duty in a PC power supply. NTCs are hence not a universally suitable replacement for inrush current limiting relays. A power user with a continuous current intake in the range of 16 A cannot be controlled with these; the heat dissipation would be
  22. 22. Publication No Cu0204 Issue Date: October 2014 Page 18 too great. NTCs can readily be used where the continuous current is moderate but the ratio to the inrush peak is quite large. A typical area of application is PC power units (Figure 33). ELECTRONIC SOLUTIONS Figure 34 – Electronic inrush current avoidance relay for transformers. Figure 35 – Inrush current of a 2kVA toroidal core transformer with an electronic inrush current limiter. The detrimental repercussions of electronic loads can also be combatted with electronics. While mechanical solutions employing the proper timing with reference to the phase may be an option, the smarter solution is of an electronic nature. One manufacturer offers special soft starters for transformers (Figure 34). Particularly transformers with toroidal cores, although their no-load currents are extremely low, have very high inrush currents which, from ratings of 400 VA onwards, may throw a fuse or a 16 A miniature circuit breaker. These dedicated relays for inductive loads with iron cores do not only spot the right point in time for switching on, which would always be the instance of voltage peak in the case of a linear inductance, but also store information on the remnant magnetism at the previous event of turning off. This means that the first inrush is not necessarily optimally attenuated, just fairly enough to avoid fuse tripping, but all subsequent instances of switching-on do not cause any inrush currents at all. These devices also avoid a repetition of the inrush due to a voltage dip or a short interruption. These interruptions may be so short that they go unnoticed. All the more do users wonder why the MCB trips – apparently for no reason at all? -1,5A -1,0A -0,5A 0,0A 0,5A 1,0A 1,5A 2,0A 2,5A 3,0A -300V -200V -100V 0V 100V 200V 300V 400V 500V 600V 5ms 6ms 7ms 8ms 9ms 10ms 11ms i→ u→ t → u i
  23. 23. Publication No Cu0204 Issue Date: October 2014 Page 19 TYPES AND PROPERTIES OF DIFFERENT SWITCHES Figure 36 – Switch meant for general application – but encountering certain limitations (see figure 54). The common understanding of a low voltage switch is that of a device for establishing and interrupting the flow of an electrical current in a mechanical manner. The contacts are being brought into touch with each other and separated again by a system driven directly by human force. Switches can be fitted into the walls, directly integrated into the device, or somewhere in between, e. g. into the connection cable or into a multiple outlet strip (Figure 36). They may be implemented as push-buttons, which only provide connection as long as they are being pushed, or as »real« switches which remain in the desired position until they are pushed back. An inter- mediate variant is the push-button switch, combining the function of the latter with the mode of operation taken from the former. Many other systems for moving the contacts are common – e.g. rocker switches, rotary switches, toggle switches, change-over switches, or selector switches, but this has no consequence on the issues dealt with in this Application Note.
  24. 24. Publication No Cu0204 Issue Date: October 2014 Page 20 TYPES AND PROPERTIES OF DIFFERENT RELAYS AND CONTACTORS In relays (Figure 39) and contactors, the contacts are brought into touch with each other by the force of an electric magnet, roughly speaking a spool of copper wire on a piece of iron, and separated again by the force of a spring. NO, NC OR BOTH At least this is what would form an »NO (normally open)« contact, but the configuration may just as well be the other way round as an »NC (normally closed)« contact. One spool can easily be utilized to operate several contacts at a time. If it operates an NO contact and an NC contact with a common central point, it forms a changeover contact. Figure 37 – Power relays by a small selection of four present and past suppliers (Schrack, Eberle, Finder, Eichhoff). Figure 38 – Interior of a power relay with two 16 A changeover contacts. If designed in such a way, one single relay may be able to carry out various switching tasks, as well as steering the inrush current limitation via an inrush resistor (see further). Note that the sequence of switching is of vital importance. A power relay will usually be designed sturdy enough to survive a short-circuit occurring while the contacts are closed, but switching into an existing short- circuit will normally destroy a relay. Take for example a relay with two NO and two NC contacts, wired to swap polarity. If the new polarity is established before the former one is interrupted, a short-circuit occurs as an intermediate state and irreversibly damages the relay. Hence, care has to be taken to meet the adequate
  25. 25. Publication No Cu0204 Issue Date: October 2014 Page 21 selection between a “break before make”, also called »Form C« contact and a »make before break«, also called “Form D” contact. WHAT IS THE DIFFERENCE? Figure 39 – Long sold so-called “miniature contactor” E3250 by Eichhoff, now still available from Tripus, this version with 2*NC and 2*NO contacts. Figure 40 – Contactor 4*NO, rated 440 V – 24 A; excitatory voltage 24 V AC, by Doepke.
  26. 26. Publication No Cu0204 Issue Date: October 2014 Page 22 Figure 41 – Contactor 4*NC, rated 440 V – 40 A; excitatory voltage 240 V AC, by Doepke. There is no clearly defined distinction between a contactor and a relay. Both include mechanical contacts operated by the electro-magnetic force of a copper spool, hence controlling a strong current and a comparatively high voltage by a small current at a potentially very low voltage. The current thus controlled may range up to 16 A in a relay; above, one would tend to speak of a contactor (Figure 39). A relay is usually ELV DC operated, while a contactor (Figure 41) works with a mains voltage AC coil, but this is not a stringent distinction either (Figure 40). SPECIAL TYPES OF RELAYS The drawback of any standard type of relay or contactor is that the copper coil has to remain energized to hold the contacts in place. When the exciting current is interrupted the contacts drop back into their resting position, driven by the force of a spring. In its resting position however, which is also the starting position, the magnetic force is substantially impaired by the air gap between the core and the yoke. This requires that the current necessary to start the yoke moving is many times higher than the current necessary to just hold it in place. So, by principle, the operating power of a relay or contactor could be cut down to a fraction once the contacts have been moved into the activated position. In a contactor, when AC operated, this is in part achieved by its nature because the closing of the yoke increases the inductance and hence reduces both the power factor and the current. Still, the effect is incomplete. REED RELAYS One way to get around this obstacle is to use a relay with a reed contact (Figure 43). These consist of a glass tube with two thin iron needles inside, the ends of which overlap but do not yet touch each other because their ends are both slightly bent. When magnetized from an external magnetic field, the ends will assume opposite polarities, attract each other and get into touch. If the external magnetic field is generated by a spool around the glass tube, the whole arrangement forms a reed relay (Figure 42).
  27. 27. Publication No Cu0204 Issue Date: October 2014 Page 23 Figure 42 – Reed relay with several glass tubes. Figure 43 – Reed relay contact (http://bwir.de/bauteile/reed-kontakt-magnetschalter). REMANENCE RELAYS The other option is a remanence relay which employs the remanent magnetism in the core to keep the yoke closed (Figure 45). The exciting current needs to flow only for a short moment. In order to release it again (Figure 44), a small current impulse of opposite polarity has to be applied, just great enough to build up the coercive field strength to remove the remanent flux. The disadvantage is the more sophisticated control circuitry requiring two different current amplitudes of opposite polarities and well-defined tolerance margins, albeit the relay itself is hardly any more sophisticated than a generic design.
  28. 28. Publication No Cu0204 Issue Date: October 2014 Page 24 Figure 44 – A remanence relay remains in the “rest” position… Figure 45 – As well as in the “work” position without any excitation power applied. LATCHING RELAYS A third option and a very similar solution is the latching relay, also called »impulse«, »keep«, or »stay« relay. It is bi-stable and hence always remains in its last position when de-energized. It may be AC operated and work in a mechanical way (Figure 46) or with permanent magnets if DC operated. It swaps from one position to the other each time it experiences a short exciting current impulse. This is made so, however, not in order to save energy but to control e. g. the lights in a staircase from multiple positions using push-buttons as light switches.
  29. 29. Publication No Cu0204 Issue Date: October 2014 Page 25 Figure 46 – Probably the most commonly used impulse relay: Eltako. MULTIPLE RELAYS Relays can also be grouped on a printed circuit board without housing, or several of them may be combined in one housing, serving similar or adjacent purposes (Figure 47). Imagine a shutter motor with one »upward« and one »downward« input terminal. Home automation systems rarely ever require only one individual relay at a time but usually a number of them for a group of tasks installed in one place. Relay output modules can save space in these cases (Figure 48). Figure 47 – Twin relay – two equal ones in one housing. Figure 48 – Relay output module of a home automation system with 4*6 A and 4*10 built-in relays (PEHA).
  30. 30. Publication No Cu0204 Issue Date: October 2014 Page 26 ZERO-CROSSING SWITCHING OF RELAYS One manufacturer released a series of relays in 2008 which are able to time the closing of the contacts more or less precisely to the point of the phase where the voltage crosses zero. Since the time delay of the relay depends on the coil voltage, furthermore its temperature (due to the change of resistance) and some of the mechanical properties, it takes a microprocessor to be integrated into the relay to achieve the necessary precision of timing. This technique is now integrated into some clocks and staircase timers of that manufacturer. ELECTRONIC POWER RELAYS (SOLID STATE) Once you start using electronic components, you could also use them all through. The so-called electronic power relays are based on thyristors. They are not very common because standards still do not approve them as a galvanic separation. However, they do a good job in applications where this is not required, such as a UPS unit. In the latter case they are used for their short response time. Thyristors (with adequate electronic control) may react no later than by the beginning of the next semi-wave after the beginning of a triggering event. Figure 49 – Electronic power relays have long been around and do good services in specific applications, but are not regarded as a galvanic separation. “ENERGY EFFICIENCY” OF RELAYS AND CONTACTORS Figure 50 – Special wiring configuration for the exciter coil to reduce the power intake. 230 V 50 Hz C= 4.7 µF 350 V= Eichhoff‘s small contactor »E 3250« ‒ alternative control wiring C~ 33 nF 250 V~
  31. 31. Publication No Cu0204 Issue Date: October 2014 Page 27 Figure 51 – Voltage surge caused by the inductance of a magnetic ballast when the starter tries to ignite a 58 W lamp. A minor drop of instantaneous current amplitude causes a peak of ≈700V. The lamp acts as a surge diverter and starts up. Figure 52 – Measurement results according to figure 50. In generic standard relays and contactors, there is still a relatively large energy savings potential by reducing the exciting current – after the contact has switched, to the magnitude necessary to hold the yoke in place. By means of a special control circuit (Figure 50) e. g. Eichhoff’s ancient “miniature contactor E3250” can be switched on and off by an NO and an NC pushbutton respectively. The magnet coil, normally meant for 230 V AC operation, is here fed with DC. The electrolytic DC capacitor C= is charged up to √2 * 230 V and discharged again when the NO pushbutton contact closes. The impulse closes the yoke and hence the contacts, and subsequently the current is limited via the AC capacitor C~. After disconnecting by the NC pushbutton the electrolytic capacitor charges up again very quickly, providing flawless functioning of the contactor about one to two seconds after opening. In this way, the continuous power consumption of 5.7 W is cut down to 0.07 W – a reduction by 98.7% (Figure 51)! If the time span before possible re-closure is halved, the energy savings will still reach 97.5% – a good compromise. The ratio of C~/C= is the factor to be varied. 0,00 A 0,25 A 0,50 A 0,75 A 1,00 A 1,25 A 1,50 A 0 V 150 V 300 V 450 V 600 V 750 V 0 ms 1 ms 2 ms 3 ms 4 ms 5 ms 6 ms i→ u→ t → u i
  32. 32. Publication No Cu0204 Issue Date: October 2014 Page 28 INRUSH CURRENTS IN SWITCHES Switches are generally rated for a maximum voltage to be applied between the contacts when open and a maximum operating current flowing across the closed contacts. What is meant by the latter is the continuous current drawn by the respective load. This rating does not take the short-term, discontinuous events into account which may – and in most cases will – arise from the process of switching. In the best case a minimum power factor is specified. This happens in order to account for the facts that  when switching on a capacitive load an extremely short, but very high inrush current will occur  when switching off an inductive load a high self-induction voltage pulse will occur which makes it hard to actually “stop” the current (inductance is a sort of “current inertia” – see Figure 52). But the latter point is not the principal problem switches are faced with nowadays. Rather, it is the impact of increased inrush currents drawn by modern electronic loads which behave largely in a capacitive manner particularly at the very first instance of switching them on. While some of these electronic loads supersede common »power guzzlers« and draw significantly lower permanent RMS currents than their precursors, the current impulses at the moment of contacting may become much higher than used to be the case in earlier days. Information on how to deal with these impacts is very sparsely disseminated by suppliers, unfortunately. EXAMPLE 1 Figure 53 – Conrad’s “power manager”, seen from underneath. Coming back to the first edition of Conrad’s »Power Manager« as shown in figure 28, it must be said that some ratings for the load currents are actually at the bottom side (Figure 52): The current rating for inductive loads is much lower than that for ohmic loads, which is a reasonable and logical approach so far, but leaves several information gaps:  It says nothing there about capacitive loads, which, after all, are the most likely to be operated and which ruined most of the individual switches after some years of use.  What is also missing is an indication as to whether the maximum current given has to be evenly distributed across the seven channels, or whether each of them could carry that much.
  33. 33. Publication No Cu0204 Issue Date: October 2014 Page 29  And if the latter is true, can the load also be switched by its allocated individual switch, or does this switch need to be switched on without load first, for the connection to be made by the main switch afterwards?  The same for switching off: Is there sufficient »breaking« capability in each of the individual small (illuminated) switches, or do you have to switch off the significantly bigger main switch first?  Is there a difference between »make« and »break« ratings? Are these equal to the current rating for a continuous load? This must be assumed, because separate values are missing, but how are inrush currents dealt with then? Unfortunately this abysmal lack of information is symptomatic for all similar cases. While it would be an intrusion to flood common users with such indigestible details, it is up to the manufacturer to select, design and arrange switches in such a manner that they just endure those inrush currents that occur in practice – be it with stronger contacts or one of the mentioned remedial measures. The quandary is, however, that said common users do not see what they are supposed to pay a higher price for. EXAMPLE 2 One of those fairly good looking, but surprisingly cheap multiple outlet strips from the DIY market (Figure 36) was used in a student’s home. It supplied a lamp, a TV set, a small radio, a microwave cooker, a printer and a coffee machine. To ensure the galvanic separation of these in part electronically controlled devices without real mains switches from the line voltage; a model with an integrated illuminated switch was employed. The good surprise was that it even bore a 2-pole switch at that price (Figure 54). The bad surprise came to the student when after 9 months suddenly the switch would no longer switch off. The rocker button could still be pushed to the »off« position and did actually interrupt the supply at that moment, but when releasing the button it snapped back into the »on« position and resumed to provide power! An analysis of the incident showed very quickly (Figure 55) that one of the contact rockers, which are normally totally loose when the rocker button is removed, dropped out, with the contacts already displaying substantial wear, but the other one had got stuck, welded to the opposed contact! Figure 54 – Switch from a multiple socket extension cord (see figure 36)…
  34. 34. Publication No Cu0204 Issue Date: October 2014 Page 30 Figure 55 – ….after a few months of use. Which load was to be blamed for this? The really great (continuous) currents are drawn by the microwave cooker and the coffee machine, but these devices had never ever been switched off via the external switch during use. After all, a coffee machine will normally have a current intake of some 6 A, but practically of an ohmic nature. In fact when a coffee machine fails it is rarely due to the internal switch. The lamp, however, was a CFL, and printers, TV sets, radios and the like are typical electronic devices which are not really turned off when turned »off«, and as soon as the general supply to them is switched on, all of the smoothing capacitors inside them are charged up together within one millisecond or two. These capacitors are dimensioned with rampant reserve capacitance to also ride through short voltage dips at full power, and they are always there, also while the respective device is idling or switched »off«. This is why the supplementary use of a »real« switch is still recommended today, even if the stand-by power consumption of such devices has been drastically reduced (sometimes far below 1 W) in recent years. The extra switch provides some extra safety against concealed faults. Now unfortunately, at the same time, this very device may fall victim to the properties of its own fosterlings. HOW MANY ON ONE CIRCUIT?
  35. 35. Publication No Cu0204 Issue Date: October 2014 Page 31 Table 3 – Maximum number of lamps that can be connected to a 10 A circuit-breaker (Osram). Niko, for example, specify a nominal current of 10 A for most switches in their catalogue, but there are also switches with a nominal current of 16 A. The former type being mainly used to operate lights. The latter are used to connect e. g. wall sockets. However, the catalogue does not provide any information about which and how many loads may be operated by these switches. Nor does Niko provide any further information. Osram give quite detailed technical specifications (Table 3). Interestingly, they state an additional requirement that, if two-pole circuit-breakers are used, the number of permissible fittings must be reduced by 20%, which is not found anywhere else. THE IMPACT APPEARS TO BE LANGUAGE DEPENDENT There is a peculiar observation to be made with Peha. Both the Dutch and French catalogues contain not only a small list of the number of compensated and uncompensated fluorescent lamps that may be connected, but also a notification concerning inrush currents of energy-saving lamps and electronic ballasts. The English and German versions, however, only make a notification concerning inrush currents. No further information concerning the number of lamps that can be connected is given. The corresponding sections of one and the same specification read as follows: Figure 56 – Peha catalogue for fluorescent lamps, Dutch version. Maximum permissible number of T8 fluorescent lamps to be operated on a B10A single-pole circuit breaker Electronic (by Osram type designation) Magnetic Single lamp Twin un- comp. parallel comp. lead-lag comp. QTP8 1*18 36 QTP8 2*18 2*25 27 32 2*23 QTIS e 1*18 17 QTIS e 2*18 2*17 27 32 2*23 QT-FIT8 1*18 17 QT-FIT8 2*18 2*8 27 32 2*23 QTP8 1*36 25 QTP8 2*36 2*17 23 32 2*23 QTIS e 1*36 17 QTIS e 2*36 2*5 23 32 2*23 QT-FIT8 1*36 17 QT-FIT8 2*36 2*8 23 32 2*23 QTi 1*28/54 26 QTi 2*28/54 2*19 15 20 2*15 QTP8 1*58 17 QTP8 2*58 2*8 15 20 2*15 QTIS e 1*58 8 QTIS e 2*58 2*5 15 20 2*15 QT-FIT8 1*58 17 QT-FIT8 2*58 2*8 15 20 2*15 Single lamp Twin lamp 18 W lamp 36 W lamp 58 W lamp
  36. 36. Publication No Cu0204 Issue Date: October 2014 Page 32 Figure 57 – Peha catalogue for fluorescent lamps, French version. Figure 58 – Peha catalogue for fluorescent lamps, English version. Figure 59 – Peha catalogue for fluorescent lamps, German version.
  37. 37. Publication No Cu0204 Issue Date: October 2014 Page 33 INRUSH CURRENTS IN RELAYS AND CONTACTORS Relays are mostly small in size and current ratings. Contactors, on the other hand, are prevalently designed for somewhat heavier work (higher currents). Occasionally information on dedicated relays and contactors for high inrush currents can be found. These devices are designed to connect loads that cause high inrush currents. There are two standards applying to contactors:  EN 60947.4 for industrial applications,  EN 61095:2009 for “electromechanical contactors for household and similar purposes” Both describe the AC utility classes. Thus, AC-1 stands for consumers with cos φ > 0.95. The higher the AC class number, the smaller the cos φ as shown in the following table: Utilization Category Type of Application AC-1 Non-inductive or slightly inductive loads, example: resistive furnaces, heaters AC-2 Slip-ring motors: switching off AC-3 Squirrel-cage motors: starting, switches off motors during running time AC-4 Squirrel-cage motors: starting, plugging, inching AC-5a Switching of discharge lamps AC-5b Switching of incandescent lamps AC-6a Switching of transformers AC-6b Switching of capacitor banks AC-7a Slightly inductive loads in household appliances: examples: mixers, blenders AC-7b Motor-loads for household appliances: examples: fans, central vacuum AC-8a Hermetic refrigerant compressor motor control with manual resetting overloads AC-8b Hermetic refrigerant compressor motor control with automatic resetting overloads AC-12 Control of resisitive loads and solid state loads with opto-coupler isolation AC-13 Control of solid state loads with transformer isolation AC-14 Control of small electromagnetic loads AC-15 Control of A.C. electromagnetic loads AC-20 Connecting and disconnecting under no-load conditions AC-21 Switching of resistive loads, including moderate overloads AC-22 Switching of mixed resistive and inductive loads, including moderate overloads AC-23 Switching of motor loads or other highly inductive loads Table 4 – AC utility classes for industrial and domestic applications. This alone already shows that these standards are old and would urgently require a revision, since they are only oriented at the fundamental power factor cos φ, not the load factor (LF or λ). Also, it may be assumed that usually an inductive cos φ is meant, because the arcing and sparking caused by these when interrupting a circuit used to be the prevalent impacts upon switching elements. This, however, does not in the least reflect today’s significantly higher impacts of capacitive and non-linear loads at contact closing. Hence, in their technical documentation contactor manufacturers normally provide a table showing the number and type of loads that can be connected to a relay or contactor. For contactors, this is almost always the case. In as far as home automation system relays are concerned, manufacturers often still remain silent. It
  38. 38. Publication No Cu0204 Issue Date: October 2014 Page 34 is then assumed that the maximum current is the one specified for utility class AC-1. For other loads, the electrician is left to his fate. NIKO Niko give surprisingly detailed data and very useful supplementary information on the variation of contact lifetime expectancy depending on quality and quantity of load. This information may very well be applied to provide a rough guideline also to correlated products by other manufacturers. Table 5 – Detailed information on relay and contactor capability ratings by Niko. MK ELECTRIC LIMITED Single-phase 230 V table Singl e- Manufacturer's relay type designation 6220s/6420s/ 6720s 7240s/ 7440s 7263s/ 7463s Rating Maximum No. of lamps / maximum total wattage 40W 57 2.28kW 115 4.60kW 172 6.88kW 60W 45 2.70kW 85 5.10kW 125 7.50kW 100W 28 2.80kW 70 7.00kW 100 10.00kW 60W 14 0.84kW 27 1.62kW 40 2.40kW 80W 12 0.96kW 23 1.84kW 35 2.80kW 15W 20 0.30kW 40 0.60kW 60 0.90kW 20W 20 0.40kW 40 0.80kW 60 1.20kW 40W 20 0.80kW 40 1.60kW 60 2.40kW 15W 30 0.45kW 70 1.05kW 100 1.50kW 20W 30 0.60kW 70 1.40kW 100 2.00kW 40W 28 1.12kW 70 2.80kW 100 4.00kW 18W 111 2.00kW 222 4.00kW 333 5.99kW 36W 58 2.09kW 117 4.21kW 176 6.34kW 7W 200 1.40kW 400 2.80kW 600 4.20kW 11W 120 1.32kW 240 2.64kW 360 3.96kW 15W 88 1.32kW 176 2.64kW 264 3.96kW 20W 66 1.32kW 132 2.64kW 200 4.00kW Motors – Maximum power Type of small motor application (AC1 - AC7a categories) 220 / 240 V single-phase with cap. 1.1 kW 2.2 kW 4.0 kW 400 V three-phase motor 4.0 kW 7.5 kW 11.0 kW Heating – Maximum power Type of small heating application (AC7b category) Number of operating cycles 230V 1-ph. 400V 3-ph. 230V 1-ph. 400V 3-ph. 230V 1-ph. 400V 3-ph. 100,000 5.4kW 16.0kW 8.6kW 26.0kW 13.6kW 41.0kW 150,000 4.6kW 14.0kW 7.4kW 22.0kW 11.6kW 35.0kW 250,000 3.5kW 10.0kW 5.6kW 17.0kW 8.6kW 26.5kW 500,000 1.6kW 5.0kW 2.6kW 7.5kW 4.0kW 12.0kW 1,000,000 1.2kW 3.5kW 1.9kW 6.0kW 3.0kW 9.0kW Type of lighting application (AC5a and AC5b categories) Electronic lamp (low consumption) Incandescent and halogen lamps Halogen lamps with transformers Fluorescent lamps with starter (single fitting with parallel compensation) Fluorescent lamps with starter (single fitting non- corrected) Electronic ballast (fluorescent lamp single setting) Table 6 – Relay and contactor capability ratings by MK Electric Limited. Type and characteristics of load P S C cos φ I î Fluorescent lamps with magnetic ballasts, lead-lag or uncompensated 1500W 60.000 Fluorescent lamps with magnetic ballasts and parallel compensation 260W 28µF 15.000 24µF 130A 18.000 80µF 195A 3.000 Fluorescent lamps with electronic twin ballasts 10*2*58W 1200W 22.000 5*200W 1000W 71A 60.000 10*200W 2000W 135A 10.000 2*200W+150W 550W 20A 180.000 300W 17A 600.000 500W 28A 400.000 12 V halogen lamps with conventional wound transformer 600VA 55A 50.000 0,60 3,5A 17A 250.000 0,60 6,6A 21A 150.000 Capacitive loads Incandescent lamps (test 5 sec. on, 55 sec. off) 230 V halogen lamps AC motors Con- tact life Remarks
  39. 39. Publication No Cu0204 Issue Date: October 2014 Page 35 This company also provides quite detailed information on contact loading capability for different types of loads. What is particularly helpful is the indication of contact life expectancy of the same load type dependent on amplitude of this load, including motors and heating. ELTAKO Table 7 – Relay and contactor capability ratings by Eltako (note that the device with a higher constant current rating does not perform any better concerning inrush currents. In case of compact fluorescent lamps a maximum permissible inrush current of 70 A for 10 ms is given. The user is left alone with the question of how to verify this. Further hints found include:  For electronic ballast gears a 40-fold inrush current has to be expected.  By using a bi-stable relay, coil power loss and heating can be avoided even in the on mode.  For steady loads of 600 W / 1200 W use current-limiting relay.  For lamps with 150 W maximum rating. Control voltage 8…230V UC 8…230V UC 10A 16A 250V 250V Incondescenl lamps including halogen, 230V 2000W 2000W Fluorescent lamp with MCG, with lead-lag or without compensation 1000W 1000W Fluorescent lamp with MCG and shunt compensation or with ECG 500W 500W Rated switching capacity
  40. 40. Publication No Cu0204 Issue Date: October 2014 Page 36 DOEPKE Table 8 – Verbose list of contactor ratings with all sorts of different types of lamps by Doepke for their series HS Contactors for 20 A to 63 A. The first amazing observation regarding Doepke is that they declare their installation contactors »energy efficient«. While at first sight this may appear like a bogus message just catching up a trend, it was already pointed out earlier that there may very well lie an energy savings potential in reducing the power required for keeping the contactor in the active position. Doepke give a rating of only 1.2 W … 2.6 W for their HS series – more than 50% less than a competitor’s product. Of course this competitor will have been selected to yield a maximum difference, but still the statement speaks for itself. On top of this they point out a special design that causes less noise – yet another feature not found in any other catalogue. The specifications they give (Table 8) are also the most detailed ones found for this collection. It may need mention that in the category “Fluorescent lamps, uncompensated or with serial compensation” the term “serial compensation” refers to an installation where every second lamp is equipped with a serial capacitor. The more adequate term would have been lead-lag compensation Current carrying capability of the new switching contacts Bela stbarMax. number of lamps per current path at 230 V / 50 Hz and max. 60°C HS20 HS25 HS40 HS63 60W 0,27A – 22 50 92 129 100W 0,45A – 13 30 55 77 200W 0,91A – 7 15 27 38 300W 1,36A – 4 10 19 26 500W 2,27A – 3 6 11 16 1000W 4,50A – 1 3 6 8 11W 0,16A 1,3µF 60 75 210 310 18W 0,37A 2,7µF 25 30 90 140 24W 0,35A 2,5µF 25 30 90 140 36W 0,43A 3,4µF 20 25 70 140 58W 0,67A 5,3µF 14 17 45 70 65W 0,67A 5,3µF 13 16 40 65 85W 0,80A 5,3µF 11 14 35 60 11W 0,07A – 2*100 2*110 2*220 2*250 18W 0,11A – 2*50 2*55 2*130 2*200 24W 0,14A – 2*40 2*44 2*110 2*160 36W 0,22A – 2*30 2*33 2*70 2*100 58W 0,35A – 2*20 2*22 2*45 2*70 65W 0,35A – 2*15 2*16 2*40 2*60 85W 0,47A – 2*10 2*11 2*30 2*40 11W 0,16A 2,0µF 30 43 67 107 18W 0,37A 2,0µF 20 32 50 80 24W 0,35A 3,0µF 15 32 50 80 36W 0,43A 4,0µF 10 32 50 80 58W 0,67A 7,0µF 6 18 36 46 65W 0,67A 7,0µF 5 18 36 46 85W 0,80A 8,0µF 4 16 33 44 18W 0,09A – 40 40 100 150 36W 0,16A – 20 20 50 75 58W 0,25A – 15 15 30 55 2*18W 0,17A – 2*20 2*20 2*50 2*60 2*36W 0,32A – 2*10 2*10 2*25 2*30 2*58W 0,49A – 2*7 2*7 2*15 2*20 20W 0,09A – 40 52 110 174 50W 0,22A – 20 24 50 80 75W 0,33A – 13 16 35 54 100W 0,43A – 10 12 27 43 150W 0,65A – 7 9 19 29 200W 0,87A – 5 5 14 23 300W 1,30A – 3 4 9 14 50W 0,61A – 16 21 38 55 80W 0,80A – 12 16 28 40 125W 1,15A – 8 11 20 28 250W 2,15A – 4 6 11 15 400W 3,25A – 3 4 7 10 700W 5,40A – 1 2 4 6 1000W 7,50A – 1 1 3 4 Current Fluorescent lamps, uncompensated or with serial compensariont Fluorescent lamps, lead-lag compensation Fluorescent lamps, with parallel compensation Fluorescent lamps, with electronic gear Transformers for ELV halogen lamps Mercury vapour high- pressure lamps uncompensated, e. g.: HQL, HPL Type of lamp Capa- citor Power rating Incandescent lamps
  41. 41. Publication No Cu0204 Issue Date: October 2014 Page 37 FINDER Here ratings are also found for the maximum permissible peak currents. While this may be of limited benefit because the common user will not necessarily know the inrush current peak of a particular load, a very useful supplement are the breaking capabilities for DC, since this mentions that these contactors can also be used for DC up to the defined magnitudes. Also the maximum loads for AC-1, AC-3, AC-7a and AC-7b can be found. Table 9 – Relay and contactor capability ratings by Finder, for devices with a maximum peak current of 80 A and 120 A respectively. HAGER Hager makes life easy. They simply write that all contactor values given are only intended for AC-1 consumers. If electricians need to install different types of load they will need to figure out for themselves. The choice of contactor depends upon a number of parameters, it says there, e. g.  the nature of the supply,  the power it is switching,  the characteristics of the load,  the control voltage required,  the number of operations. The use of lZ060 (heat dissipation inserts) between all contactors installed or between contactors and adjacent devices is required. LEGRAND Legrand take reference to the EN 61095 standard. They also provide a table showing not only different types of consumers (and how many of them can be installed), but also the maximum switched power depending on the number of switching operations a day for a lifetime of 10 years with 200 days of operation per year. Rated current 25A 25A Maximum peak current 80A 120A L-N 250V 250V L-L 440V 440V Rated load AC1 / AC7a (per pole @ 250V) 6250VA 6250VA Rated current AC3 / AC-7b 10A 10A Rated Ioad AC 15 (per pole @ 230V 1800VA 1800VA Single-phase motor rating (230 V AC) 1kW 1kW Rated current AC-7c – 10A Incandescent or halogen lamps 230V – 2000W Compact fluorescent lamps (CFL) – 200W Electronic ballast fluorescent tubes – 800W Magnetic ballast compensated fluorescent tubes – 500W 30V 25A 25A 110V 5A 5A 220V 1A 1A 1000mW 1000mW 10V 10V 10mA 10mA Breaking capacity DC 1 Minimum switching load Rated voltage AC
  42. 42. Publication No Cu0204 Issue Date: October 2014 Page 38 Table 10 – Detailed overview provided by the catalogue of Legrand. Contactor current rating → 16A 20A 40A 63A No. of switchings → n Maximum total power to be connected ≤50 3,5kW 4,5kW 9,0kW 14,0kW 75 3,0kW 3,5kW 7,5kW 12,0kW 100 2,5kW 3,0kW 6,0kW 9,5kW 250 1,5kW 2,0kW 4,0kW 6,0kW 500 1,0kW 1,0kW 2,5kW 4,5kW ≤50 10,0kW 13,0kW 26,0kW 41,0kW 75 9,0kW 11,0kW 22,0kW 36,0kW 100 7,0kW 9,0kW 17,0kW 26,0kW 250 3,0kW 4,0kW 8,0kW 13,0kW 500 2,0kW 3,0kW 6,0kW 9,0kW Lamp ratings → P Maximum number of units to be 40W 40 47 118 156 60W 32 37 87 115 75W 27 30 72 96 100W 21 23 52 71 150W 13 15 36 48 200W 11 12 26 35 300W 8 8 18 25 500W 4 5 11 15 1000W 2 2 7 5 20W 16 19 45 64 50W 11 12 29 42 75W 9 10 25 34 100W 7 8 20 28 150W 4 5 15 19 Method of compensatio n none parallel lead-lag none parallel lead-lag none parallel lead-lag none parallel lead-lag 15W 24 16 – 28 18 – 75 40 – 105 60 – 18W 24 16 32 28 18 38 75 40 85 105 60 120 20W 24 16 32 28 18 38 75 40 85 105 60 120 36W 22 16 18 26 18 21 65 40 45 93 60 65 40W 22 16 18 26 18 21 65 40 45 93 60 65 58W 15 11 11 17 13 13 40 30 29 58 43 40 65W 15 11 11 17 13 13 40 30 29 58 43 40 115W 8 6 7 10 6 9 22 14 18 33 20 24 140W 8 6 7 10 6 9 22 14 18 33 20 24 Tande m 4*18W – – 16 – – 19 – – 48 – – 67 Singlelamps LightingHeating(AC.7a) Incandescent and halogen lamps 12V halogen lamps with ferro- magnetic transformer Fluorescentlampswithferro- magneticcontrolgear,bymethodof compensation 230V single- phase 400V three- phase dependentondailynumber ofswitchings
  43. 43. Publication No Cu0204 Issue Date: October 2014 Page 39 SCHNEIDER ELECTRIC Table 11 – Schneider Electric also offer quite a detailed list. Contactor current rating → 16A 25A 40A 63A 100A Lamp ratings → P Maximum number of units to be connected La 40W 38 57 115 172 250 60W 30 45 85 125 187 75W 25 38 70 100 150 100W 19 28 50 73 110 150W 12 18 35 50 75 200W 10 14 26 37 55 300W 7 10 18 25 37 500W 4 6 10 15 22 1000W 2 3 6 8 12 20W 15 23 42 63 94 50W 10 15 27 42 63 75W 8 12 23 35 52 100W 6 9 18 27 40 150W 4 6 13 19 28 Method of compensation none parallel lead-lag none parallel lead-lag none parallel lead-lag none parallel lead-lag none parallel lead-lag 15W 22 15 – 30 20 – 70 40 – 100 60 – 150 90 – 18W 22 15 30 30 20 46 70 40 80 100 60 123 150 90 180 20W 22 15 30 30 20 46 70 40 80 100 60 123 150 90 180 36W 20 15 17 28 20 25 60 40 43 90 60 67 135 90 100 40W 20 15 17 28 20 25 60 40 43 90 60 67 135 90 100 58W 13 10 10 17 15 16 35 30 27 56 43 42 84 64 63 65W 13 10 10 17 15 16 35 30 27 56 43 42 84 64 63 115W 7 5 6 10 7 10 20 14 16 32 20 25 48 30 37 140W 7 5 6 10 7 10 20 14 16 32 20 25 48 30 37 Tandem 4*18W – – 15 – – 23 – – 46 – – 69 – – 100 18W 14 18 – 34 21 – 57 40 – 91 60 – – – – 35W 3 4 – 9 5 – 14 10 – 24 15 – – – – 55W 3 5 – 9 5 – 14 10 – 24 15 – – – – 90W 2 3 – 6 4 – 9 8 – 19 11 – – – – 135W 1 2 – 4 2 – 6 4 – 10 6 – – – – 180W 1 2 – 4 2 – 6 5 – 10 7 – – – – 70W 8 6 – 12 9 – 20 16 – 32 25 – – – – 150W 4 6 – 7 9 – 13 16 – 18 25 – – – – 250W 2 2 – 4 3 – 8 6 – 6 9 – – – – 400W 1 2 – 3 4 – 5 8 – 8 12 – – – – 1000W – 1 – 1 2 – 2 4 – 4 6 – – – – 18W 74 111 222 333 500 36W 38 58 117 176 260 58W 25 37 74 111 160 2*18W 36 55 111 166 250 2*36W 20 30 60 90 135 2*58W 12 19 38 57 85 7W 133 200 400 600 900 11W 80 120 240 360 540 15W 58 88 176 264 396 20W 44 66 132 200 300 23W 38 57 114 171 256 Compact fluorescent lamps(small ratings) Incandescent lamps including halogen 12V halogen lamps with ferro-magnetic transformer T8fluorescentlampswithferro- magneticcontrolgear,bymethodof compensation Singlelamp orlead-lagpairs T8fluorescent lampswith electronic ballast Single lamp Twin lamp Sodiumvapourlamps low pressure high pressure
  44. 44. Publication No Cu0204 Issue Date: October 2014 Page 40 SUMMARY OF RESULTS Table 12 – Summary of fabricators‘ ratings – 1) It only speaks of “compensated” here; 2) Depending on contact lifetime expectancy. Table 13 – Various derating factors for various types of load given by various relay and contactor suppliers, derived from Table 12. Table 12 gives an overview of the various fabricators ratings. Note that these results are to some degree simplified and generalized to give the best possible overview. For instance, some manufacturers offer only contactors with minimum current ratings of 20 A, 25 A or 40 A. These were listed as 16 A here, since they will need to be selected for 16 A circuits if it is decided to go for this respective fabricator who does not offer any Switches Relays and contactors Peha NL FR EN DE 10A – – – – – – 2000VA – – – – – 16A – – – – – 28 2000VA 13 2000W – 21 19 10A – – – – – – – – – – – – 16A – – – – – 12 – – – – 7 6 10A – – – – – – – – – – – – – 16A – – – – – – – – – – – – – 10A – 14 14 – – – 1000VA – – – – – 16A – – – – – – 1000VA 13 – – 15 13 10A – 14 1) 14 1) – – – 500VA – – – – – 16A – – – – – – 500VA 6 500W 1) – 11 10 10A – 14 1) 14 1) – – – 1000VA – – – – – 16A – – – – – – 1000VA 20 500W 1) – 2*11 2*10 10A – – – – – – – – – – – – 16A – – – – – – – 15 800W – – 25 10A – – – – – – – – – – – – – 16A – – – – – – – – 10 500W 1) – 16 15 10A – – – – – – – – – – – – – 16A – – – – – – 29 – – 800W – – – 10A – – – – – – – – – – – – 16A – – – – – – 1400W – 200W – – ≈1000W 20 /40 2) 22 (2*11) Hager Legrand Schnei- der Finder 6 (or 1*600W) Niko Niko MK Electric Eltako Doepke 100 W incandescent light bulbs 100 W halogen ELV incandescent light bulbs with conventional transformers 100 W halogen ELV incandescent light bulbs with electronic transformers 58 W fluorescent lamps with uncompensated magnetic ballasts How many of these can be operated on one switch, or one contact, respectively? Total wattage of compact fluorescent lamps on one switch 4*18 W (or 2*36W) fluorescent lamps with twin electronic ballast(s) 58 W fluorescent lamps with magnetic ballasts and parallel compensation 58 W fluorescent lamps with electronic ballasts 70A (10ms) 4*18 W fluorescent lamps in “tandem” mode (or 2*36W) with magnetic ballasts and “lead-lag” compensation 2*10 (twin ballasts) 4 (+4*7µF) 58 W fluorescent lamps with magnetic ballasts and “lead-lag” serial compensation 22 (11 pairs) Switches Relays and contactors Peha Niko MK Electric Eltako Doepke Finder Legrand Schnei- der 10A – 0,8696 – 0,8696 – – – – 16A – 0,5435 0,7609 0,5435 0,3533 0,5435 0,5707 0,5163 10A – 0,2609 – – – – – – 16A – 0,1630 0,3261 – – – 0,1902 0,1630 10A – – – – – – – – 16A – – – – – – – – 10A 0,9380 – – 0,4348 – – – – 16A 0,5863 0,9213 – 0,2717 0,5444 – 0,6281 0,5444 10A 0,4078 0,2680 – 0,2174 – – – – 16A 0,2549 0,1675 – 0,1359 0,1092 0,1359 0,2003 0,1821 10A 0,4200 0,6409 – 0,4348 – – – – 16A 0,2625 0,4005 – 0,2717 0,3859 0,1359 0,4245 0,3859 10A – 0,4783 – – – – – – 16A – 0,2989 – – 0,2242 0,2174 – 0,3736 10A – – – – – – – – 16A – – – – 0,2717 0,1359 0,4348 0,4076 10A – – – – – – – – 16A – – 0,6935 – – 0,2174 – – 10A – – – – – – – – 16A – – 0,3804 – – 0,0543 – 0,2717 Derating factors against the respective current ratings given by the respective suppliers Total wattage of compact fluorescent lamps on one switch 58 W fluorescent lamps with magnetic ballasts and “lead-lag” serial compensation 58 W fluorescent lamps with electronic ballasts 4*18 W fluorescent lamps in “tandem” mode (or 2*36W) with magnetic ballasts and “lead-lag” compensation 4*18 W (or 2*36W) fluorescent lamps with twin electronic ballast(s) 100 W halogen ELV incandescent light bulbs with conventional transformers 100 W halogen ELV incandescent light bulbs with electronic transformers 58 W fluorescent lamps with uncompensated magnetic ballasts 58 W fluorescent lamps with magnetic ballasts and parallel compensation 100 W incandescent light bulbs
  45. 45. Publication No Cu0204 Issue Date: October 2014 Page 41 lower ratings. Also, the lamp ratings were partially multiplied, 2 * 100 W replaced with 1 * 200 W or 4 * 18 W replaced with 2 * 36 W, respectively. From these, a number of rough de-rating factors could be derived as compiled in Table 13. These simply represent the ratios of the summed device ratings as shown in table 12 by contact voltage x contact current taken from the relay or contactor rating, respectively. FINDINGS AND CONCLUSIONS Electricians have to actually look at the table for the brand of contactors used to figure out the maximum number of lamps that can be connected. A contactor rated e. g. 16 A of one brand will not necessarily replace a 16 A contactor of another brand for the same type and number of loads. For 230 V halogen lamps of 300 W, for example, 7 lamps may be connected to a 16 A contactor by Schneider Electric. With Legrand it is 8. With Doepke it is only 4 with the HS20 contactor (nominal current of 20 A). This is all very much design dependent. It can for instance depend on the contact material, to mention only one parameter a manufacturer can vary within very wide limits. In the residential installation segment, no problem is likely to arise regarding the use of contactors. Few fluorescent lamps are used in homes. The number of simultaneously connected CFLs is also relatively small. In tertiary applications, such as open-space offices, and industrial applications, there is a greater chance that large groups of lamps will be connected in parallel. In these cases, the electrician will have to take the characteristics of the contactors used into account. The use of relays (in home automation systems, for example) will create more of a problem in the residential segment. These relay modules are usually composed of several smaller relays for AC-1 loads of 4 A, 6 A, 8 A, 10 A. In these cases, the electrician must be on the alert for overload of the relay contacts. Niko provide an example of this, where the relay of the Nikobus output module can only connect 4 parallel-compensated fluorescent lamps. After having compiled partially quite verbose tables, fabricators may be confronted with some challenges when a substantial part of lighting equipment will go to LED. While both lamps and control gear are fabricated in a more or less standardized scheme or at least tiered in certain ever-repeating wattages, such trend is not yet visible with LED lighting equipment.

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