Fuses vs. Circuit Breakers


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Why should engineers consider circuit breakers instead of fuses in their designs?

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  • There are basic differences between circuit breakers and fuses. Identical design does not mean interchangeability. The time/current curve referring to the trip characteristics is the major difference between thermal circuit breakers and simple blade fuses. Resettability is certainly an advantage of the circuit breaker and so is the fact that the breaker - unlike the fuse - does not age. In combination with a manual release button the cbe may also be used for disconnecting circuits during repair work. It is the thermal circuit breakers that especially improve the protection of cables and loads by reducing the trip current with rising ambient temperature. The maximum current decreases in the same degree as does the capacity of the load.
  • The trip curve in the data sheet shows the max. possible tolerance of the trip time in dependence of the current. The grey band shows the range at 21°C. The dotted lines show the lower tolerance limit of the trip curve at max. operating temperature and the max. possible trip time at minimum operating temperature. Attention: the curve always shows a tolerance band. The trip curve of the individual breaker lies somewhere within the band and may be moved to either side with rising or falling temperature.
  • The trip curve band shown here for the blade fuse is taken from the ISO 8820 standard. Most manufacturers of fuses indicate just these values in their data sheets. The tolerance limits of DIN 72581 are not entirely identical, but comparable. The comparison shown here clearly indicates the difference between a thermal CBE and a fuse: At the same rated current the CBE responds more delayed, i.e. inrush peaks are tolerated, although it trips at the same overload. Therefore the circuit breaker is ideally suited for protection of lamps, motors and capacitive loads (electronic components).
  • The diagram shows the temperature behaviour of normally used cables in the automotive industry. The line shows when the critical temperature for the cable is reached (at which current, after which time). The critical temperature of the cable is defined as the temperature at which the isolation of the cable may be damaged irreversibly.
  • As an example I chose the curve of the 1.0mm 2 cable. Standardisation is easy with this curve. Protection is normally done with 10A. In order to achieve a standardised characteristic curve as in the data sheet of the circuit breaker the current values have to simply be divided by 10. Therefore referencing to the trip curve band of the circuit breaker is easy.
  • This diagram shows a comparison of the characteristic curves of circuit breaker, fuse and cable. Current rating of CBE and fuse are identical. In both cases the current rating is defined as being the limit which causes the protective element to trip. Rated current of the cable is standardised so as to be in accordance with the following table: 0.75 mm 2 = 7.5A 1.0 mm 2 = 10A 1.5 mm 2 = 15A 2.0 mm 2 = 20A 2.5 mm 2 = 25A These are the usual cross sections in automotive industry, taking the allowed operating temperature into account. The green curve of the cable is above the tolerance band for the trip time of the CBE. Thus the cable is at any rate protected against overheating. (The upper dotted line is negligible as it refers to –30°C and at this temperature the cable curve would also be moved to the right. The trip curve of the CBE is as temperature-dependent as the cable curve.)
  • Trip time behaviour and pulse resistance The different time behaviour is the reason why CBEs allow to better utilize the cable capacities. Blade fuses often have to be oversized in order to prevent them from ageing through the inrush pulse or even from blowing. In this case the cable has to be adapted to the fuse value, i.e. a bigger cross section is necessary in order to ensure protection by the considerably oversized fuse in the event of an overload or short circuit The CBE is hardly prone to inrush peaks. We may therefore choose a smaller current rating and a smaller cable cross section is required.
  • The example shows that a pulse of 100ms and 4 times rated current (of the fuse) might cause the fuse to blow. The characteristic curve of the fuse is therefore moved horizontally until it is made sure that the pulse will in not cause the fuse to blow. In our example the fuse requires double the current rating of a circuit breaker so as not to be damaged by the inrush current of the load. The cable has to be selected in a size to as to make sure that it won’t be damaged when continuously loaded with the rated current of the fuse. Thus a fuse requires the double cable cross section.
  • When fuses are replaced by circuit breakers of the same rating in the electrical system of a car, reliability is increased as nuisance trippings become more unlikely. The delayed trip behaviour of the circuit breaker makes it less sensitive to current peaks. Besides, delayed fuses achieve their trip time behaviour by means of tin drops on the fuse element. With high temperatures the tin diffuses into the copper of the fuse element (= bronze). This causes the fuse element to melt because of reduced melting temperature. This process is irreversible. So if a fuse is loaded with current peaks it might be pre-damaged.
  • The CBE is not sensitive against inrush currents. Therefor it is possible to replace a fuse with a CBE having a lower rated current. E.g. replacing a 15A fuse with a 10A CBE. This is not a problem because a fuse can only be used up to 75% of it’s rated current - according to ISO 8820 and the datasheet of blade type fuses. The reason for this restriction is the aging problem of fuses. The CBE is increasing the security because the load is better protected and the tripping characteristic for higher currents is as fast as the tripping behavior of the fuse.
  • Two types of circuit breakers - two philosophies The two automotive E-T-A circuit breakers type 1610 and type 1170 have different trip curves. Compared to the 1610 the curve of type 1170 is a bit more delayed and thus more similar to a cable’s characteristic curve. Type 1170 allows the maximum utilisation of a cable’s ampacity. Together with its other benefits - 400A rupture capacity, tease-free and trip-free mechanism - it makes type 1170 a fully-featured circuit breaker for cars. Type 1610 has a slightly faster curve. MAN makes use of this fact: the combination of re-defined current ratings (the 10A unit has a 7.5A bimetal) with a faster curve helps to achieve a trip time behaviour as required by the DIN72581. This means maximum safety combined with the benefit of a circuit breaker - resettability. Type 1610-41 may thus be considered an “automatic, reusable fuse”.
  • Fuses vs. Circuit Breakers

    1. 1. E-T-A Elektrotechnische Comparison Fuses vs. Circuit Breakers
    2. 2. E-T-A Elektrotechnische Part 1: General advantages of CBEs Part 2: 1610/1170 versus Blade Type Fuses Comparison Fuses vs. Circuit Breakers
    3. 3. E-T-A Elektrotechnische Part 1: General advantages of CBEs Part 2: 1610/1170 versus Blade Type Fuses Comparison Fuses vs. Circuit Breakers
    4. 4. E-T-A Elektrotechnische What is a fuse? A fuse is a device that protects against damage from excessive current. It contains a short piece of wire made of an alloy that melts readily. The flow of current through a fuse causes the wire to heat up and melts when excessive current passes through the fuse. This action burns out the fuse and breaks the circuit. It also interrupts the flow of electricity because a fuse is always connected in series with the circuit it protects. A burned-out fuse which is commonly called a "blown" fuse must be replaced for the circuit to function. Comparison Fuses vs. Circuit Breakers
    5. 5. E-T-A Elektrotechnische Who invented the fuse? ?
    6. 6. E-T-A Elektrotechnische The fuse was invented (and patented) in 1880 by Thomas Alva Edison, the famous inventor of the incandescent light bulb. Comparison Fuses vs. Circuit Breakers
    7. 7. E-T-A Elektrotechnische Different types of fuses Blade Type Fuses Glass Fuses SMT Fuses Bolt-In Fuses Comparison Fuse Types
    8. 8. E-T-A Elektrotechnische CBEs: Usable many times Fuses: Usable once only Therefore prices cannot be compared on a one circuit breaker/one fuse basis. ON - OFF ON - OFF ............ .............ON - OFF ON - OFF Comparison Fuses vs. Circuit Breakers
    9. 9. E-T-A Elektrotechnische Convenient resetting of CBEs reduces downtime and service repair costs. There is no need of spares. And: Risk of using temporary inappropriate substitutes is eliminated, warrenty costs are reduced. Damned! Has anybody seen a spare fuse! Comparison Fuses vs. Circuit Breakers
    10. 10. E-T-A Elektrotechnische Most CBEs have a status indication. Fuses don‘t! E-T-A Type 1658 ON OFF Comparison Fuses vs. Circuit Breakers
    11. 11. E-T-A Elektrotechnische Many types of CBEs are usable as ON/OFF switches Fuses aren‘t! The E-T-A series 3120 is used to switch the grain mill ON and OFF, at the same time protecting the electric motors from overheating by overcurrents. Example Comparison Fuses vs. Circuit Breakers
    12. 12. E-T-A Elektrotechnische CBEs: single and multipole versions are available. Fuses: only single pole versions available.. Some Examples E-T-A 8340-F, three pole E-T-A 8345, two pole E-T-A 3140 three pole Comparison Fuses vs. Circuit Breakers
    13. 13. E-T-A Elektrotechnische CBEs: No hazard when the circuit breaker is unintentionally switched onto a short circuit. Fuses: Installing a fuse with the load connected may cause an open intensive arc which is a potential hazard for personnel. This fuse was plugged in on an existing short circuit. The arc destroyed the terminal block. Comparison Fuses vs. Circuit Breakers
    14. 14. E-T-A Elektrotechnische Many CBEs are available with auxiliary contacts. Fuses aren‘t! E-T-A 2210-S with two integral auxiliary contacts Terminals of the auxiliary contacts Comparison Fuses vs. Circuit Breakers
    15. 15. E-T-A Elektrotechnische CBEs: No shifting of characteristic curves. Fuses: Ageing may shift the characteristic curves which may cause nuisance tripping (expensive downtime and stoppages) Time curve of a 10A blade type fuse (Littlefuse) Comparison Fuses vs. Circuit Breakers
    16. 16. E-T-A Elektrotechnische CBEs: Well adjusted to the load, even in the event of high inrush currents from capacitors and motors. Fuses: Trip upon inrush currents; otherwise a higher current rating, possibly requiring a larger wire size, must be used, in which case protection from low overloads is no longer ensured Diagram of start-up current of an unloaded asynchronous motor Diagramm: E-T-A Laboratory
    17. 17. E-T-A Elektrotechnische Thermal and thermal-magnetic CBEs: Because of its temperature dependent characteristic curve the circuit breaker perfectely adapts to the temperature dependent capability of the load. Fuses: The characteristic curve is independent from the ambient temperature.
    18. 18. E-T-A Elektrotechnische You‘ll find more manufacturers of fuses in our competitor database. Manufacturers of fuses
    19. 19. E-T-A Elektrotechnische Part 1: General advantages of CBEs Part 2: 1610/1170 versus Blade Type Fuses Comparison Fuses vs. Circuit Breakers
    20. 20. E-T-A Elektrotechnische Reset button / status indication Manual release button with colour coding Comparison Fuses vs. Circuit Breakers
    21. 21. E-T-A Elektrotechnische Test current A Operating time s min. max. 1.1 IN 360 000 (100h) ∞ 1.35 IN 0.75 600 2 IN 0.15 5 3.5 IN 0.04 0.5 6 IN 0.02 0.1 Trip time characteristics The time characteristics of the blade fuse is stipulated in standard 72581 and ISO 8820. Thermal circuit breakers show a delayed response compared to fuses. ISO8820-3 Comparison Fuses vs. Circuit Breakers
    22. 22. E-T-A Elektrotechnische Trip time characteristic The time/current behaviour of circuit breakers depends on the type. This is the trip time curve of type 1610. Comparison Fuses vs. Circuit Breakers Data sheet 1610
    23. 23. E-T-A Elektrotechnische The trip time behaviour of a circuit breaker depends on the type. Trip curve comparison of type 1610 and blade fuse. Data sheet 1610 Trip time characteristic Comparison Fuses vs. Circuit Breakers
    24. 24. E-T-A Elektrotechnische 5 10 50 100 500 0.1 1 10 100 1000 10000 0,35 0,5 0,75 1,0 1,5 2,0 2,5 Heat-up time (s) of a cable from 20°C up to max. temperature of 160°C current (A) t Temperature behaviour of automotive cables Comparison Fuses vs. Circuit Breakers
    25. 25. E-T-A Elektrotechnische 5 10 50 100 500 0.1 1 10 100 1000 10000 1,0 Current (A) t Trip curve of the 1mm² cable - rated load 10A Temperature behaviour of automotive cables Heat-up time (s) of a cable from 20°C up to max. temperature of 160°C Comparison Fuses vs. Circuit Breakers
    26. 26. E-T-A Elektrotechnische The green line shows the trip time required by a cable with corresponding rating to reach 160°C at a certain current. Both the fuse and the circuit breaker are fast enough. Data sheet 1610 Trip time characteristic Comparison Fuses vs. Circuit Breakers
    27. 27. E-T-A Elektrotechnische Insensitive to inrush peaks. Overload is switched off reliably. A smaller rating is possible with the same load. Trip time characteristics Advantages of the circuit breaker t I(t) Comparison Fuses vs. Circuit Breakers
    28. 28. E-T-A Elektrotechnische Three ways to find your rating Comparison Fuses vs. Circuit Breakers
    29. 29. E-T-A Elektrotechnische Circuit breakers allow the use of thinner cables with the same rated current. Fuses have to be oversized - and so do the cables. Less copper through thinner cables Comparison Fuses vs. Circuit Breakers
    30. 30. E-T-A Elektrotechnische Replace fuses by circuit breakers with the same current rating Current peaks are tolerated with unchanged protection. Circuit breakers do not age. tin Comparison Fuses vs. Circuit Breakers
    31. 31. E-T-A Elektrotechnische Disconnection with smaller overcurrent. Fast in the event of a short circuit. Replace fuses by circuit breakers with smaller current rating Comparison Fuses vs. Circuit Breakers
    32. 32. E-T-A Elektrotechnische Cable characteristics Cable characteristics Comparison Fuses vs. Circuit Breakers
    33. 33. E-T-A Elektrotechnische33 CBs are resettable CBs help to reduce cable cross sections CBs offer enhanced safety CBs increase the reliability