Electrical Measurment Safety


Published on

Understanding hidden hazards and new safety standards.

Published in: Technology
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide
  • Suggestions for presentations: 1 hour presentation: Use all slides except numbers 12, 13 and 14. Hands-on training presentation (1.5 hour, requires a digital multimeter): Use full presentation, including three slides with hands-on exercises (“Safety Inspection: Test leads and probes”, “Safety Inspection: Checking fuses on Amps inputs” and “Safety Inspection: Overload protection on volts inputs...”).
  • This meter had the original fuse replaced with an automotive fuse. When the operator attempted to measure 480 volts with the leads in the amps inputs, the fuse offered no protection. It is better to leave the circuit unfused and open than to use the wrong fuse. Note that the test leads and probes, though damaged, survived more or less intact. This fact, plus the mechanical ruggedness of the meter body, which helped to contain the explosion, contributed to the fact that the operator was not hurt.
  • This meter accidentally contacted Medium Voltage. The operator worked in a West Virginia mine. He disconnected the 13.8 kV buss bar, but did not know that the buss bar was being fed from another connection. He went to check a control circuit behind the buss bar. When he got close to the buss bar, about 4-6” away, there was an arc-over to his probe tips. The resulting plasma fireball singed his beard, but otherwise did not harm him. When he’d recovered sufficiently, he took the meter back to his distributor to see if the warranty still applied...
  • An electrician was loaned this low-cost meter one day because his quality meter was not available. He accidentally went across 480 volts with the leads in the amps jacks. Both fuses are 250 volts. The fuse didn’t open in time to prevent major damage to the meter. However, while the meter is not as dramatically damaged as the previous ones, the electrician was injured - he had severe burns on his forearm, upper arm and shoulder and had his arm in a sling when we met him. A good part of the reason is the cheap quality of the probes and leads (see next slide).
  • There are two lessons here: One is that the safety quality of the leads and probes is just as important, if not more important, than that of the meter itself. The other is that it can happen to anyone: this electrician was a seasoned journeyman with 30-odd years of experience.
  • This photo shows the environment - a typical electrical room - in which the following accident occurred. (See next two slides for more detail of the results). Two experienced senior electricians had just installed a fused disconnect (an enclosure with an external switch to open the circuit and with fuseholders on the inside). They were checking to see if the phases were correctly installed. To do that, they opened a second disconnect enclosure. They planned to use a meter with one probe on Phase A in disconnect #1 and one probe on the presumed Phase A in disconnect #2 - if the phases were the same, the meter would read 0V. However, the leads were not long enough to reach across the room to the other disconnect - so they attached a wire to one probe as a lead extension. The first electrician, whose mind was not on what he was doing, then put the wrong probe on the terminal - it was the probe with the wire on it. When the second electrician contacted his terminal with the wire, he had created a phase-to-phase fault . There was an instant fireball. He fell to his knees and crawled away. His clothes were melted. He suffered severe burns and was rushed to the hospital. He survived and his sight was saved because he had been wearing glasses.
  • Shot of inside of disconnect enclosure where fireball occurred. Note that the damage occurred on the line side of the fuses. The load side is virtually undamaged. This shows a critical mistake that was made - measuring on the line instead of the load side of a circuit protective device. You should always look for the lowest energy point to make the measurement.
  • Closeups of damage at line side of fuse holders.
  • Safety hazards are broken into two broad categories, “operator error” which are avoidable and “electrical environment” (later slide) which are unavoidable. 1. The mA/amps jacks on a DMM connect to a very low impedance test circuit inside the meter. When the low impedance of the DMM’s mA/amps input jacks accidentally are placed across a power circuit, they in effect form a short-circuit. See graphics slide entitled “Misuse of DMM in ammeter mode”. You can demonstrate the input impedance of the amps inputs by measuring it. Put the meter into ohms mode and attach leads. Use the red lead (connected to the V/Ohm input) and put the probe tip into the 10 A and the mA input (typical reading is 0.1 ohm and 10 ohm respectively). An OL reading indicates an open fuse. 2. Older analog meters would typically self-destruct if they contacted power circuit voltages while in ohms mode. 3. Sometimes it is necessary to emphasize what should be obvious. We have seen and heard examples of people not understanding that DMMs are low-voltage measuring devices. Solenoid testers on the other hand are very vulnerable to contact with MV, and there are instances of electricians suffering fatal injuries when accidentally measuring MV with their “wiggies”.
  • Safety hazards are broken into two broad categories, “operator error” which are avoidable and “electrical environment” (later slide) which are unavoidable. 1. The mA/amps jacks on a DMM connect to a very low impedance test circuit inside the meter. When the low impedance of the DMM’s mA/amps input jacks accidentally are placed across a power circuit, they in effect form a short-circuit. See graphics slide entitled “Misuse of DMM in ammeter mode”. You can demonstrate the input impedance of the amps inputs by measuring it. Put the meter into ohms mode and attach leads. Use the red lead (connected to the V/ohm input) and put the probe tip into the 10 A and the mA input (typical reading is 0.1 ohm and 10 ohm respectively). An OL reading indicates an open fuse. 2. Older analog meters would typically self-destruct if they contacted power circuit voltages while in ohms mode. 3. Sometimes it is necessary to emphasize what should be obvious. We have seen and heard examples of people not understanding that DMMs are low-voltage measuring devices and that 1000 Volt is called “Low voltage”. Solenoid testers on the other hand are very vulnerable to contact with MV, and there are instances of electricians suffering fatal injuries when accidentally measuring MV with their “wiggies”.
  • Objective: Get the trainee to think about the safety integrity of test leads and probes, often the weak point in the overall safety of the meter. Module 2 on safety focuses on the meter itself. Test leads are easy to neglect. However, a safety-designed meter with cheap or beat-up test leads, or leads not designed for high energy circuits, is like a new car with bad tires. Double insulation, finger guards, shrouds and recessed input jacks protect against electric shock from accidental contact with live circuits. Shrouds protect against the possibility that the probes could be connected across live voltage and the banana plugs be pulled from the meter. CAT ratings (III-1000 V) are visible on the leads and probes. Test lead resistance is in the 0.1-0.3 ohm range. A single lead can be tested by looping it between the volts and com inputs.
  • Objectives: Measure low input impedance of amps inputs. Quick fuse test. The test is mostly the same, the reading may differ per DMM: 80SeriesV: If a test lead is plugged into the mA/µA or A terminal and the rotary switch is turned to a non-current function, the Meter chirps and flashes “LEAd” if the fuse associated with that current terminal is good. If the Meter does not chirp or flash “LEAd”, the fuse is bad and must be replaced. Fluke 179: <0.5V is okay, OL: fuse bad. Always check the meters manual for the correct procedure (Note that this test does not test for the wrong fuse being installed, as could happen if the original fuse was replaced with a low energy fuse. From a safety point of view, it is preferable that a blown fuse not be replaced, and the circuit left open, if the correct fuse is not available.)
  • Objective: Demonstrate the existence of overload protection. The trainee is now measuring 120 Vac. There is the real possiblity of damage. In this mode, and having been sensitized to safety issues, he or she is asked to successively select Vdc, mVdc, Ohms and even Amps. Typically, trainees are at least a little bit hesitant, and rightfully so. It is not intuitively obvious that the meter will not be damaged. Overload recovery is automatic. The key point to make is that the meter is protected against overvoltages (up to the meter’s rated voltage) as long as the leads are in the volts inputs.
  • IEC 1010 was recently renumbered as IEC 61010 to correlate with the European standards numbering system (EN 61010). The content is the same,and either IEC 1010 or IEC 61010 is acceptable. IEC 61010 is adopted by national standards organizations (such as ANSI in the U.S.) with the addition of a “National Forward” which makes minor additions or changes to take into account the differences between electrical systems and local regulations in various countries and areas. However, by and large, IEC 61010 is a uniform international standard. The major effect of IEC 61010 is to provide much greater protection against transients than the old standard, IEC-348. Transients are a major hazard for personnel as well as a major source of equipment damage.
  • The concept of Categories was not included in the old standard, IEC 348. Categories correlate with available fault current (short circuit current). Categories are defined more in terms of current than voltage: the higher the fault current, the higher the category. If anything, it can be confusing to think in terms of voltage. For one thing, all categories apply to low voltage only (<1000 V); for another, within each category (from CAT IV to CAT I) there are “working voltages” – 1000 V, 600 V, 300 V, 150 V, 50 V The old standard, IEC 348 was based on steady-state voltages. IEC 61010 is based on over-voltage transients, as well as steady-state voltages . Impulse (transient) testing and specified clearance/creepage are both required by IEC 61010. Transient tests are the basic functional test, while clearance/creepage are the basic design spec. To pass the test, a meter must be hit with twenty transients in a row (ten positive, ten negative). The shape and peak value of the transient are specified in the standard.
  • 1. Proximity to source of power determines the installation category (category for short). The introduction of categories is a new concept introduced with IEC 61010 which distinguishes it from IEC 348. 2. Address some common misunderstandings: First of all, emphasize that everything in IEC 61010 refers to low voltage (<1000 V). Even when we talk about the utility level, we are talking about the low voltage connection to the facility. If, as is common in industrial plants, the utility PCC (point of common coupling) is at medium voltage, that portion of the system is not covered by IEC 61010. Secondly, the concept of installation category is different from the concept of voltage level. It is possible for the dc voltage in a copier to be higher that the ac voltage feeding the facility, but the energy available and the potential for electrical explosion (arc and blast) are much different in the two locations. Category does correspond to the level of short circuit current available at that location. The standard recognizes that within a given category, higher voltages require higher levels of protection. This point is introduced later in the presentation.
  • IEC 61010 is principally concerned with the effects of transients from lightning, the worst-case scenario. The reasoning behind IEC 61010 is that the lightning transient will be dampened as it travels through the cabling and various devices of the building. CAT IV - includes (low voltage) outdoor lines and the run between the meter and panel. CAT III - includes the “permanently installed” (i.e., not cord and outlet connected) motor. Three-phase distribution is CAT III. CAT II - includes receptacle outlet loads. The line side of the power supply in electronic equipment would be CAT II, while the electronic circuitry itself would be CAT I. CAT I - The copier has a step-up transformer. Therefore it could have high voltages, but it will not have high energy. The standard acknowledges that transients can come from the load itself, not just from lightning or utility activity. It states that equipment only belongs in a particular category “if it does not cause overvoltages increasing the level specified for that category.” In other words, if a normally CAT III piece of equipment generates transients which are at a CAT IV level, the equipment (and the electrical environment) become CAT IV.
  • Electricians are accustomed to thinking in terms of short circuit currents because one of the key specifications for circuit breakers and fuses is their kAIC rating. KAIC stands for kilo-amps interrupting capacity-the kilo-amps of fault current that will be safely interrupted. Source impedance refers to the total impedance between the load and the source, including the impedance of the wiring. The greater the distance, the greater the impedance. The TVSS (transient voltage surge suppressor) industry has by necessity done a good deal of customer education on this issue. In brief, it is evident that a surge suppressor installed at a service entrance, or at a load panel or at the outlet will be required to survive much different energy levels.
  • Ohm’s Law states that amps = volts / ohms. For the same voltage a 2 ohm source will have 6 times the current and six times the power of a 12 ohm source. A 6 kV test impulse for a CAT II environment has much less energy than a 6 kV impulse for a CAT III environment, even though the CAT II-1000 V voltage is higher than the CAT III-600 V. This is definitely a potential source of confusion. Here is the problem: if a customer looks only at the voltage rating without understanding the category concept, he could actually choose the CAT II-1000 V instrument thinking that it was “safer” than the CAT III-600 V instrument.
  • Under the previous standard, we were trained to look at the voltage rating alone. Now we must understand that with IEC 61010 we must look at both the CAT designation and “working voltage” together. The combination will tell us the transient withstand rating of the test instrument, and, from the users’ point of view, that is where the rubber meets the road. Most IEC-348 meters will not qualify as CAT III meters. And for the test leads or other accessories: The chain is only as strong as the weakest link
  • CAT IV-1000 V is the highest level of safety for which IEC has passed design and testing specifications. Why is the III-600 V / II-1000 V better: It is an improvement over the CAT II-600 V design because it uses 1000 V components and 1000 V creepage (spacing along the surface) specs. It can measure ac or dc to 1000 V It is an improvement over II-600 V because it can withstand the CAT III-600 V transient of 6 kV with a 2 ohm source (36 times more power than the CAT II-1000 V 6 kV spike with a 12 ohm source ). It helps eliminate the confusion over categories and voltage ratings. The end-user is protected whether he selected the meter based on category or on voltage rating.
  • IEC-1010 is the basis for national standards: ANSI/ISA-S82.01-94 (USA) CAN C22.2 No. 1010.1-92 (CAN) EN61010-1: 2001 (EUR) NRTL (National Recognized Testing Lab), such as UL and CSA, test and certify that products meet their safety specs, which are based on the national standards. UL 3111, for example, is based on the ANSI standard which follows IEC 61010. UL 1244 was based on IEC 348. While the system of approval by an independent listing agency is far from perfect, it is much superior to a manufacturer “self-certifying” by stating that products are “designed to” a standard.
  • This is what we often find when we test products.
  • 1) Arc Flash - The transient due to the lightning strike was large enough to cause an arc to strike inside the meter. The circuit(s) or measures that were designed into the meter to protect against this event happening have just failed and now there is a direct short between the two terminals. The resistance of an arc is very low. If one of these electrical transients , or voltage spikes causes an arc inside a hand held meter while it is connected to the terminals; for example, a 550 HP, 480 V motor, a lethal chain of events can begin. Once the arc has formed, any protection circuitry or dielectric spacings inside the instrument have been shorted and all of the available short circuit current flows through the test leads, and the arc. 2) Arc Flash - The path of the current through the arc begins at the motor terminals and ends up back at the motor terminals. The current can be several thousands of amps for long enough to begin melting the tips of the test probes. At this point in time, the source(s) of current into the arc are; the 480 V panel that supplies the motor, and the 480 V motor. A typical 550 HP elevator motor draws 264 FLA (full load amps) at 480 V and will supply 6 to 7 times this much current into a fault (1564 - 1850 amps) . The circuit breaker or fuse protection in front of the motor will let through several thousand amperes before clearing. At these current levels the tip of test leads superheat and the handheld test probes melt away, drawing an arc from the contact point to the probe tip. When the user sees this blue-white flash, a natural reaction is to pull back, away from the motor terminals. This action draws the arc farther away from the contact point. We now have two large arcs, one from each contact point. In the case of the 550 HP motor , this current can be 6 to 7 times the full load amps of the motor (264 FLA) plus the let through from the fuses protecting the motor. Currents as high as 6000 amperes or more can flow for several milliseconds. 3) Arc Blast - As the user pulls the probes away from the motor terminals, an arc is drawn from each probe tip to the motor terminal it was measuring. If these two arcs join to form one, there is a very low impedance path directly between the motor terminals. This arc has a temperature of approximately 6000 °C, the same as the arc of an arc welder, but with several thousand times more energy and potential for destruction. 4) Arc Blast - As this arc grows it super heats the surrounding air creating a pressure front of expanding air and moves according to the laws of physics that govern the force, and direction of the fireball. As we see in this slide, the results can be a very bad day for our service technician. All of the energy available in the electrical system will fuel this arc and form a fireball that reaches the same temperature as the arc of an arc welder. This plasma fireball is capable of moving around and can burn through steel. The effect on anyone that it comes into contact with is severe at best, and deadly at worst.
  • Foreseeable misuse - One of the most common misuses of handheld DMM’s (digital multi-meters), that can result in a chain of events similar to that described above, results from taking a voltage measurement with the test leads in the mA or A terminals. If the instrument is unfused or if the current interrupt rating of the fuse in the instrument is less than the available short circuit current, then an arc is established inside the instrument. Some handheld multi-meters are inappropriate for use in industrial environments because of the low current interrupt rating of the fuses used or because of internal spacings limitations. Knowing the ratings and limitations of your instruments and using them accordingly will help to protect you from electrical shock or other hazards when taking electrical measurements. Key points to make: Think of the meter as a branch circuit in the palm of your hand. Don’t you want proper fault protection on that branch circuit? Always use a fuse specified by the manufacturer.
  • Some customers are confused about why we use a fuse with an IR (interrupting rating) of 17 kAIC when the fuses and breakers on their distribution systems have a much higher IR This slide explains the calculations used to design the fuse used in 1000 V meters manufactured by Fluke (860, 80 Series, 20 and 70 Series III rated CAT III-600 V/CAT II-1000 V). It is simply an exercise in Ohm’s Law: Amps = 1000 V / .060 ohms = 16.7 kA The major assumption is that the meter will not measure voltages which exceed 1000 V at the same time that the leads are in the amps inputs. As long as that is the case, the fuse will interrupt the maximum possible fault current. The unlimited source represents a worst-case scenario, but it doesn’t affect the circuit, because the worst-case fault current that the meter will see is limited by the the impedances in the test leads, shunt resistor, etc. The fuse was tested at a Bussman facility which simulated an unlimited test source (about 200,000 A), and passed. It also passed in tests done by the South African mines.
  • Some manufacturers mark their meters as CAT III-750 V. The IEC 61010 standard does not allow spacing in between 600 V and 1000 V; i.e., a meter can only “meet” CAT III-750 V by meeting CAT III-1000 V, in which case any manufacturer would presumably mark it CAT III-1000 V. Some manufacturer’s meters have 1000 V marked on the front, with no reference to CAT I, II or III. Only on the back and in the manual does it become clear that the meter is CAT I -1000 V (it is dual rated as CAT III-300 V). It is fair to say that there appears to be some “confusion” among manufacturers with regard to these relatively new standards., which is why it is important for you to educate yourself with regard to the new safety standards.
  • The Fluke 70/20 Series meters, originally designed according to IEC 348, when re-rated according to IEC 1010, come up as CAT II-600 V meters. It’s probably accurate to say that they were the premier meters of their generation from a safety point of view - yet they do not qualify as CAT III meters. Therefore, it is recommended that these older meters, including the many 77II-class meters being used to test power circuits, be replaced with CAT III-600 V meters as soon as possible. While there is no regulatory obligation to do so, ultimately it is in the user’s best interest.
  • The first three bullets are self-explanatory. The “one-handed” measurement technique ensures that the lowest impedance path is not through the heart. It works like this: first one lead is attached (ground lead first, if the measurement is phase to ground); then the measurement is made with the other lead. A fault path from phase to phase would exist through the meter, not through the person doing the measurement, as would be the case if he/she were holding one probe in each hand when testing phase to phase voltage. Similarly, a fault from phase to ground would travel along the side of the body, and chances of survival would be greater. Of course, that “extra” hand should be kept where it won’t accidentally make contact with a conducting surface. “One-handed measurement” is nothing but the electricians’ old “one hand in the pocket” trick. The three-point test method implicitly tells electricians to never assume that their test equipment is always working right. Get into the habit of testing your test equipment .
  • Never exceed the working voltage, even if it has a range for it. This particularly true for scopes. Overvoltage Category Examples: CAT IV · Refers to the “origin of installation”, i.e... where low voltage connection is made to utility power.· Electricity meters, primary overcurrent protection equipment.· Outside the building and service entrance, service drop from the pole to building, run between the meter and panel.· Overhead line to detached building, underground line to well pump. CAT III · Equipment in fixed installations, such as switchgear and three phase motors.· Bus and feeder in industrial plants.· Feeders and short branch circuits, distribution panel devices.· Lighting systems in larger buildings.· Appliance outlets with short connections to service entrance. CAT II · Appliances, portable tools, and other household and similar loads.· Receptacle outlets and long branch circuits.1 Outlets more than 10 meters (30 feet) from CAT III source. Two outlets more than 20 meters (60 feet) from CAT IV source. CAT I · Protected electronic equipment.· Equipment connected to source circuits in which measures are taken to limit transient voltages to an appropriately low level.· Any high-voltage, low-energy source derived from a high-winding resistance transformer, such as the high-voltage section of a copier.
  • Example of how channel A may be connected to the input of the firing circuit in an ASD control unit. Note that the differential probe depicted does not need a separate ground connection.
  • This information is from ANSI C62.41 - IEEE recommended practice on surge voltages in low-voltage ac power circuits. In general, the category definitions or descriptions in the IEC standard itself are so general as to be of little educational use. It is helpful to point out that even a 120 V wire run from the house to a garage or storage shed qualifies as CAT IV, because of the potential exposure to lightning. This emphasizes the key concept that category refers to location . Even an underground line is CAT IV, because lightning, when it travels through the earth, tends to seek a low impedance path.
  • From ANSI C62.41 - IEEE recommended practice on surge voltages in low-voltage ac power circuits CAT III - Some examples of typical locations: Three-phase wiring is almost always CAT III or higher. The line side of a panelboard (whether fed by 3-phase or single phase) is CAT III. The load side, at the load side of the 15/20/30 A circuit breaker, could be considered as either CAT III (because permanently installed) or as CAT II (because of the extra protection offered by the breaker). However, this is more or less a moot point, since an electrician working in this environment will typically be making measurements on line and load side and will therefore need a CAT III protected meter.
  • From ANSI C62.41 - IEEE recommended practice on surge voltages in low-voltage ac power circuits CAT II - Some examples of typical locations: The distance specs are included simply to emphasize the point that the standard is built around the natural damping of transients as they travel through the impedances, including the cable impedances, of a system.
  • This is a good place to point out that the standard is not just written for meter manufacturers but for lab instrumentation as well.
  • Electrical Measurment Safety

    1. 1. Electrical measurement safety Understanding hidden hazards and new safety standards
    2. 2. <ul><li>Goals </li></ul><ul><li>Awareness of electrical measurement hazards </li></ul><ul><li>Understand international safety specifications for DMM’s and Scopes </li></ul><ul><li>Understand the four installation overvoltage categories </li></ul><ul><li>Learn how to minimize and avoid electrical measurement hazards </li></ul><ul><li>Outline </li></ul><ul><li>What electrical power can do to a DMM </li></ul><ul><li>Common safety hazards </li></ul><ul><li>Meter safety inspection </li></ul><ul><li>IEC safety standards </li></ul><ul><li>Arc blast </li></ul><ul><li>Meter and Scope safety check list </li></ul>Goals of this education program
    3. 3. Handheld test tool safety Last known earthly residence of automotive fuse used to replace original fuse Test leads survived intact How not to save time...
    4. 4. Handheld test tool safety Test leads destroyed 13.8 kV arced over to test probes. Insides were barbecued. This DMM had a hot date with 13.8 kV.
    5. 5. Handheld test tool safety Probe tips burned off 250V fuse didn’t open in time The wrong meter to use on a power circuit. Poor quality leads and probes led to injury.
    6. 6. Handheld test tool safety The electrician suffered severe burn injuries on his hand and arm. Fingerprints burned into probes
    7. 7. Handheld test tool safety Typical work environment
    8. 8. Handheld test tool safety Aftermath of an accident
    9. 9. Handheld test tool safety If it melts metal, what does it do to people?
    10. 10. <ul><li>Arc from transients (lightning, load switching) </li></ul><ul><ul><li>Protection: Independent certification to meet CAT III-1000 V or CAT IV 600 V </li></ul></ul><ul><li>Voltage contact while in continuity or resistance </li></ul><ul><ul><li>Protection: Overload protection in Ohms up to the meter’s voltage rating </li></ul></ul><ul><li>Measuring voltage with test leads in current jacks </li></ul><ul><ul><li>Protection: High energy fuses rated to the meter’s voltage rating Use meters / testers without current jacks </li></ul></ul><ul><li>Shock from accidental contact with live components </li></ul><ul><ul><li>Protection: Test Leads double insulated, recessed / shrouded, finger guards, CAT III – 1000 V. Replace when damaged </li></ul></ul><ul><li>Using meter or tester above rated voltage </li></ul><ul><ul><li>Protection: Good karma </li></ul></ul>Common DMM / tester hazards
    11. 11. Common DMM safety hazards <ul><li>Measuring voltage while test leads are in the current jacks: short-circuit! </li></ul><ul><ul><li>Protection: Fluke meters use high energy fuses. </li></ul></ul><ul><li>Contact with ac or dc power source while in Ohms mode. </li></ul><ul><ul><li>Protection: Use a meter with “Overload Protection”. Functions are self-protected to the meter’s rated voltage. </li></ul></ul><ul><li>Using meter above rated voltage, i.e., on voltage circuits over 1000V. </li></ul><ul><ul><li>Protection: Good karma </li></ul></ul>Three common errors that are avoidable
    12. 12. Safety inspection <ul><li>Check test lead resistance: Step 1: Insert leads in V/  and COM inputs. Step 2: Select  , touch probe tips. Good leads are 0.1 - 0.3  . How do you check a single test lead? </li></ul><ul><li>Visually check for: </li></ul><ul><li>CAT III-1000 V/CAT IV-600 V rating </li></ul><ul><li>Double insulation </li></ul><ul><li>Shrouded connectors, finger guards </li></ul><ul><li>Insulation not melted, cut, cracked, etc. </li></ul><ul><li>Connectors not damaged: no insulation pulled away from end connectors </li></ul><ul><li>Probe tips: not loose or broken off </li></ul>Test leads and probes
    13. 13. Safety inspection <ul><li>Step 1: Plug test lead in V/  input. Select  . </li></ul><ul><li>Step 2: Insert probe tip into mA input. Read value. </li></ul><ul><li>Step 3: Insert probe tip into A input. Read value. </li></ul><ul><li>Is the fuse okay? What would an open fuse read? </li></ul>Checking meter fuses on most meters
    14. 14. Safety Inspection With leads in V/  and COM inputs: Step 1: Select V and put probes in a live outlet. Will you damage the meter if you... Step 2: Select mV Step 3: Select  Step 4: Select A. Overload protection is only to DMM’s rated voltage. Overload protection on volts inputs
    15. 15. New IEC Safety Standards
    16. 16. International Electrotechnical Commission <ul><li>IEC 61010 (2 nd edition) is the basis for: </li></ul><ul><ul><li>ANSI/ISA-S82.01-94 (US) </li></ul></ul><ul><ul><li>CAN C22.2 No. 1010.1-92 (CAN) </li></ul></ul><ul><ul><li>EN61010-1:2001 (EUR) </li></ul></ul><ul><li>EN 61010 (2 nd edition) is the new standard for low voltage “test, measurement and control equipment”. </li></ul><ul><li>EN 61010 (2 nd edition) provides much improved protection against “overvoltage impulse transients” - voltage spikes </li></ul><ul><li>2 nd edition mandatory since 1-1-2004 in Europe and adopted in July 2004 in US/Canada </li></ul>
    17. 17. IEC 61010 key concepts <ul><li>CATEGORIES: CAT I to CAT IV </li></ul><ul><ul><li>The greatest danger from transients is in the high categories, because they could trigger an arc blast. </li></ul></ul><ul><li>IMPULSE TESTING: No failure allowed </li></ul><ul><ul><li>Meters must be tested by being hit with a specified number of transients, with specified peak voltages. </li></ul></ul><ul><li>INTERNAL SPACING: increased </li></ul><ul><ul><li>Clearance (distance through the air) and </li></ul></ul><ul><ul><li>Creepage (surface distance) are increased. </li></ul></ul>Protection against overvoltage transients
    18. 18. Overvoltage category <ul><li>The level and energy of voltage impulses is dependent on the location. The closer the location is to the power source, the higher the available fault current, the higher the category </li></ul><ul><li>IEC 61010 defines four locations or categories: </li></ul><ul><ul><li>CAT IV “Origin of installation” Utility level and any outside cable run </li></ul></ul><ul><ul><li>CAT III Distribution wiring, including “mains” bus, feeders and branch circuits; permanently installed loads. </li></ul></ul><ul><ul><li>CAT II Receptacle outlet circuit; plug-in loads. </li></ul></ul><ul><ul><li>CAT I Protected electronic circuits </li></ul></ul>
    19. 19. Category locations
    20. 20. Overvoltage category <ul><li>The higher the short circuit fault current available, the higher the category </li></ul><ul><ul><li>High energy transients are much more dangerous, because they can trigger an arc blast </li></ul></ul><ul><li>The greater the source impedance, the lower the category </li></ul><ul><ul><li>Transients are dampened by system impedance as they travel from the point where they were generated. </li></ul></ul><ul><li>TVSS (transient voltage surge suppression) devices are sized larger (more joules) at the panel than at the receptacle outlet. </li></ul>Common sense ways to think of categories
    21. 21. Determining the true voltage withstand rating <ul><li>WITHIN each Category: </li></ul><ul><li>There are designated “working voltages” (50, 150, 300, 600, 1000 V). </li></ul><ul><li>A higher voltage has a higher transient withstand </li></ul><ul><li>CAT IV example: </li></ul><ul><ul><li>CAT IV – 600 V: 8 kV impulse </li></ul></ul><ul><li>CAT III example: </li></ul><ul><ul><li>CAT III – 600 V: 6 kV impulse </li></ul></ul><ul><ul><li>CAT III – 1000 V: 8 kV impulse </li></ul></ul><ul><li>CAT II example: </li></ul><ul><ul><li>CAT II – 600 V: 4 kV impulse </li></ul></ul><ul><ul><li>CAT II – 1000 V: 6 kV impulse </li></ul></ul>
    22. 22. When is 600 V more than 1000 V? <ul><li>CAT III-600 V or CAT II-1000 V? </li></ul><ul><li>The greater the source impedance,the lower the Category: </li></ul><ul><li>CAT IV-600 V: </li></ul><ul><ul><li>8 kV impulse </li></ul></ul><ul><li>CAT III-600 V: </li></ul><ul><ul><li>6 kV impulse </li></ul></ul><ul><ul><li>2 ohm test source </li></ul></ul><ul><li>CAT II-1000 V: </li></ul><ul><ul><li>6 kV impulse </li></ul></ul><ul><ul><li>12 ohm test source </li></ul></ul><ul><ul><li>A CAT III-600 V 6k V test impulse has 6 times the current of a CAT II-1000 V 6 kV test impulse! </li></ul></ul>
    23. 23. First the CAT, then the voltage <ul><li>Voltage rating by itself can be misleading. </li></ul><ul><ul><li>CAT III-1000 V (8 kV transient) is safer than CAT III-600 V (6k V transient) </li></ul></ul><ul><ul><li>But CAT III-600 V is safer than CAT II-1000 V </li></ul></ul><ul><li>First know the category you are working in, then choose the appropriate voltage rating. </li></ul><ul><li>If you ever measure power circuits, you should use a CAT III-600 V or CAT IV 600 V/CAT III-1000 V meter. </li></ul><ul><ul><li>And CAT IV 600 V/CAT III-1000 V test leads and probes. </li></ul></ul>
    24. 24. Look for CAT III or CAT IV markings CAT III- 600 V CAT III-1000V CAT IV-600 V
    25. 25. Levels of CAT protection CAT Transient with Fuse and Clearance Creepage 2  Source overload (air) (surface) Rating III-1000 V 8000 V 1000 V 16.0 mm 16.0 mm IV-600 V III-600 V 6000 V 1000 V 11.5 mm 14.0 mm II-1000 V II-600 V 6000 V 600 V 11.5 mm 11.5 mm
    26. 26. <ul><li>IEC sets standards but does not test or inspect for compliance. </li></ul><ul><ul><li>A manufacturer can claim to “design to” a standard with no independent verification. </li></ul></ul><ul><ul><li>To be UL-Listed, CSA or TUV-Certified , a manufacturer must employ the listing agency to TEST the product’s compliance with the standard. </li></ul></ul><ul><ul><li>Look for the listing agency’s emblem on the meter. </li></ul></ul>“ Listed” vs. “designed to”
    27. 27. “ Designed to EN 1010-1” Brand A Brand B Brand C Markings CAT II – 750 V CAT III D of C to 1000 V Input IEC 1010-1 CAT II – 1000 V Cat III – 1000 V CAT III – 1000 V Creepage clearance 3.7 mm 2.5 mm 7.5 mm Doesn’t Doesn’t Doesn’t comply comply comply with 5.7 mm with 16 mm with 16 mm Transient tests Input protection Display Input protection components window components opened breakdown opened under high @ CAT II level voltage Tested @ But can the product pass testing...
    28. 28. 1 Flashover inside meter 3 Arcing at the terminals 4 Arc blast 2 Fault current in test leads
    29. 29. Misuse of DMM in ammeter mode
    30. 30. Fuse protection on amps inputs
    31. 31. What’s the bottom line? <ul><li>If you work on power circuits, you need a CAT III-600 V or CAT IV-600 V/ CAT III 1000 V meter. </li></ul><ul><li>Look for the CAT rating and voltage rating marked near the input jacks. </li></ul><ul><ul><li>CAT or voltage rating alone can be misleading </li></ul></ul><ul><li>Look for independent certification. UL 3111 </li></ul>CAT IV-600 V CAT III-1000 V
    32. 32. What’s the bottom line? <ul><li>If you use a scope on power circuits, you need a CAT III-600 V scope and scope probes. </li></ul><ul><li>Look for the CAT rating and voltage rating marked near the input jacks. </li></ul>CAT III-600 V
    33. 33. What’s the bottom line? <ul><li>Safety must be built-in </li></ul><ul><li>An industrial grade meter devotes 10 % - 15 % of components exclusively to protection. </li></ul><ul><li>Built-in protection against the most common safety hazards: </li></ul><ul><ul><li>High voltage transients and danger of arc-over </li></ul></ul><ul><ul><li>Voltage contact while in continuity or resistance mode </li></ul></ul><ul><ul><ul><li>High integrity components </li></ul></ul></ul><ul><ul><li>Voltage measurement while test leads are plugged into amps jacks </li></ul></ul><ul><ul><ul><li>High energy fuses </li></ul></ul></ul>CAT IV-600 V CAT III-1000 V Overload protection on all functions 1000V high energy fuses
    34. 34. What about my old meter? <ul><li>Unless a meter was specifically designed to meet CAT III-600 V or higher, it is not safe to use on power circuits. Most meters produced before 1997 do not meet the standard. </li></ul>Old er Fluke 70 Series-III CAT II-600 V UNDER RATED New 170 Series CAT IV-600 V CAT III-1000 V PROPERLY RATED Original Fluke 70 Series NOT RATED Newer meters also have additional features and capabilities Larger displays Back light 1000 Vac capability Capacitance Frequency Magnetic hangers Temperature 3X dc accuracy 2X ac accuracy Min / Max Record Probe holders Battery door
    35. 35. Meter safety checklist <ul><li>Insist on these safety features: </li></ul><ul><li>Fused current inputs (high energy fuses). </li></ul><ul><li>Overload protection on the ohms function. </li></ul><ul><li>Test leads that have shrouded connectors and finger guards. </li></ul><ul><li>Recessed input jacks. </li></ul><ul><li>Meet the latest safety standards (CAT III-600 V or CAT IV 600 V/CAT III 1000 V) and are independently certified. </li></ul>
    36. 36. Meter safety checklist <ul><li>Watch for: </li></ul><ul><li>Cracked or oily case </li></ul><ul><li>Broken input jacks </li></ul><ul><li>No meter is safe when improperly used. </li></ul><ul><li>Use meters within their rating. </li></ul><ul><li>Use meters designed for measurements on power circuits. </li></ul><ul><li>Use replacement fuses approved by the manufacturer. </li></ul>
    37. 37. Test lead safety checklist <ul><li>Don’t let test leads be a weak point </li></ul><ul><li>CAT III-1000 V or CAT IV 600 V/ CAT III 1000 V rating </li></ul><ul><li>Double insulation </li></ul><ul><li>Shrouded connectors </li></ul><ul><li>Arc Flash Hazard consideration using specialized probes and PPE materials </li></ul><ul><li>Finger guards </li></ul><ul><li>Insulation not damaged: not melted, cut, cracked, stretched </li></ul><ul><li>Connectors: no insulation pulled away from end connectors </li></ul><ul><li>Probe tips: not loose or broken off (too short) </li></ul>
    38. 38. Safety first <ul><li>Safe practices include but are not limited to: </li></ul><ul><li>Whenever possible, work on de-energized circuits. Follow proper lock-out/tag-out procedures. </li></ul><ul><li>Use well maintained tools and appropriate safety gear </li></ul><ul><ul><li>Safety glasses, insulated tools, insulating gloves, flash suits, insulating mats, etc. </li></ul></ul><ul><li>Don’t work alone. </li></ul><ul><li>Practice safe measurement techniques. </li></ul><ul><ul><li>Always connect the grounded lead first, hot second. </li></ul></ul><ul><ul><li>Disconnect the hot lead first, grounded lead second. </li></ul></ul><ul><li>Use the three-point test method. </li></ul><ul><ul><li>Test known circuit, measure target circuit, then re-test known circuit. </li></ul></ul>
    39. 39. Oscilloscope safety <ul><li>Category ratings </li></ul><ul><li>Select a scope and probes and clamps for the worst case category </li></ul><ul><li>Voltage ratings </li></ul><ul><li>Working voltage </li></ul><ul><li>Transient voltage </li></ul>Overvoltage Working voltage Peak impulse Test source category (dc or ac – rms to grnd) transient (Ohm = V/A) (20 repetitions) CAT I 600V 2500 V 30 ohm source CAT I 1000V 4000 V 30 ohm source CAT II 600V 4000 V 12 ohm source CAT II 1000V 6000 V 12 ohm source CAT III 600V 6000 V 2 ohm source CAT III 1000V 8000 V 2 ohm source
    40. 40. Oscilloscope safety <ul><li>Line powered bench scopes </li></ul><ul><li>Use a differential or isolation probe to separate the earth ground connection of the scope from high energy circuits that are also referenced to earth ground. </li></ul><ul><li>Isolate bench scope grounds only in conjunction with differential/isolation probes. </li></ul>Differential Input + – 50Ω BNC Output
    41. 41. Oscilloscope safety Typical use of a differential probe Typical use of a differential probe DC VOLTAGE DC VOLTAGE TO MOTOR
    42. 42. Oscilloscope safety <ul><li>Battery powered scopes - </li></ul><ul><li>Have inherent ground isolation for superior common mode noise rejection </li></ul><ul><li>Some come standard with CAT III 600 volt probes for measurements in high energy circuits </li></ul>
    43. 43. Oscilloscope safety <ul><li>Probes </li></ul><ul><li>Beware of bench scope probes - they are usually CAT I - 500 volts </li></ul><ul><li>Don’t use CAT I or II divider probes on CAT III circuits </li></ul><ul><li>Don’t use probes with exposed metal parts </li></ul><ul><li>Don’t use probes without specified ratings </li></ul><ul><li>Read the manual for safe probe connections - they may vary greatly between instruments </li></ul>
    44. 44. Oscilloscope safety <ul><li>Safe practices </li></ul><ul><li>De-energize circuits </li></ul><ul><li>Use protective gear </li></ul><ul><li>Do not exceed instrument voltage and category ratings </li></ul><ul><li>Use dc coupling - ac coupling may not reveal dangerous voltages </li></ul><ul><li>Use 3-point test method </li></ul><ul><ul><li>Test known live circuit </li></ul></ul><ul><ul><li>Test target circuit </li></ul></ul><ul><ul><li>Test known live circuit again </li></ul></ul><ul><li>Avoid holding or touching the scope if possible </li></ul>
    45. 45. Oscilloscope safety <ul><li>Current clamps </li></ul><ul><li>Clamps have category and working voltage ratings </li></ul><ul><li>Do not exceed them </li></ul>CAT III circuits - Make sure you use Cat III leads AND scope AND current clamp
    46. 46. What’s the bottom line? <ul><li>If you use a scope on power circuits, you need a CAT III-600 V scope and scope probes. </li></ul><ul><li>CAT II scopes and probes are mainly for loads that plug into a receptacle outlet. </li></ul>CAT III-600 V
    47. 47. CAT IV <ul><li>Equipment of overvoltage category IV is for use at the origin of the installation (utility service). </li></ul><ul><ul><li>Outside and service entrance </li></ul></ul><ul><ul><li>Service drop from pole to building </li></ul></ul><ul><ul><li>Run between meter and panel </li></ul></ul><ul><ul><li>Overhead line to detached building </li></ul></ul><ul><ul><li>Underground line to well pump22 </li></ul></ul>
    48. 48. CAT III <ul><li>Premises wiring: “mains”circuits, i.e., bus and feeders and distribution panels </li></ul><ul><li>Permanently installed loads: motors, lighting systems, drives, load centers </li></ul><ul><li>Typically separated from utility service by at least a single level of transformer isolation </li></ul><ul><li>Does not include receptacle plug-in loads, except in the case of heavy appliance outlets with “short” connections to service entrance </li></ul>
    49. 49. CAT II <ul><li>Loads that plug in at receptacle outlet </li></ul><ul><li>Examples of such equipment are appliances, portable tools and other household and similar loads </li></ul><ul><li>All outlets at more than 10 m (30 ft) from Category III </li></ul><ul><li>All outlets at more than 20 m (60 ft) from Category IV </li></ul>
    50. 50. <ul><li>Equipment in which measures are taken to limit transient overvoltages to an appropriately low level </li></ul><ul><ul><li>Examples are protected electronic circuits. A copier that has an internal step-up transformer and 1000 Vdc is still a CAT I-1000 V machine, because the current levels are so low </li></ul></ul>CAT I