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Electrical Measurment Safety
- 1. Electrical measurement safety Understanding hidden hazards and new safety standards
- 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. 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. 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. Handheld test tool safety The electrician suffered severe burn injuries on his hand and arm. Fingerprints burned into probes
- 7. Handheld test tool safety Typical work environment
- 8. Handheld test tool safety Aftermath of an accident
- 9. Handheld test tool safety If it melts metal, what does it do to people?
- 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. New IEC Safety Standards
- 24. Look for CAT III or CAT IV markings CAT III- 600 V CAT III-1000V CAT IV-600 V
- 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
- 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. 1 Flashover inside meter 3 Arcing at the terminals 4 Arc blast 2 Fault current in test leads
- 29. Misuse of DMM in ammeter mode
- 30. Fuse protection on amps inputs
- 41. Oscilloscope safety Typical use of a differential probe Typical use of a differential probe DC VOLTAGE DC VOLTAGE TO MOTOR
Notas do Editor
- 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.