Working Safely with Digital Multimeters, Part 1
As distribution systems and loads become more complex, the possibilities of transient overvoltages increase. Motors, capacitors and power conversion equipment, such as variable speed drives, can be prime generators of spikes. Lightning strikes on outdoor transmission lines also cause extremely hazardous high-energy transients. If you’re taking measurements on electrical systems, these transients are “invisible” and largely unavoidable hazards. They occur regularly on low-voltage power circuits, and can reach peak values in the many thousands of volts. In these cases, you’re dependent for protection on the safety margin already built into your meter. The voltage rating alone will not tell you how well that meter was designed to survive high transient impulses.
Early clues about the safety hazard posed by spikes came from applications involving measurements on the supply bus of electric commuter railroads. The nominal bus voltage was only 600 V, but multimeters rated at 1000 V lasted only a few minutes when taking measurements while the train was operating. A close look revealed that the train stopping and starting generated 10,000 V spikes. These transients had no mercy on early multimeter input circuits. The lessons learned through this investigation led to significant improvements in multimeter input protection circuits.
Test Tool Safety Standards
To protect you against transients, safety must be built into the test equipment. What performance specification should you look for, especially if you know that you could be working on high-energy circuits? The task of defining safety standards for test equipment is addressed by the International Electrotechnical Commission (IEC). This organization develops international safety standards for electrical test equipment.
Meters have been used for years by technicians and electricians, yet the fact is that meters designed to the IEC 1010 standard offer a significantly higher level of safety. Let's see how this is accomplished.
The real issue for multimeter circuit protection is not just the maximum steady state voltage range, but a combination of both steady state and transient overvoltage withstand capability. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large currents. If a transient causes an arc-over, the high current can sustain the arc, producing a plasma breakdown or explosion, which occurs when the surrounding air becomes ionized and conductive. The result is an arc blast, a disastrous event which causes more electrical injuries every year than the better known hazard of electric shock.
The most important single concept to understand about the standard is the Overvoltage Installation Category. The standard defines Categories I through IV, often abbreviated as CAT I, CAT II, CAT III and CAT IV. The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (ac resistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher energy transients. Thus, a multimeter designed to a CAT III standard is resistant to much higher energy transients than one designed to CAT II standards.
Within a category, a higher voltage rating denotes a higher transient withstand rating. For example, a CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1000V rated meter thinking that it is superior to a CAT III-600 V meter.
It’s Not Just the Voltage Level
A technician working on office equipment in a CAT I location could actually encounter dc voltages much higher than the power line ac voltages measured by a motor electrician in a CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, are clearly a lesser threat, because the energy available to an arc is quite limited. This does not mean there is no electrical hazard present in CAT I or CAT II equipment. The primary hazard is electric shock, not transients and arc blast. Shocks can be every bit as lethal as arc blast.
To cite another example, an overhead line run from a house to a detached work shed might be only 120 V or 240 V, but it’s still technically CAT IV. Why? Any outdoor conductor is subject to very high-energy lightning-related transients. Even conductors buried underground are CAT IV, because although they will not be directly struck by lightning, a lightning strike nearby can induce a transient because of the presence of high electromagnetic fields. When it comes to Overvoltage Installation Categories, the rules of real estate apply: it’s location, location, location.
Transients — The Hidden Danger
Let’s take a look at a worst-case scenario in which a technician is performing measurements on a live three-phase motor control circuit, using a meter without the necessary safety precautions.
Here’s What Could Happen:
1. A lightning strike causes a transient on the power line, which in turn strikes an arc between the input terminals inside the meter. The circuits and components to prevent this event have just failed or were missing. Perhaps it was not a CAT III rated meter. The result is a direct short between the two measurement terminals through the meter and the test leads.
2. A high-fault current–possibly several thousands of amps — flows in the short circuit just created. This happens in thousandths of a second. When the arc forms inside the meter, a very high-pressure shock wave can cause a loud bang — very much like a gunshot or the backfire from a car. At the same instant, the tech sees bright blue arc flashes at the test lead tips — the fault currents superheat the probe tips, which start to burn away, drawing an arc from the contact point to the probe.
3. The natural reaction is to pull back, in order to break contact with the hot circuit. But as the tech’s hands are pulled back, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there is now another direct phase-to-phase short, this time directly between the motor terminals.
4. This arc can have a temperature approaching 6000°C (10000°F), which is higher
than the temperature of an oxyacetylene cutting torch! As the arc grows, fed by available short circuit current, it superheats the surrounding air. Both a shock blast and a plasma fireball are created. If the technician is lucky, the shock blast blows him away and removes him from the proximity of the arc; though injured, his life is saved. In the worst case, the victim is subjected to fatal burn injuries from the fierce heat of the arc or plasma blast. In addition to using a multimeter rated for the appropriate measurement category, anyone working on live power circuits should be protected with flame resistant clothing, should wear safety glasses or, better yet, a safety face shield, and should use insulated gloves. Transients aren’t the only source of possible short circuits and arc blast hazard. One of the most common misuses of handheld multimeters can cause a similar chain of events.
Let’s say a user is making current measurements on signal circuits. The procedure is to select the amps function, insert the leads in the mA or amps input terminals, open the circuit and take a series measurement. In a series circuit, current is always the same. The input impedance of the amps circuit must be low enough so that it doesn’t affect the series circuit’s current. For instance, the input impedance on the 10 A terminal of a Fluke meter is .01 Ω. Compare this with the input impedance on the voltage terminals of 10 MΩ (10,000,000 Ω).
If the test leads are left in the amps terminals and then accidentally connected across a voltage source, the low input impedance becomes a short circuit! It doesn’t matter if the selector dial is turned to volts; the leads are still physically connected to a low-impedance circuit.* That’s why the amp terminals must be protected by fuses. Those fuses are the only thing standing between an inconvenienc — blown fuse — and a potential disaster.
Use only a multimeter with amps inputs protected by high-energy fuses. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer. These fuses are rated at a voltage and with a short circuit interrupting capacity designed for your safety.
*Some multimeters have an input alert which gives a warning beep if the meter is in this configuration.
Please tune into tomorrow’s Chemical Equipment Daily for part two of this two-part piece. For more information, please visit www.fluke.com.