Using proper test tools and understanding a location’s impact and measurement categories can help increase safety from transient overvoltage hazards.
From Fluke Corp.
As distribution systems and loads become more complex, the possibilities of transient overvoltages increase. Motors, capacitors and power conversion equipment, such as variable-frequency drives (VFDs), can be prime spike generators. Lightning strikes on outdoor transmission lines also cause 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 won’t 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 600V, but multimeters rated at 1,000V lasted only a few minutes when taking measurements while the train was operating.
A close look revealed that the train stopping and starting generated 10,000V 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. Which performance specification should you look for, especially if you know that you could be working on high-energy circuits? The International Electrotechnical Commission (IEC) develops international safety standards for electrical test equipment and should be the organization to turn to for guidance.
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 that causes more electrical injuries every year than the better-known hazard of electric shock. (See sidebar below.)
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, etc. (see Figure 1). 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 a system’s impedance (AC resistance).
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, e.g., a CAT III-1,000V meter has superior protection compared to a CAT III-600V-rated meter. The real misunderstanding occurs if someone selects a CAT II-1,000V-rated meter thinking that it is superior to a CAT III-600V meter.
It’s Not Just the Voltage Level
In Figure 2, a technician working on office equipment in a CAT I location could encounter DC voltages much higher than the power line AC voltages measured by the motor electrician in the CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, clearly are a lesser threat because the energy available to an arc is limited.
This doesn’t 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, which will be discussed later, 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 120V or 240V, 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.
Fluke Corp. is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. Based in Everett, Washington, Fluke Corp. manufactures, distributes and services electronic test tools.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.