Multimeter damage is not just a matter of voltage

In motorcycling, if you have a "$10 head," choose a "$10 helmet." With multimeters, since "voltage is voltage," as long as your meter has a high enough voltage rating aren't you safe?

Not exactly. Failed units typically see much higher voltage than the user thought was being measured--not misuse but a momentary high-voltage spike that hit without warning. Distribution systems and loads are becoming more complex, giving greater possibility of transient overvoltages. And lighting striking transmission lines can produce extremely hazardous transients.

Voltage rating will not tell you how a meter survives high-transient impulses--the issue is combined steady-state and transient overvoltage capability. The new International Electrotechnical Commission (IEC) 1010 performance standard today offers higher levels of safety. Its most important concept is the Overvoltage Installation Category, CAT I through CAT IV. The higher the CAT, the greater the power available and potential for higher-voltage transients. Lower CATs mean more system impedance to both attenuate transients and limit currents that could feed on arc-overs caused by transients. CAT I is protected devices (lab or office equipment). CAT IV is outdoor conductors or connections to utilities.

Within each category, a higher voltage rating denotes a higher transient-withstand rating. But a CAT II-1,000V-rated meter is not superior to a CAT III-600V meter. Why? For a 6,000V transient, Ohm's Law says the 2(omega) CAT III test source (see table) tolerates six times the current of the 12(omega) CAT II source, equating to 36 times the power into the meter. And test leads also should be certified to a voltage at least as high as your meter.

To speak with a Fluke applications engineer, call 800-443-5853 or fax 800-FLUKEFAX.