Is Your Digital Multimeter Truly Safe?

When testing an electrical circuit, there is always the danger of overvoltages occurring with the risk of serious injury. With this in mind, standards have been established for instruments to be used in various applications from electronic component testing when repairing copiers or VCR's, to testing on main power sources that employ thousands of volts of power.

The IEC (International Electrotechnical Commission) established standards in 1988 for the safety of electrical equipment for measurement, control and laboratory use. These were used to establish various national standards including the US ANSI/ISA-S82.01-94. The standards now specify categories of overvoltage based on the distance the piece of equipment is from the power source. Higher categories present greater potential danger and are those located closest to the power source. The following categories are part of the standard:

1. Category I refers to signal level voltages such as telecommunications and other electronic equipment

2. Category II refers to local level voltages such as portable equipment and certain appliances

3. Category III refers to distribution level voltages including mains and branch circuits

4. Category IV refers to voltages contained in overhead and underground power systems and are beyond the scope of the ANSI/ISA standards

When considering the purchase of a meter, check to see which category voltage the meter is made to test. Also, check to see if the design of the meter will meet the IEC-61010-1 standards so that the "creepage" and "clearance" distances within the meter will withstand higher overvoltage occurrences. Remember, just because a meter indicates that it will test 1000V, always check to see how it is rated for peak impulse transient protection. Also, check to be sure that the meter has met the stringent testing procedures established by independent agencies such as UL, CSA or VDE.

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