Electricians working on panel and distribution systems rely on their tools (and 
training) to keep them from becoming a direct connection between hot wires and ground. Personal protective equipment, such as flame-resistant clothing, safety glasses or a face shield, and properly rated gloves help protect the worker from these serious arc and electrocution hazards.
But ironically, the most sophisticated tool in an electrician’s pouch – the digital multimeter (DMM) – may be the one tool that subjects a worker to the greatest risk.
It’s mind-boggling that electricians are very careful to make sure their hand tools and personal protective equipment meet safety standards. But when it comes to digital multimeters, they often use the least expensive units they can find.
Outside of incorrectly using the meter, the greatest risk comes from transient voltage spikes that can push 10 times the normal voltage or more through power lines in an instant, without warning. Transient voltage events are becoming more frequent as motors, capacitors and variable-speed drives are installed. Lightning can also cause transient voltage spikes.
If you are testing a circuit and a high transient voltage event occurs, the voltage can overpower the safety factor designed into the digital multimeter. These transient power surges also expose the worker to shock or arc-burn risks that can cause serious injury or death.
When working with electricity, nothing is more important than overvoltage protection. The categories tell you where you can use the test instrument and the overvoltage rating tells you how much protection the test instrument provides.
These definitions were derived from the international safety standard for insulation coordination as it applies to measurement, control and laboratory equipment. These measurement categories are explained in more detail by the International Electrotechnical Commission (IEC). Please refer to either of their publications: IEC 61010-1 or IEC 60664.
Category I is the signal level for telecommunications and electronic equipment that has low-energy equipment with transient limiting protection.
Category II is the local level for fixed or non-fixed powered devices – everything from lighting to appliances to office equipment such as copiers.
Category III is the distribution level – fixed primary feeders or branch circuits. These circuits are usually separated form Category IV, whether utility service or other high-voltage source, by a minimum of one level of transformer isolation. This includes feeders and short branch circuits, distribution panels and heavy appliance outlets with “home run” connections to service entrance.
Category IV is the primary supply level. Greenlee is leading the way in offering protection certified to conform with the IEC’s proposed standard. This includesoutside and service entranceservice drop from pole to building, overhead line to detached building, and an underground line to a well pump.
While the Category rating indicates where the digital multimeter can be safely used and the level of protection from the transient voltages, there are other significant steps that should be followed to ensure proper usage. There are basic checks that should be made before taking a measurement with a DMM. End-users should first make sure the function switch is properly set. Then check to make sure the test leads are connected to the proper jacks. Once the DMM is set to its highest range, and if the measurement value is unknown, make sure the test leads are in good condition and the fuse in the DMM is good. A DMM measurement should also be checked on a known (energized) voltage source before taking a voltage measurement to make sure the unit is functioning, is set to the proper voltage, and that the test leads are at an open circuit.
Abbreviations, Symbols and Prefixes
Abbreviations are used on a digital multimeter to provide quick recognition using a letter or combination of letters to represent common electrical quantities such as voltage, current or resistance. Abbreviations also save space on the DMM when identifying functions such as a minimum and maximum. Finally, abbreviations provide standardized communication in the industry.
Symbols are used on a digital multimeter to provide quick recognition using a graphic element to represent a quantity, unit, component or function. Symbols save space when identifying DMM measurement quantities, functions, or components. Symbols also provide standardized communication in the industry.
When taking measurements with a DMM, prefixes simplify notation of large and small numbers. Prefixes represent common electrical quantities when combined with abbreviations and indicate value on function switch position and display.
Usage Guidelines
Here are some additional guidelines to remember when using a DMM. Use recommended personal protective equipment. Ensure that there are no atmospheric hazards. Use one hand when working on live circuits. Ensure that the measurement is within limits of the DMM. Ensure that the DMM function switch setting and test lead connections are correct.
Use Recommended Personal Protective Equipment
Safety equipment is of the utmost importance when measuring power. Before beginning, you must wear the necessary safety gear. A good hard hat and either safety glasses or goggles are essential for jobsite safety. Safety shoes that have insulating soles with an ANSI Z41 EH (electrical hazard) rating are another necessity, so if you accidentally touch a live conductor, there won’t be a ground through you to the floor. Also, wearing ear plugs and fire-resistant clothing, along with standing on a rubber safety mat or wearing safety gloves, are always good ideas for stopping a current before it stops you.
Finally, you’ll also find the new multimeters combine various tools to free up space in your toolbag. Some of them offer non-contact testing for certain jobs. Others feature a voltage/continuity tester with a ground-fault circuit function and built-in flashlight. The cost of a new meter that does more and meets today’s standard is a very small price when compared to the shock or arc hazard and the chance to upgrade to a tool that may make work simpler.
Chera M. Ellis is Director of Marketing for Greenlee. She can be reached at 815-397-7070 or at cellis@greenlee.textron.com.
DMM Abbreviations
AC = Alternating Current or Voltage
DC = Direct Current or Voltage
V = Volts
mV = Millivolts
kV = Kilovolts
A = Amperes
mA = Milliamperes
μA = Microamperes
W = Watts
k W = Kilohms 
M W = Megohms
DET - AC Detective Technology
Hz = Hertz
kHz = Kilohertz
μF = Microfarads
nF = Nanofarads
° F = Degrees Fahrenheit
° C = Degrees Celsius
HAR = Harmonics
COM = Common
OL = Overload
MAX = Maximum
M
IN = Minimum
AVG = Average
