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Photographgraph of a mini digital multimeter in use by the author

How to use Digital Multi Meters or DMMs Safely
ElectricAPedia ^©

Safety During Electrical System Inspections

Using DMMs and VOMs Safely

This website discusses How to use Digital Multi Meters or DMMs Safely - Safety Procedures for Inspecting Residential Electric Panels, volt meters, VOMs, electrical test equipment. This is a chapter of our electrical safety procedures article discusses safety hazards at residential electrical panels and suggests safety procedures for the electrical inspector, home inspector, or other professionals who examine residential electrical systems. Safe electrical inspection procedures and safe use of volt meters, DMMs, multimeters, and similar electrical test equipment is discussed at the end of the article. Original text: DF, as ASHI Technical Journal Staff, January 1992, with updates February 2006. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Our site offers impartial, unbiased advice without conflicts of interest. We will block advertisements which we discover or readers inform us are associated with bad business practices, false-advertising, or junk science. Our contact info is at inspect-ny.com/appointment.htm.

Safe Use of Electrical Test Equipment: DMMs & VOMs

In some circumstances, when examining electrical panels, receptacles, or other electrical devices in a building, ASHI, CREIA, CAHI, or other home inspectors may elect to make current and voltage measurements. For inspectors who elect to use these tools, make sure that the tools themselves do not become a source of damage, or injury. The following tips are based on a short article by Leonard Ogden in CEE News. Shown here is my little Jensen analog VOM which has served flawlessly for decades. At the top of this page is our still smaller pocket digital DMM DM78 made by CircuitMate.

How to Test digital volt meter DMM or Volt Ohm Meter VOM meter condition

Use only DMM's (digital multi meters) or VOMs (volt-ohm meters, the analog predecessors to DMMs) designed for high energy measurements. A good meter has overload protection built in and a strong, well-insulated case. Don't use a meter having cracked or loose parts. In selecting a meter, look for recessed input jacks to reduce shock risks at the connectors.

Frequently check for damage to the meter itself, or for loose, cut, or worn test leads. If you can see the conductor in the leads, replace them. Check that there is low resistance between the leads themselves - a partial indication of good condition. Use well insulated test leads that have finger guards.

A simple volt ohm meter (VOM) such as the Triplett^TM 310 shown here can be used to test for unexpected and unsafe voltage at a component. Set the VOM in the highest AC-voltage range.

One probe of the VOM is used to contact the surface of the electric panel (or any component to be examined), the other probe is touched to a reliable ground source, [NOTE: Once having tested at the highest voltage range, greater accuracy may be obtained by choosing more sensitive ranges which permit readings to be taken in the upper portion of the scale.

Disconnect the test probes (or shut off the voltage source) before changing the voltage range setting on the VOM.] or in the example shown, to the neutral side of the circuit.

For example, if the VOM meter indicates more than 1or 2 volts between a service panel cover and ground, there's a safety problem. Most low-cost analog-type meters such as the one described provide additional ranges used to read lower voltages with more sensitivity.

Some VOM models provide alligator clips for the ends of the test probes. These clips permit measuring high voltage without handling the probes. Always shut off the power before connecting the alligator clips.

Measuring Amperage or Current

This clamp-on multimeter made by Triplett^TM can measure amperage draw (we used it when servicing and testing air conditioning system compressors) but it also includes probes permitting the device to be used as a standard, if slightly awkward probing VOM as well.

One feature we liked on this analog meter was the adjustable scale which permits measuring voltages in ranges of 1-5 volts, 5-25 volts, 25 to 125 volts, 100 60 500 volts (our scale for inspecting residential electrical equipment), high voltages from 250V up to 1250 volts, and as well, an ohms scale - making this a versatile analog multimeter. After nearly 20 years we still make occasional use of this nice analog meter. Here's closeup of the adjustable scale on this meter.

Electrical Inspection techniques Using DMMs and VOMs

Insulate yourself from possibly live electrical parts by careful selection of clothing, leather boots, and where appropriate, insulated gloves. If you cannot operate your equipment while wearing gloves you have a safety problem.

Use the proper voltage range and other control settings on the meter. If you attempt a voltage measurements with test leads in the amps or current jack (a big but common mistake) and if your probes or meter are not fused, the resulting short across the voltage source can cause an explosion in the meter.

Amp measurements using voltage clamps avoid extra risks of having to handle live wires. [See sketch at left.] When measuring amps without a current clamp, make sure power is off before connecting into the circuit. When disconnecting the meter, always unplug the red (hot) lead first.

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

Original author of the sidebar on testing VOM DMM condition: Kenneth Kruger, R.A., P.E. AIA ASCE, is an ASHI Member and ASHI Director in Cambridge, MA. He provided basis for this article penned by DJ Friedman.
See "How to Use DMM's Safely," Leonard Ogden, CEE News, 888 Seventh Ave., New York, NY 10106, Dec 1990 p.10.
Dr. Jess Aronstein, consulting engineer, Poughkeepsie NY, 1991
Daniel Friedman - editing & publishing author, photographs.
Technical reviewers are invited to comment or ask questions - contact us

More Information on Electrical Inspection Procedures, Electrical Hazards, Building Diagnostic Inspections and Repairs

* Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
Ampacity of an Electrical Service: How to determine the electrical service size or ampacity entering a building
Circuit Breaker, a bad one fails to trip failure at aluminum bus-to-circuit breaker connection - field report and photographs
Electrical Panels, How to Inspect in Buildings, safety for electrical inspectors, electrical panel, fusing, wiring defects, defective products. Inspection Class Presentation
Federal Pacific Electric (FPE) Stab-Lok Circuit Breaker Panel Hazards Website - Latent fire hazards, in-depth authoritative research, documents, advice on Stab-Lok electric panel and circuit breaker failures and what to do when this equipment is found in buildings.
Lightning Strike Risk Assessment, Protection Systems & Services
Multi-wire branch circuit inspection and defects
Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
"Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
"Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
"Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
Electrical System & Wiring Hazard Inspection, Detection, Cause, Remedy, Prevention - Main Electrical Page

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