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How to use Digital Multi Meters or DMMs Safely
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DMM - digital multimeter safety: this article discusses safety procedures to follow when using digital multi meters or DMMs or VOMs. We describe safety procedures for inspecting residential electric panels and building wiring using DMMs, volt meters, VOMs, and similar electrical test equipment. These DMM/VOM safety procedures aid in addressing safety hazards found at residential electrical panels and electrical wiring systems and are intended 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
are discussed. Original text: DF, as ASHI Technical Journal Staff, January 1992, with updates through 2012. Also see SAFETY: ELECTRICAL INSPECTION SAFETY and see DMM Digital Multimeter, How to Use. To measure amps or current see AMPS MEASUREMENT METHODS; to measure volts or voltage levels see VOLTS / AMPS MEASUREMENT EQUIP.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Safe Use of Electrical Test Equipment: DMMs & VOMs
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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 (volt ohm meter) which has served flawlessly for decades. At the top of this page is our still smaller pocket digital DMM
DM78 made by CircuitMate™. Other examples of VOMs and ammeters or current measuring instruments are shown below. |
How to Test digital volt meter DMM or Volt Ohm Meter VOM meter condition
High energy measurement instruments: Use only DMM's (digital multi meters) or VOMs (volt-ohm meters, the analog predecessors to DMMs) designed for high energy
measurements.
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.
 Insulate yourself from possibly live electrical parts
by careful selection of clothing, leather boots, and where appropriate,
insulated gloves.
Wear gloves: If you cannot operate your equipment while wearing
gloves you have a safety problem. (Photo at left).
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.
The photo (left) shows our Sperry Digisnap™ DSA-500 clamp-on ammeter that also functions as a digital VOM multimeter and continuity tester. Here, missing safety gloves, the DMM user was measuring 0.11 Amps current draw at a control on a gas-fired warm air furnace. The openings at the bottom of the meter show that before attempting a current (amps) measurement the user removed the test leads from the instrument.
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Don't use a DMM or VOM meter having cracked or loose
parts. In selecting a meter, look for recessed input jacks to
reduce shock risks at the connectors.
A simple volt ohm meter (VOM) such as the TriplettTM 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.
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This clamp-on multimeter made by TriplettTM
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 close up of the adjustable scale on this meter.
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Electrical Inspection techniques Using DMMs and VOMs
Amp measurements using voltage clamps avoid extra risks
of having to handle live wires. [See sketch at left.]
To measure amps or current see AMPS MEASUREMENT METHODS.
Also see DMM Digital Multimeter, How to Use for the proper DMM or VOM function and range settings.
When
measuring amps without a current clamp, make sure power is off before
connecting into the circuit.
When disconnecting the multimeter or voltmeter, always unplug the red (hot) lead first.
Also see SAFETY: ELECTRICAL INSPECTION SAFETY
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Safe Use of Electrical Test Equipment: DMMs & VOMs
8.3 During an electrical system inspection the inspector is NOT required to
8.3.A. insert any tool, probe, or testing device inside the panels
Do Not Grab an Shake SEC Entry Mains in the Panel
In his final electrical seminar in St. Louis a decade ago, Bob Smith,
a lecturer from SHRC, the Small Homes Research Council, told ASHI home inspectors and candidates
that he always verifies the quality of the service connection at
the main breaker by grabbing the two hot entrance wires and giving
them and the panel a good shake.
Watch out: Don't do this! Readers would
not be warned here if this questionable advice had not been presented
at that seminar. Electricians and trained experts work with live electrical wires. Home inspectors should not do so.
October 22, 1988 - El Cajon, CA - a young electrician died
when he accidentally
electrocuted himself. Acting police Lt. Carl Case said 19-year-old
Sean M. Smith was working under a house, lying on his back working
on an electrical addition to the house, when his wire [strippers]
accidentally connected with a live wire. He said Smith's boss,
Troy Beatty, heard the victim yell. Beatty found Smith unconscious
under the house, pulled him out, performed CPR, and summoned help. -- IAEI News, November/December 1990 p.40.
These electrical inspection suggestions are not a complete inventory of all electrical components that should be inspected; these notes focus on identification of conditions that may present special electrical hazards for the electrical inspector.
Contact Us by email to suggest changes, corrections, and additions to this material.
General Electrical Safety Suggestions describes important basic safety procedures, clothing, gloves, eye protection, and other safety equipment for home inspectors and electrical inspectors.
Safety Warnings for Electrical Inspectors & Home Inspectors Using Electrical Test Equipment on Building Electrical Systems & Devices
See Safety Hazards and Safe Inspection Procedures at Residential Electric Panels.
Safety Warning: The ASHI Standards of Practice and other home inspection standards for electrical inspections do not require the inspector to insert any instrument into the service panel. Therefore this testing is optional. It's also a dangerous procedure that can damage electrical equipment or worse, cause electrical shock, or even death, and should not be undertaken unless the person conducting the examination is trained and competent to avoid electric shock. If the inspector is not trained for this procedure
s/he should never insert any instrument or tool into electrical equipment. See Safety Hazards and Safe Inspection
Procedures at Residential Electric Panels.
Simpson Instruments adds the following safety advice for users of VOMs and DMMs and similar test devices, and other instrument manufacturers offer similar cautions: [6]
- The test instrument should only be used by personnel qualified to recognize shock hazards
and trained in the safety precautions required to avoid possible injury.
- Do not work alone when measuring circuits where a shock hazard might exist. Notify a nearby person that you are making, or intend to make such measurements.
- Locate all voltage sources and accessible current paths before making measurement
connections. Be sure that the equipment is properly grounded and
the right rating and type of fuse(s) is installed. Set the Instrument to the
proper range before applying power. Voltage might appear unexpectedly in
defective equipment. An open bleeder resistor can result in a capacitor retaining
a dangerous charge. Turn the power off and discharge all capacitors
before connecting or disconnecting the Instrument.
- Inspect the test leads for cracks, breaks or crazes in the insulation, probes
and connectors before each use. If any defects are noted, replace the test
leads immediately.
- Do not measure in a circuit where corona is present. Corona can be identified
by a pale blue color emanating from sharp metal points in the circuit, or
by a buzzing sound, or by the odor of ozone. In rare instances, such as
around germicidal lamps, ozone might be generated as a normal function.
Ordinarily, the presence of ozone indicates the presence of high voltage and
probably an electrical malfunction.
- Hands, shoes, floor and workbench must be dry. Avoid making measurements
under humid, damp or other environmental conditions that could affect
the dielectric withstanding voltage of the test leads or the Instrument.
- For maximum safety, do not touch test leads or Instrument while power is
applied to the circuit under test.
- Use extreme caution when making measurements where a dangerous combination
of voltages could be present, such as in an RF amplifier.
- Do not make measurements using test leads of lesser safety than those originally
furnished with the Instrument.
- Do not touch any object which could provide a current path to the common
side of the circuit under test or power line ground. Always stand on a dry
insulated surface capable of withstanding the voltage being measured.
- Do not connect to an electrically energized circuit in a hazardous area.
- No General Purpose VOM is to be used to make electrical measurements on
blasting circuits or blasting caps.
Fluke adds this safety advice: To avoid possible electric shock or personal
injury, follow these guidelines: [5]
- Use this Meter only as specified in this
manual or the protection provided by the
Meter might be impaired.
- Do not use the Meter if it is damaged. Before
you use the Meter, inspect the case. Look for
cracks or missing plastic. Pay particular
attention to the insulation surrounding the
connectors.
- Make sure the battery door is closed and latched before operating the Meter.
- Make sure the battery door is closed and
latched before operating the Meter.
- Remove test leads from the Meter before
opening the battery door.
• Inspect the test leads for damaged insulation
or exposed metal. Check the test leads for
continuity. Replace damaged test leads before
you use the Meter.
- Do not apply more than the rated voltage, as
marked on the Meter, between the terminals or
between any terminal and earth ground.
- Never operate the Meter with the cover
removed or the case open.
- Use caution when working with voltages above
30 V ac rms, 42 V ac peak, or 60 V dc. These
voltages pose a shock hazard.
• Use only the replacement fuses specified by
the manual.
- Use the proper terminals, function, and range
for measurements.
- Avoid working alone.
- When measuring current, turn off circuit power
before connecting the Meter in the circuit.
Remember to place the Meter in series with the
circuit.
- When making electrical connections, connect
the common test lead before connecting the
live test lead; when disconnecting, disconnect
the live test lead before disconnecting the
common test lead.
- Do not use the Meter if it operates abnormally.
Protection may be impaired. When in doubt,
have the Meter serviced.
• Do not operate the Meter around explosive
gas, vapor, or dust.
- Use only 1.5 V AA batteries, properly installed
in the Meter case, to power the Meter.
- When servicing the Meter, use only specified
replacement parts.
- When using probes, keep fingers behind the
finger guards on the probes.
- Do not use the Low Pass Filter option to verify
the presence of hazardous voltages. Voltages
greater than what is indicated may be present.
First, make a voltage measurement without the
filter to detect the possible presence of
hazardous voltage. Then select the filter
function.
- Only use test leads that have the same voltage,
category, and amperage ratings as the meter
and that have been approved by a safety
agency.
- Use proper protective equipment, as required
by local or national authorities when working
in hazardous areas.
- Comply with local and national safety
requirements when working in hazardous
locations.
Additional Advice to Avoid Damaging VOM or DMM & Equipment
Fluke adds this advice: To avoid possible damage to the Meter or to the
equipment under test, follow these guidelines: [5]
- Disconnect circuit power and discharge all
high-voltage capacitors before testing
resistance, continuity, diodes, or capacitance.
- Use the proper terminals, function [be sure the function switch is set to the proper position] , and range
for all measurements.
- Do not remove batteries while the Meter is
turned on or a signal is applied to the Meter’s
input jacks.
- Before measuring current, check the Meter's
fuses. (See "Testing the Fuses" in the Users
Manual on the accompanying CD.)
- Do not use the LoZ mode to measure voltages
in circuits that could be damaged by this
mode’s low impedance (≈3 kΩ). (Model 289
only)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- Electrical shock injury statistics: www.healthatoz.com - September 2008;
- Daniel Friedman - Publisher & Editor of InspectAPedia.com®
- N. Srinivasan, MSEE, is a senior member of IEEE with 30 years experience in the electrical industry. Mr. Srinivasan is in Vienna VA.
- Louis P. Babin generously contributed technical editing about the effects of doubling ampacity in an electrical circuit (September 2007)
- ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson,
and the
- ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E.,
with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors www.ashi.com.
- [4] Digisnap DSA-500 snap-around digital multimeter, A.W. Sperry Instruments Inc., 2150 Joshua's Path, Suite 202, Hauppage NY 11788, Tel: 800-645-5398, Email: cat@awsperry.com, Website: www.awsperry.com
- [5] Fluke Corporation, 6920 Seaway Blvd, Everett, WA 98203, USA, PO Box 9090 Everett, Washington 98206, Tel: +1(425) 347-6100, Technical support: 1(800) 44-FLUKE (1(800) 443-5853), Website: www.fluke.com,
Fluke Europe B.V,
PO Box 1186 Eindhoven, The Netherlands, Tel: +31 (0)40 2 675 200 +31 (0)40 2 675 222, Website: www.fluke.eu
- Digital 287/289 Digital Multimeter, Users Manual, retrieved 9/5/21, original source: http://assets.fluke.com/manuals/287_289_umeng0200.pdf, [copy on file as Fluke_287_289_umeng0200.pdf]
- [6] Simpson Electric, P.O. Box 99, 520 Simpson Avenue, Lac du Flambeau, WI 54538-0099 Tel: 715-588-3311, customer service: 715-588-3947, Email: support@simpsonelectric.com, Website: www.simpsonelectric.com/
- Simpson 260® Series 6XLM
Volt-Ohm-Milliammeter Instruction Manual, retrieved 9/5/2012, original source: http://www.simpsonelectric.com/uploads/File/datasheets/260-6xlm.pdf, [copy on file as Simpson_260-6xlm.pdf]
- [7] tif 300cc Tic Tracer voltage detector, Tif Instruments Inc., 9101 NW 7th Avenue, Miami, Florida 33150
- [8] Greenlee® GT-16 adjustable voltage detector, Greenlee Textron Inc., Website: greenlee.com, Tel: 800-435-0786 , Email: echsupport@greenlee.textron.com,
 Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com 11/06
- Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee. Mr. Hankey has served in other ASHI professional and leadership roles. Contact Roger Hankey at: 952 829-0044 - rhankey@hankeyandbrown.com. Mr. Hankey is a frequent contributor to InspectAPedia.com.
- Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at ap@octoberhome.com
- 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.
- "Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
- "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.
- ...
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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