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Application Note

Measuring live voltages andcurrent in todays high energyenvironments can result in asevere hazard to equipment andusers if proper precautions arenot applied. Given the risk oftransients, surges, and old-

fashioned human error, it alwayspays to follow safe work practicesand use test instruments rated forthe voltage or current youremeasuring.

Whenever possible, work onde-energized circuits and followproper lockout, tag-out proce-dures. If you have to work onlive circuits, following the stepsbelow will improve yourmeasurement practices and helpreduce any hazard.Setup

1. Assess the environment beforetaking the measurement.2. Do not work alone in haz-

ardous areas.3. Wear the appropriate Personal

Protective Equipment (PPE) asdetermined by NFPA 70 E andthe local recommendations ofhealth and safety personnel.

4. Make sure your test instrumentis rated for the measurementenvironment.

5. Be familiar with and knowhow to use your equipmentprior to any hazardousmeasurements.

Practices1. Measure at the lowest energy

point.2. Keep your eyes on the area

youre probing and keep bothhands free as conditionsrequire.

3. For single phase, connectneutral first hot second.

4. Use the three point testmethod discussed below.

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Safety considerationsfor live measurements

Keep your eyes on the job at hand

5. Use test probes with a mini-mum amount of exposed metalsuch as .12 in (4 mm) metal tipprobes.

6. Keep one hand in your pocketunless you must use bothhands for a good measurement.

Setup

Environmental analysis

Before you open an equipment

cabinet, look over your workenvironment. How do you planto use your meter? Where willyou mount it? Do you have clearaccess to the equipment inquestion? Have you been trainedon or are you knowledgeable inthe use of your meter? Are envi-ronmental hazards present, suchas tree branches or water? Doyou have enough light andventilation?

Avoid working in dark areas.If you choose to work in a darkarea, turn on the test tools back-light to brighten the display foreasier viewing. And, if youreworking in a deep or recessedpanel, use a test probe extenderand probe light to illuminate thearea to be probed. Be sure youcan clearly view the point ofmeasurement. The probe exten-der facilitates measurement bykeeping your hands further awayfrom the inside of the panel,reducing potential danger.

Also make sure you have ahelper knowledgeable in electri-cal safety or let someone elseknow where youre working. Itsnever good practice to workalone on high energy circuits.

When working in a deep or recessed panel, use a test probe extender and probe light to illuminate the area.

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2 Fluke Corporation Safety considerations for live measurements

you make your measurement always keep your eyes onyour test probes.

3. When taking single-phasemeasurements, always con-

nect the neutral lead first, thehot lead second. After takingyour reading, disconnect thehot lead first, the groundedlead second.

4. When testing for voltage, usethe three point test method.1. Test a similar known live

circuit first2. Test the circuit to be

tested3. Re-test the first known

live circuit.This process verifies your testinstrument is working properly

an important part of yourpersonal safety.5. When making measurements

in or around high energy threephase distribution panels, usetest probes with a minimumamount of exposed metal atthe probe tips, such as .12 in(4 mm) metal tip probes. Thisreduces the risk of an acciden-tal arc flash from probe tipsbeing inadvertently shortedtogether between phases.

6. Keep one hand in your pocketor out of the panel and themeasurement circuit. You dont

want to offer a closed circuit.Whenever possible, use aproperly rated alligator clip toattach the black test lead tothe circuit under test. Thisgives you a free hand to probewith the red test lead.

What to wear

When an arc flash occurs, PersonalProtective Equipment (PPE) is theonly thing that can defend youfrom horrific pain, injury andeven death. When measuringsystems of 1000 volts or less,including 480 V and 600 V threephase circuits, follow the detailedPPE standards developed by theAmerican National StandardsInstitute (ANSI), the CanadianStandards Association (CSA), andthe International Electro-Technical Commission (IEC).

The safety specificationsinclude, among other considera-tions, eye and hearing protection,insulated hand tools, insulated

gloves, and fire resistant clothing.In the United States, NFPA stan-dard 70E and NEC article 110.16define where and when PPEmust be used.

Voltage category ratings

In the four-category ratingsystem, the closer you are tothe power source, the higherthe category and the greaterthe danger. CAT III covers distri-bution level wiring, including

480 or 600 volt circuits such as3-phase bus and feeder circuits,motor control centers, load cen-ters and distribution panels.

Given the trend toward usinghigher voltage levels in modernbuildings, most commercial andindustrial electricians work in aCAT III or CAT IV environment sometimes, without even know-ing it. And thatcan lead to realdanger.

Make sure your test instru-ments are rated to a minimum ofCAT III 600 V. Most digital multi-meters (DMMs) display theseratings near the voltage/current

input terminals.If your DMM and your leadsdont have the appropriateratings, DONT MAKE THEMEASUREMENTS with them.If have to make these measure-ments, upgrade and purchaseinstruments rated for thatenvironment. Using an olderinstrument (manufactured priorto 1996) or a new instrumentwithout the proper voltage andcategory rating will put you andyour employer at risk.

Practices

1. It is a good practice to meas-ure voltage at the point of thelowest available energy. Forexample, if you are measuringvoltage on a breaker panel,identify the lowest-ratedbreaker available, and makeyour measurement there. Thisway, you have more protectionbetween yourself and thepotential hazard

2. Effective steps should be takento obtain the best readingwithin the necessary envelopeof safety. If conditions requirethat both of your handsremain free for a safe meas-urement, set the instrumentdown; use the instrumentsbail stand (if it has one). Betteryet, use a magnetic hanger tohang the unit at eye level onthe edge of the panel. Donttry to watch the meter while

Fluke Meters designed toolder standards do not showcategory rating on front ofinstrument

1000 V CAT III and 600 VCAT IV meters designed towithstand 8000 V transients

New

Use meters with these markings:1000 V CAT III or 600 V CAT IV

Old

Do not use meters without properCAT markings on 480 V circuits

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3 Fluke Corporation Safety considerations for live measurements

Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800) 36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web access: http://www.fluke.com

2005 Fluke Corporation. All rights reserved.Printed in U.S.A. 1/2005 2412973 A-US-N Rev A

Fluke. Keeping your worldup and running.

Fluke safety: helping toreduce hazards

Fluke offers a complete line of

CAT IV 600 V/CAT III 1000 Vdigital multimeters with backlitdisplays for the measurementapplications described here.Fluke accessories like the TPAKMagnetic Hanger, the L210 ProbeExtender and Probe Light, theTP238 Test Probes and the i400Current Clamp are rated for usein high energy environmentscomplying to US andInternational standards.

Remember, using the righttools helps you do your job more

safely and efficiently.For more information and

resources about good electricalmeasurement practices, includingweb links to all the standardsmentioned in this article, visitwww.fluke.com/safety.