Fluke 373 Application Note Proper Use Of Clamp Meters In Commercial And Residential Settings

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Application Note

There’s nothing so annoying as

a breaker that keeps tripping,

usually at the most inopportune

times. More annoying yet is not

being able to figure out why

as the production line stands

silent waiting for you to work

your magic. The pressure’s on!

In this application note, we’ll

discuss how to utilize all the

capabilities of your clamp meter

so you can keep your world up

and running.

We all know that clamp

meters are used to measure

circuit loading. But, with a

bit of ingenuity, you can also

use clamps to tell you which

breaker controls which outlets,

as well as to measure individual

loads (for both load and ground

currents, if any). This can help

you solve load problems quickly

and preserve your reputation as

an ace troubleshooter.

Clamp meters measure

current by determining the

magnetic field around a cur-

rent-carrying conductor. There

is simply no other practical way

to measure current on electri-

cal wiring systems. Breaking

these circuits open to make a

series circuit measurement is

impractical and may even be

a career ending move if you

inadvertently take critical loads

offline. Usually, measurements

are taken at the panel and

include loading and balance

on three-phase feeders. With

the prevalence of harmonic

loads, neutral measurements at

panelboards are also manda-

tory. Current measurements can

also be used to diagnose motor

health.

Beyond these basic mea-

surements that clamps were

specifically designed for,

modern digital clamp meters

have voltage and resistance

measurement capability as

well. That means it’s possible

to make most, if not all, of the

common, everyday measure-

ments using a clamp meter.

If an electrician could take

just one test tool on the job,

it would make sense for him

to choose his clamp meter.

Furthermore, that clamp meter

should be a true-rms model.

The alternative is an average-

sensing model, which costs

less, but will not measure the

Proper use of clamp

meters in commercial

and residential settings

From the Fluke Digital Library @ www.fluke.com/library

current accurately. Whenever

there are electronic loads

(computers, TVs, lighting, motor

drives, etc.) on a circuit, the

average-sensing meter could

be inaccurate. The greater the

electronic load, the greater the

inaccuracy. True-rms clamps

will always be accurate (assum-

ing of course that you’ve kept it

calibrated). So unless you feel

comfortable saying that you’re

not likely to run into those

kind of loads, get a true-rms

clamp. That way you can think

about the job, not the test tool.

In commercial sites especially,

accurate true-rms clamps are

mandatory.

2 Fluke Corporation Proper use of a clamp meter in commercial and residential settings

Clamp meters in

residential applications

For residential electricians,

clamps are a necessity to mea-

sure loads on individual branch

circuits at the service panel.

While a spot check of current

is often sufficient, sometimes it

doesn’t provide the full picture

as loads are switching on and

off, going through cycles, etc.

Voltage should be stable in an

electrical system, but current

can be very dynamic. To check

the peak or worst-case loading

on a circuit, use a clamp with

a min/max function which is

designed to catch high cur-

rents that exist for longer than

100 ms, or about eight cycles.

These currents lead to intermit-

tent overload conditions which

can cause nuisance tripping of

circuit breakers.

Take measurements on the

load side of the circuit breaker

or fuse. The breaker will open

the circuit in the event of an

accidental short circuit. This is

especially important with any

kind of direct-contact voltage

measurement. Even though

clamp jaws are insulated and

therefore have a level of pro-

tection that doesn’t exist with

direct-contact voltage measure-

ment, it’s still a good idea to be

cautious.

A common problem in

residential electrical work is

mapping outlets to breakers. A

clamp can be useful in identify-

ing which circuit a particular

outlet is on. First take a base-

line reading, at the service

panel, of the existing current

on the circuit. Then put the

clamp in min/max mode. Go to

the outlet in question, plug in

a load—a hair dryer is ideal—

and turn it on for a second or

two. Check the clamp to see

if the max current reading has

changed. A hair dryer will

typically draw 10-13 amps, so

there should be a noticeable

difference. If the reading is the

same, you’ve got the wrong

breaker.

Clamp meters in

commercial environments

Clamp meters are used at the

panelboard to measure circuit

loading on feeders as well as

on branch circuits. Measure-

ments on branch circuits should

always be made at the load

side of the breaker or fuse.

• Feeder cables should be

checked for balance as well

as loading: current on all

three phases should be more

or less the same, to minimize

the return current on the

neutral.

• The neutral should also be

checked for overloading.

With harmonic loads, it’s

possible for the neutral to be

carrying more current than

a feeder—even if the feeders

are balanced.

• Each branch circuit should

also be checked for possible

overloading.

• Finally, the grounding circuit

should be checked. There

should be minimal current on

the ground.

Testing for leakage

currents

To check if there is leakage

current on a branch circuit,

put both the hot and neutral

wires in the jaws of the clamp.

Any current that is measured

is leakage current, i.e., current

returning on the ground circuit.

The supply (black wire) and

return (white wire) currents

generate opposing magnetic

fields. The currents should be

equal (and opposite) and the

opposing fields should cancel

each other out. If they don’t,

that means that some cur-

rent, called leakage current, is

returning on another path, and

the only other available path is

the ground.

If you do detect a net cur-

rent between the supply and

return, consider the nature

of the load and the circuit. A

mis-wired circuit can have up

to half of the total load current

straying through the ground

system. If the measured cur-

rent is very high, you probably

have a wiring problem. Leakage

current may also be caused by

leaky loads or poor insulation.

Motors with worn windings or

moisture in fixtures are common

culprits. If you suspect exces-

sive leakage, a de-energized

test using a megohmmeter will

help evaluate the integrity of

the circuit’s insulation and help

identity if and where a problem

exists.

Measuring individual

loads

To measure individual loads,

you can use a break-out cord

at the receptacle. This is simply

an extension-type cable where

the outer insulation is stripped

so that the black, white and

green wires are exposed. It’s

a lot easier than taking the

receptacle out to get at a wire.

Plug the load into the cable and

the cable into the outlet. To

measure load current, clamp the

black wire. Make the ground

current check directly on the

green cable or on the black and

white wire together.

Motors and motor

control circuits

One of the most challeng-

ing places to make current

measurements is in a control

circuit cabinet, especially if it

uses IEC-style components.

European-originated IEC-style

components are much more

compact than the equivalent

NEMA parts, and the wiring

can be packed pretty tight. The

tapered jaw and “backlight”

function of the Fluke 370 Series

clamp meters are well suited to

this measurement task.

Three-phase induction

motors are commonly used in

commercial buildings to drive

fan and pump loads. Motors

can either be controlled by

electromechanical starters or by

electronic variable speed drives.

3 Fluke Corporation Proper use of a clamp meter in commercial and residential settings

Variable speed drives are more

and more common, since they

save considerable energy.

The Fluke 376 is the ideal

clamp to make these motor and

drive measurements:

• Loading: The current draw

of the motor, measured as

an average of the three

phases, should not exceed

the full load amperes rating

of the motor (times the

service factor). On the other

hand, a motor that is loaded

below 60 percent of full load

amperes—and many of them

are—is less and less effi-

cient, and the power factor

decreases as well.

• Current balance: Current

imbalance can be an indica-

tion of problems with the

motor windings (for example,

different resistances on field

windings due to internal

shorts). Generally speaking,

imbalance should be under

10 percent. (To calculate

imbalance, first calculate

the average of the three

phase readings; then find

the highest deviation from

the average and divide by

the average.) The extreme

of current imbalance is

single phasing, when there

is no current on one of the

three phases. This is usually

caused by an open fuse.

• Inrush current: Motors

started across the line (by

mechanical starters) will

have an inrush current (vari-

able speed drives do not

have inrush current). The

inrush current is approxi-

mately 500 percent on older

motors to as high as 1,200

percent on energy-efficient

motors. This inrush current,

if it is too high, is a common

cause of voltage sags as well

as nuisance tripping. The

“inrush” function of the Fluke

376 clamp meter is a unique

capability, designed to trig-

ger on the inrush current and

capture its true value.

• Peak loading (shock loads):

Some motors are subject

to shock loads, which can

cause enough of a current

surge to trip the overload cir-

cuit in the motor controller.

Think of a saw running into

a knot. The min/max function

can be used to record the

worst-case current drawn by

shock loads.

Whether in residential or com-

mercial settings, the clamp

meter is the electrician’s indis-

pensable test tool.

Work safely

The high voltage and currents

present in electrical power

systems can cause serious

injury or death by electrocution

and burns. Consequently, only

trained, experienced electricians

who have knowledge of electri-

cal systems in general and the

equipment under test should

perform testing and modifica-

tion of electrical systems.

Fluke cannot anticipate all

possible precautions that you

must take when performing the

measurements described here.

At a minimum, however, you

should:

• Use appropriate safety equip-

ment such as safety glasses,

insulated gloves, insulating

mats, etc.

• Be sure that all power has

been turned off, locked out,

and tagged in any situation

where you will be in direct

contact with circuit compo-

nents. Be certain that the

power can’t be turned on by

anyone but you.

• Read and understand all

of the applicable manuals

before applying the infor-

mation in this application

note. Take special note of

all safety precautions and

warnings in the instruction

manuals.

• Do not use instruments on

applications for which they

are not intended, and always

be aware that if the equip-

ment is used in a manner not

specified by the manufac-

turer, the protection provided

by the equipment may be

impaired.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2009-2012 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 7/2012 1989065C_EN

Modification of this document is not permitted

without written permission from Fluke Corporation.

Fluke. Keeping your world

up and running.®