Fluke 376 Application Note Clamp Meter Readings

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Application Note

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

Clamp meter readings:

problems and solutions

There is no smell of burnt insu-

lation, the load appears normal,

the coupling is intact, ambi-

ent temperature is not above

the nameplate specified value

of 40 °C (104 °F). As expected,

however, the motor is too hot to

touch. The next obvious trouble-

shooting tactic is to measure

the amperage the motor draws

while running. Since a small

increase in current flow to a

motor produces a proportionately

larger amount of heat, motor

amperage exceeding name-

plate values should be carefully

investigated as a possible cause.

These overload trips, though

often caused by motor loading

issues, can also indicate bearing

failure, insulation breakdown or

voltage unbalance.

After you ensure that all safety

requirements for working near

energized components are satis-

fied, open the fuse disconnect

door in preparation for using

your clamp meter to measure

current. Once again, a visual

inspection reveals no obvious

problems. You begin to clamp

around a motor phase conductor

at the bottom of a fuse—and now

the problems begin.

Clamping around a phase

conductor

When you use a clamp meter it

is necessary to clamp around

only one phase conductor at a

time. When current flows to a

conductor it produces a magnetic

field around the conductor. The

more current, the stronger the

magnetic field. The magnetic

field induces a voltage into the

clamp portion of the meter. The

meter reading will be propor-

tional to the amount of voltage

induced into the clamp. There-

fore, reading the current flow

through a conductor requires

that only the magnetic field

produced by that one conduc-

tor be induced into the clamp.

Clamping around two or more

conductors will cause the

magnetic fields to cancel each

other and the resultant reading,

if not “zero,” will be completely

erroneous.

So, what is the problem

when clamping around only

one conductor? Have you ever

tried getting a clamp around a

10 AWG (American wire gauge)

solid conductor with inadequate

conductor length inside the

box? Have you ever tried pulling

away one 2 AWG conductor from

two others in an attempt to get

that clamp to fit? Sometimes

two of the conductors can be

reached fairly easily, but clamp-

ing around the third conductor

may be difficult. In some cases,

the clamp you are using may

simply not be big enough to go

around the conductor you need

to measure. You may need to

clamp around two conductors in

parallel. Each set of measure-

ments will produce its own

unique problems.

A motor trips on overload.

Visual and physical inspection reveals no abnormalities.

2 Fluke Corporation Clamp meter readings: problems and solutions

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

©2010 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 10/2010 3949064A A-EN-N

Modification of this document is not permitted

without written permission from Fluke Corporation.

Fluke. Keeping your world

up and running.®

Safety is always a prime

consideration. Even when you

wear the appropriate shock and

arc-flash protection, having to

push and pull on live conductors

is not an enjoyable task and can

be dangerous. Conductors come

loose from underneath terminals.

Conductor insulation scraped off

during installation can come in

contact with the metal enclosure

when clamping around conduc-

tors. The phase-to-ground fault

can instantly escalate into a

phase-to-phase arcing fault with

potentially catastrophic arc-flash

results. There are no guarantees

anytime you are exposed to

energized conductors and circuit

parts. And, probing about those

conductors with the clamp meter

can only compound the problem.

The solution to the above

problems is a flexible clamp.

Weaving a flexible clamp around

a conductor means not pulling

and tugging on it to move it from

adjacent conductors. Whether

dealing with the 10AWG solid

conductor or a 2 AWG (or larger)

stranded conductor, installing

the flexible clamp is simpler

and unquestionably much safer.

The problem has always been

that flexible clamps were typi-

cally found only on equipment

such as data loggers and power

quality analyzers, and were not

available for the standard clamp

meter used by professionals in

the field. Not true anymore, as

Fluke now offers true-rms ac/

dc clamp meters that utilize the

iFlex™ flexible current probe.

One model even offers remote

display.

Determining inrush

current

When an ac motor starts it will

draw more current than its full

load value on the nameplate.

How much more current depends

on the load on the motor when

it starts and the code letter on

the nameplate. Excessive start-

ing torque requirements or low

system voltages can result in

excessive inrush current during

When taking clamp meter

readings you should be ready for

problems. Remember that safety

is number one. And make sure

you have the right measurement

tool for the job—one that allows

for safe and accurate readings,

can measure the current range

you need, and even provide

remote display for both safety

and convenience.

start-up. Determining inrush

current can be tough with a

digital meter. Values change

rapidly and determining the

actual inrush value can be dif-

ficult. Min, max, average and

inrush recording features of the

Fluke models allow the meter

to capture variations automati-

cally. Also, electronics filter out

noise and capture motor start-

ing current exactly as the circuit

protection sees it. That means

what you see on the meter is

the same value sensed by the

overloads, time-delay fuses and

inverse-time circuit breakers.

Another problem is that inrush

current values may exceed the

maximum value read by the

meter. For example, a 460 volt,

250 horsepower motor should

draw no more than 302 amps

at full load. Inrush currents for

such a motor starting under full

load may be 1,500 to 2,000

amps. Determining the inrush

value can be important in diag-

nosing motor overload problems.

Obviously the meter must be

capable of reading such values.

Most clamp meters read a maxi-

mum value of 400 amps. Some

of the new Fluke clamp meters

have a 1000 amp limitation

with the fixed jaw and all have

a 2500 amp maximum current

measurement with the iFlex™.

That means you can measure

the running current on a 460

volt, 500 horsepower motor!

Beyond motors

Certainly not all clamp-on

ammeter readings are taken on

motors. Current readings must

be taken on all types of non-

linear loads. Line side currents

on uninterrupted power supply

(UPS) systems and variable-

system drives (VFDs) must

be measured. Lighting loads

containing electronic bal-

lasts, branch circuits supplying

computers, printers and copy

machines must all be measured.

The new Fluke true-rms clamp

meters feature a low pass filter,

allowing for accurate readings

on such nonlinear equipment.