Application Note

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Application Note

Leakage current

measurement basics

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

In any electrical installation, some current will flow

through the protective ground conductor to ground. This

is usually called leakage current. Leakage current most

commonly flows in the insulation surrounding conductors

and in the filters protecting electronic equipment around

the home or office. So what’s the problem? On circuits

protected by GFCIs (Ground Fault Current Interrupters),

leakage current can cause unnecessary and intermittent

tripping. In extreme cases, it can cause a rise in voltage

on accessible conductive parts.

The causes of leakage

current

Insulation has both electrical

resistance and capacitance—

and it conducts current through

both paths. Given the high

resistance of insulation, very

little current should actually

leak. But, if the insulation is old

or damaged, the resistance is

lower and substantial current

may flow. Additionally, lon-

ger conductors have a higher

capacitance, causing more

leakage current. That’s why

GFCI breaker manufacturers

recommend one-way feeder

length be limited to 250 feet

(76.2 m), maximum.

Electronic equipment, mean-

while, contains filters designed

to protect against voltage

surges and other disruptions.

These filters typically have

capacitors on the input, which

adds to the overall capacitance

of the wiring system and the

overall level of leakage current.

Minimizing the effects

of leakage current

So, how can you eliminate or

minimize the effects of leakage

current? Quantify the leakage

current and then identify the

source. One way of going about

this is to use a leakage current

clamp meter. These are very

much like the clamp meters

used for measuring load cur-

rents, but deliver significantly

better performance when

measuring currents below

5 mA. Most clamp meters sim-

ply won’t register such low

currents.

Once you place the jaws of a

clamp meter around a conduc-

tor, the value of current it reads

depends on the strength of the

alternating electromagnetic field

surrounding the conductors.

To accurately measure low

current levels, it is essential

that the mating faces of the

jaws are protected from dam-

age, are kept clean and are

closed completely together

without an air gap when

Summary of content (3 pages)

PAGE 1
Leakage current measurement basics Application Note In any electrical installation, some current will flow through the protective ground conductor to ground. This is usually called leakage current. Leakage current most commonly flows in the insulation surrounding conductors and in the filters protecting electronic equipment around the home or office. So what’s the problem? On circuits protected by GFCIs (Ground Fault Current Interrupters), leakage current can cause unnecessary and intermittent tripping.
PAGE 2
SupplySupply Load Load SupplySupply Load Load narrow band-pass filter for removing currents at other frequencies. Measurement of leakage current to ground mA A mA A mA A mA Load Neutral mA A Figure 1 Test single-phase circuits by clamping the phase and neutral conductor. The measured value will be any current flowing to ground.
PAGE 3
Exposed Conductive Parts mA A Load along with the phase conductors. The measured value will be any current flowing to ground. (See Figure 2) D-H Measuring leakage current through the ground conductor mA To measure the total leakage flowing to the intended ground connection, place the clamp around the ground conductor.