How Much Leakage is Bad?
The chart, Figure 4, shows the effect of
electrical shock on the human body. You
may be surprised to learn that the most
likely range of fatal current is between 30
and 250 mA. This is the area where the
breathing and heart are most severely
affected. The chart is based on a person
weighing 150 pounds. Smaller people,
especially children, are much more
susceptible to shocks at lower current
levels.
There should be no leakage at all, but
this is not practical in real terms.
Components, such as RF decoupling
capacitors and antenna matching
transformers, allow some current to flow.
Underwriter Laboratories has established
guidelines for safe leakage currents. The
maximum UL allowable limit (since 1972)
is 500 microamps. Consumer electronic
devices manufactured before 1972 were
allowed 750 microamps. The leakage test
on the PR57 is calibrated directly in
microamps to compare with the
appropriate standard.
Measuring Leakage at Lower Levels
Some equipment must be kept at a
leakage current which is considerable
less than consumer products. Equipment
such as clinic and hospital equipment
must have less than 100 or even 10
microamps.
This requires more resolution than
provided by the PR57 meter. Simply
place an AC current meter (such as the
Sencore DVM37 or DVM56A) in series
with the Safety Leakage Probe. This
allows you to measure the leakage within
0.1 microamps with digital accuracy.
Checking the Calibration of the
Leakage Test
The current-limiting resistor in the Safety
Leakage Probe is also a calibration
resistor. The calibration of the PR57
leakage test can be tested at any time so
you will know that the readings obtained
during the test are correct.
To test the calibration of the PR57
leakage scale:
1.
Plug the PR57 into a properly
grounded three-wire AC outlet.
2.
Depress the AC VOLTS OUTPUT
button and turn the PR57 “ON”.
Adjust the AC VOLTS control for a
reading of 117 volts on the PR57
meter.
3.
Plug the leakage probe into the
Probe jack and depress the HI SIDE
LEAKAGE pushbutton. Place the
probe tip into the small blade
opening of the ISOLATED OUTPUT
socket in the PR57. The meter
should read 780 microamps without
the button pushed on the Safety
Leakage Probe.
Testing Leakage on Three-Wire Units
The PR57 leakage test is designed to
operate exactly the same on a unit with a
two-wire AC line cord or a 3-wire
(grounded) AC line cord. The ground
connector on the ISOLATED OUTPUT
jack is connected to earth ground through
the power cord of the PR57. This assures
safe operation of the unit under test as its
chassis is maintained at earth potential.
This ground path does not affect the
safety leakage test. The leakage between
the internal circuits and the chassis on
these three-wire units is read exactly the
same as two-wire units. You simply touch
the Safety Leakage Probe to each
metallic cabinet part and read the
leakage directly on the PR57 meter. Do
not defeat the third-wire ground on either
the device under test or the PR57.
Testing for Line Cord and Extension
Cord Leakage
An increasingly common cause of
electrical fires is leakage between the
conductors of line cords and extension
cords. Leakage of this type may
eventually short and/or cause sparks
which may ignite a fire.
The instrument to use to test for leakage
in line cords or extension cords is your Z-
Meter. Measure the leakage between the
two conductors on a two wire cord, or the
leakage between the three conductors on
a three wire cord. In either case, the
leakage should be absolutely zero (with
the cord unplugged from the wall or
device that it supplies power to). If any
leakage is detected, the cord is defective
and should be replaced.
Powerite is a registered trademark of Sencore, Inc.
Form #4550
Printed in U.S.A.
Fig. 5: The built-in calibration resistor in the Safety Leakage Probe allows a quick check
of the leakage scale calibration by inserting the Safety Leakage Probe into the Isolated
Output and checking for a full scale leakage reading.
