B&K Precision 1655A - Manual - Manual (Page 27)

25
STANDARD LEAKAGE TEST
As previously mentioned, most equipment with a 2-wire ac
power plug is the “hot chassis” type. The plastic or wood cabinet
normally insulates the chassis so the user cannot touch it. Other
exposed metal parts such as antennas, antenna terminals, knob
shafts, screw heads, handles, etc. are also insulated (or isolated
with very high resistance) to protect the user from electrical shock
Most equipment manufacturers specify a leakage test after
reassembly of the equipment to insure the continued protection of
the user from electrical shock.
The standard leakage test requires a leakage probe consisting
of a test lead in series with a 1.5kΩ, 10 watt resistor parallel by a
0.15µF capacitor. One end of the leakage probe is connected to
an earth ground. Voltage across the resistor is measured on an ac
voltmeter as the leakage probe is touched to each exposed metal
part of the equipment. An isolation transformer must not be used
during the standard leakage test, and the entire test must be
repeated with the ac power plug reversed, even if an adapter is
required to reverse the connections of a polarized plug.
The maximum permissible leakage specified by OSHA, UL,
and CSA is 500µA. Some manufacturers specify a lower limit for
a greater margin of safety.
THE SIMPLER METHOD
The Model 1655A AC Power Supply includes a built-in
leakage tester that greatly simplifies the leakage test (Fig. 12).
The equipment under test remains connected to the
ISOLATED OUTPUT, just as if it was for servicing. The voltage
was probably already preset to 120 volts at the conclusion of
servicing.
The permanently attached leakage probe of the ac power
supply is merely touched to each exposed metal part of the
equipment under test (antenna, terminals, tuner knob, knob shafts,
handle, screw heads, etc.)
No separate voltmeter nor conversion of results is required.
Just press the LEAKAGE button and read leakage directly from
the LEAKAGE scale of the meter in microamps (in milliamps for
higher values of leakage). See the manufacturer’s service
literature for the maximum permissible leakage. However,
leakage in excess of 500µA is considered hazardous in any
equipment. Locate and correct the cause of leakage.
The expanded scale meter gives good resolution in the normal
100 to 500µA range. Reading give up to 5mA are measured on a
compressed scale. The amount of excess leakage can be a clue to
the type of leakage path. A full scale meter reading probably
indicates a direct short to the power line, but the meter is
protected from damage.
The leakage probe may also be used to verify proper wiring of
equipment with polarized plug; there should be very low leakage
from the chassis to common and full scale leakage to hot.
Leakage of equipment with a 3-wire power plug is performed
in the same manner as for 2-wire equipment.
OPERATING INSTRUCTIONS Leakage Tester
USING THE MODEL 1655A AS A LEAKAGE TESTER