B&K Precision 1655A - Manual - Manual (Page 20)

18
THE SOLUTION
Don’t Ground the Chassis
Simply connecting a test lead from the chassis to a good earth
ground is not a satisfactory solution. If the chassis is “hot”, this
places a direct short from the “hot” side of the power line to earth
ground. Hopefully, this would only trip the circuit breaker for the
wall outlet. But a wall outlet is a very high energy source, and
before the circuit breaker is tripped, the grounding test lead may
“weld” or “disintegrate”, or severe damage may occur to the
equipment under test.
Use An Isolation Transformer
For servicing “Hot Chassis” equipment, always connect an
isolation transformer between the wall outlet and the equipment
under test. The Model 1655A or 1653A AC Power Supply
includes such an isolation transformer, among its many other
features.
As shown in Fig. 8, when an isolation transformer is used, the
chassis of the equipment under test is electrically isolated from
both sides of the power line. The chassis of the equipment under
test may not be grounded by connecting a test lead from the
chassis to a convenient earth ground (such as an earth ground
terminal of the test equipment).
Summary
Any piece of equipment with a 2-wire power plug may be the
“hot chassis” type, even if it is equipped with a polarized plug.
There is not disadvantage to using an isolation transformer, even
if the equipment under test already includes its own isolation
transformer. Therefore, for safety, treat all equipment with a 2-
wire power cord as “hot chassis” and use the ISOLATED
OUTPUT of the 1655A or 1653A AC Power Supply for servicing
such equipment.
PROCEDURE
(refer to Fig. 9)
1. Turn off the 1655A or 1653A AC Power Supply and set
AC Volts control to MIN.
2. Connect the power plug of the ac power supply to a 120
volt, 60 Hz ac wall outlet. The wall outlet must be the 3-
wire type with the third wire returned to a good earth
ground.
3. Connect any ac powered test equipment to be used to an
ac wall outlet (do not use the ISOLATED OUTPUT
receptacles of the ac power supply for test equipment).
4. Connect the ac power plug to the equipment under test to
the ISOLATED OUTPUT receptacle of the ac power
supply. This provides the isolation as shown in Fig. 8.
WARNING
CONNECT ONLY ONE “HOT CHASSIS” EQUIPMENT AT
A TIME. Unless both have an identical line-to-chassis
relationship, there is a shock hazard between two chassis.
5. Turn the POWER ON.
6. Select VOLTS function and adjust AC VOLTS control to
desired voltage (typically 120V).
7. It is recommended that the chassis of the equipment under
test be grounded with a test lead.
Troubleshooting: Use As Isolation Transformer OPERATING INSTRUCTIONS