Service manual

Appendix C - Earth continuity testing
Equipment required
An earth continuity tester or appliance tester capable of sourcing
8-25A derived from an AC source with a no-load voltage not
exceeding 12V, for example, CLARE A433R-IO.
IIt is recommended that the calibration and operation of the
instrument is checked frequently enough to ensure its accuracy.
DANGER:
SWITCH OFF THE COMPUTER, DISCONNECT IT FROM
THE MAINS SUPPLY, AND DISCONNECT ANY EXTERNAL
PERIPHERALS AND CABLES BEFORE CARRYING OUT
THIS TEST.
WARNING:
DO NOT MOVE THE TEST PROBE WHILST APPLYING
THE TEST CURRENT.
Before testing
Check the mains lead and plug for any physical damage and
replace if necessary.
Test procedure
Consult the instructions supplied with the test equipment. The
test should be performed on a fully assembled computer
and the equipment must be tested with the mains lead it
will be operated with.
Using the Earth continuity tester, check the continuity
between the power supply cord plug earth/ground pin and
the following points:-
1 the chassis upper wrap
(Note that if you need to remove the paint from the
upper wrap in order to make a good connection, do
this in a place where it will not be visible to the
customer. We recommend an area within the cooling
slots on the side of the upper wrap.)
2 the lower wrap (metalwork) on the base of the unit
3 the rear panel fixing screws (six screws)
4 the fixing bolts for
• printer/parallel port (D-type)
• Analogue RGB port (D-type)
• any other expansion cards (eg SCSI).
The resistance measured between the earth pin and each
of the above test points shall not exceed 0.15 Ohm. This
value includes an allowance for the resistance of the
mains cable. The duration of each test shall not
exceed 10 seconds. No waiting period between tests is
necessary.
DANGER:
THE FOLLOWING TESTS INVOLVE HIGH CURRENTS BUT
LOW VOLTAGES. ALL NECESSARY PRECAUTIONS MUST
BE TAKEN TO ENSURE OPERATOR SAFETY DURING
TESTING.
WARNING:
YOU MUST NOT ALLOW THE
TEST PROBE TO TOUCH ANY
OF THE CONNECTORS' SIGNAL
PINS AS THIS WILL DAMAGE
THE EQUIPMENT.
Appendix C - Earth continuity testing Issue 1, November 1991 C-1