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...6 SETTING UP
6.3 Electrical Checks
Carry out the following electrical checks:
6.3.1 Model 4234 Power Supply Unit Output
Warning. This unit is part of the certified
intrinsically safe system. Appropriate safety precautions
must be taken to prevent any incendive electrical
discharges in the hazardous area when carrying out this
task
Testing the output may only be carried out with the hazardous
area cable disconnected and a dummy load resistor fitted
across the output. Never operate the unit to supply an open
circuit.
1) Electrically isolate the power supply unit.
2) Remove the cover from the power supply unit.
3) Disconnect the output wires to the hazardous area at
terminals TB2+ and TB2–.
4) Connect a 10Ω (2W ±5%) dummy load resistor across
terminals TB2+ and TB2–.
Warning. Ensure that proper electrical safety
precautions are taken at all times when undertaking this
procedure.
5) Switch on the power supply unit and check that it is stable
at 350mA.
6) On completion of tests isolate the unit, remove the dummy
load resistor and reconnect the output wires to the
hazardous area.
7) Replace the cover on the unit.
18
6.3.2
Zener Barrier Units
The zener barriers in the 6553 Monitor Unit are checked at the
time of manufacture. To ensure absolute safety when fitting a
new instrument, check that the barriers in the monitor are
properly earthed by carrying out a routine test before using the
analyzer system.
Warning.
• This unit is part of the certified intrinsically safe
system. Appropriate safety precautions must be taken
to prevent any incendive electrical discharges in the
hazardous area when carrying out this task.
• If these tests reveal a faulty zener barrier, the barrier
MUST be replaced by a new unit. The barrier is a
sealed unit and no repair is permitted. The correct
zener barriers are certified intrinsically safe to EX (ia)
IIC and no other type may be substituted.
1) Electrically isolate the 6553 monitor unit.
2) Remove the outer case from the monitor.
3) Disconnect the cable connected to terminal 3 of the barrier
unit.
4) Using a low voltage ohmmeter, measure the resistance
between terminals 1 and 3. This must be less than 18.15Ω.
If in excess of this value - change the barrier.
5) Using a low voltage ohmmeter, ensure that the resistance
between terminals 2 and 4 of the barrier unit and the
application plant safety earth is less than 1Ω.
6) Connect the wire to terminal 3 on the barrier unit.
7) Fit the outer case to the 6553 Monitor Unit.
6.3.3 Checking System Earth
Check that the resistance between earth terminals on the
analyser system and the application plant system safety earth
does not exceed one ohm.
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