x/EN MT/Ma7
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MiCOM P74
1.3 Method of repair
P741, P742, P743 relays
If the relay should develop a fault whilst in service, depending on the nature of the fault, the
watchdog contacts will change state and an alarm condition will be flagged. Due to the
extensive use of surface-mount components faulty PCBs should be replaced as it is not
possible to perform repairs on damaged circuits. Therefore, either the complete relay or just
the faulty PCB, identified by the in-built diagnostic software, can be replaced. Advice about
identifying the faulty PCB can be found in the Problem Analysis.
The preferred method is to replace the complete relay as it ensures that the internal circuitry
is protected against electrostatic discharge and physical damage at all times and overcomes
the possibility of incompatibility between replacement PCBs. However, it may be difficult to
remove an installed relay due to limited access in the back of the cubicle and rigidity of the
scheme wiring.
Replacing PCBs can reduce transport costs but requires clean, dry conditions on site and
higher skills from the person performing the repair. However, if the repair is not performed by
an approved service centre, the warranty will be invalidated.
Before carrying out any work on the equipment, the user should be familiar with the
contents of the safety section/safety guide Pxxx-SG-4LM-2 or later issue, the technical
data section and the ratings on the equipment rating label
1.3.1 Replacing the complete relay
The case and rear terminal blocks have been designed to facilitate removal of the complete
relay should replacement or repair become necessary without having to disconnect the
scheme wiring.
Before working at the rear of the relay, isolate all voltage and current supplies to the relay.
Note: The relay range has integral current transformer shorting switches
which will close when the module containing the block is removed.
Disconnect the relay earth, IRIG-B (Central unit only) and fibre optic connections, as
appropriate, from the rear of the relay.
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