P92x/EN CM/E11 Techncal Guide
Commissioning Guide
Page 26/28 MiCOM P921-P922-P923
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8.2.4 Measurement accuracy
If the power system is energised, the values measured by the relay can be compared
with known system values to check that they are in the approximate range expected. If
they are then the analogue/digital conversion and calculations are being performed
correctly by the relay. Suitable test methods can be found in Section 6 of this chapter.
Alternatively, the values measured by the relay can be checked against known values
injected into the relay via the test block, if fitted, or injected directly into the relay
terminals. Suitable test methods can be found in Section 7 of this chapter. These tests
will prove the calibration accuracy is being maintained.
8.3 Method of repair
If the relay should develop a fault while energised, depending on the nature of the
fault, the watchdog contacts will change state and an alarm will be generated. Due to
the use of surface-mount components, faulty PCBs should be replaced as it is not
possible to perform repairs on damaged circuits. Thus either the complete relay or
just the faulty PCB, identified by the relay’s diagnostic software, can be replaced.
Refer to Chapter P92x/EN FT of this Technical Guide for more information on alarms.
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: to avoid such difficulties, the MiCOM P921-
P922-P923 relays are designed to be removed while energised for fast replacement
of the live part of the relay, thus minimising the absence of protection.
BEFORE CARRYING OUT ANY WORK ON THE EQUIPMENT, THE USER SHOULD BE
FAMILIAR WITH THE “SAFETY” SECTION OF CHAPTER “P92x/EN IN”, OF THIS
MANUAL. THIS SHOULD ENSURE THAT NO DAMAGE IS CAUSED BY INCORRECT
HANDLING OF THE ELECTRONIC COMPONENTS.
8.3.1 Replacing the complete relay
The MiCOM P921-P922-P923 relays can be removed and replaced if necessary
without having to disconnect the rear terminals.
This is possible while the relay is energised. It is however recommended that all
auxiliary supplies are isolated before working on the relay.
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