P74x/EN MT/N
1, P742, P743
(MT) 11-
1. MAINTENANCE
1.1 Maintenance period
It is recommended that products supplied by General Electric Information receive periodic
monitoring after installation. As with all products some deterioration with time is inevitable. In
view of the critical nature of protective relays and their infrequent operation, it is desirable to
confirm that they are operating correctly at regular intervals.
GE protective relays are designed for a life in excess of 20 years.
P74x current differential relays are self-supervising and so require less maintenance than
earlier designs of relay. Most problems will result in an alarm so that remedial action can be
taken. However, some periodic tests should be done to ensure that the relay is functioning
correctly and the external wiring is intact.
If a Preventive Maintenance Policy exists within the customer’s organisation then the
recommended product checks should be included in the regular programme. Maintenance
periods will depend on many factors, such as:
• operating environment
• accessibility of the site
• amount of available manpower
• importance of the installation in the power system
• consequences of failure
1.2 Maintenance checks
It is recommended that maintenance checks are performed locally (i.e. at the substation
itself).
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
1.2.1 Alarms
The alarm status LED should first be checked to identify if any alarm conditions exist. If so,
press the read key repeatedly to step through the alarms.
Clear the alarms to extinguish the LED.
1.2.2 Opto-isolators
The opto-isolated inputs can be checked to ensure that the relay responds to their
energisation by repeating the commissioning test.
1.2.3 Output relays
The output relays can be checked to ensure that they operate by repeating the
commissioning test.
1.2.4 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 that is expected. If
they are then the analogue/digital conversion and calculations are being performed correctly
by the relay.
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.
These tests will prove the calibration accuracy is being maintained.
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