ABB Relion 670 series, Installation And Commissioning Manual

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The neutral-point voltage to an earth-fault protection IED is checked. The voltage
is normally 0.1 to 1V secondary. However, voltage can be considerably higher due
to harmonics. Normally a CVT secondary can have around 2.5 - 3% third-harmonic
voltage.
17.3.2.8 Restoring
Maintenance is very important to improve the availability of the protection system
by detecting failures before the protection is required to operate. There is however
little point in testing healthy equipment and then putting it back into service with
an open terminal, with a removed fuse or open miniature circuit breaker with an
open connection, wrong setting, and so on.
Thus a list should be prepared of all items disturbed during test so that all can be
put back into service quickly and without overlooking something. It should be put
back into service item by item and signed by the responsible engineer.
Section 17
1MRK 502 029-UEN A
Commissioning and maintenance of the fault clearing system
238
Installation and commissioning manual