Phase Failure Relay aplications :

MOTOR PROTECTION
The phase failure relay has the sensitivity to detect the voltage unbalance caused by a
motor single phasing. In most instances even a motor with no load and no other loads
connected to the open phase will be detected when single phasing. In using the phase failure relay it
must be realised that it only monitors the supply voltage at the point of connection and
therefore provides no protection for single phasing due to faulty switchgear, connections or
blown fuses etc., on the load side of this point. phase failure relay is ideal for detecting phase
failure and unbalance of HT lines and for this purpose 220V relays are also available. As the
phase failure relay detects the presence of negative sequence voltage a relay connected on the load side
of a three phase transformer will detect a blown fuse or high impedance in one phase of the
supply to the transformer, regardless of the transformer winding connections.
Pump motors on the end of a HT line with fuse protection and therefore high probability of
single line outages are an obvious application for the phase failure relay.
PHASE REVERSAL PROTECTION
The phase failure relay can be used for transportable cranes, refrigerated transporters, etc., which
on being connected from one supply to another require phase reversal protection. The
advantage of the phase failure relay for this application is the added protection of phase failure and
undervoltage.
3 PHASE UNDERVOLTAGE
It is a useful undervoltage relay in that phase angles are also monitored. As an example
three single phase undervoltage relays may not detect an open phase which is connected to
a healthy phase through a low impedance load. The phase failure relay however will detect this
condition due to the change in phase angles. In using the phase failure relay as an undervoltage
relay it must be remembered that only the B-W voltage is monitored directly. The other two
Æ to Æ voltages are monitored for correct phase angle and voltage balance with respect to
the B-W voltage. The difference in percentage undervoltage expressed as phase to phase
and phase to neutral must also be appreciated. As a guide, when two Æ to Æ voltages are
equally reduced as shown - the relationship is as follow