The Crompton phase sequence and phase failure protector relays are
designed to monitor the correct phase rotation or sequence of threephase,
three or four-wire supply systems and provide protection against
incorrect phase sequence, loss of one phase and under-voltage.
Operation
Rotating machines are particularly vulnerable to incorrect phase sequence. Three-phase
motors can rotate in the wrong direction, potentially leading to physical damage or the
risk of injury to personnel, yet voltage and current readings may appear normal. If one
phase is lost because of a blown fuse, electric motors can continue to operate (singlephasing)
which can result in severe electrical or mechanical damage. For permanent
installations, this relay should be used to monitor the incoming supply, protecting all
equipment against incorrect connection at initial installation or after maintenance work.
Rotating machines that cannot tolerate reverse rotation or pose significant risk to
personnel under this condition should be individually protected with this relay.
The possibility of incorrect supply connection is much more likely in portable
equipment or marine applications.
The phase sequence and phase failure protectors continuously monitor the three-phase
supply. With the correct phase sequence applied, the front panel LED will illuminate
and the output relay will be energised. An incorrect sequence or missing phase will
de-energise the relay and the LED will be extinguished. If the supply drops below 85%
of its nominal voltage, this condition will also cause a trip.
Note: If one phase is lost due to a blown fuse, some loads can re-generate the missing
voltage. This relay can be used as a phase failure relay providing the regenerated
voltage in the open phase is less than 70% of the nominal supply voltage. If there is
the possibility of a higher regenerated voltage, the phase balance relay 252-PSF should
be used. |
