The 250 series phase balance protector module provides continuous
surveillance of a three-phase, three- or four-wire system and monitors
the correct phase rotation or sequence of three-phase supply systems.
The module protects against phase loss, reversal or sequence, phase
unbalance and system 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 (single-phasing)
which can result in severe electrical or mechanical damage. This relay has the added
advantage that it will detect the phantom or regenerated phase that can be caused by a
single-phase failure on some equipment or when running motors at low load levels.
An unbalanced supply voltage can lead to temperature rises in motors. An unbalanced
voltage as little as 10% can increase operating temperature to 150% of normal.
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 protector continuously monitors the three-phase supply. With the correct phase
sequence applied and all three voltages balanced within the required limits, the front panel
LED will illuminate and the output relay will be energised. An incorrect sequence, missing
phase, out of balance or under-voltage condition will de-energise the relay and the LED will
be extinguished.
The set point control allows adjustment of the voltage matching between 5% and 15%.
The time delay function operates only for the voltage unbalance condition. The delay can be
used to prevent nuisance tripping due to short term unbalance situations. Incorrect phase
rotation, a missing phase or an under-voltage condition trip the relay immediately. |
