Single-phasing occurs when one phase of a three-phase system is intentionally or accidentally opened. It is considered as the worst case of Voltage Unbalance and can either be in the primary side or secondary side of a distribution transformer. However, this post will only discuss single-phasing on the secondary side of the transformer.
Secondary single-phasing can be very damaging to electrical loads, especially three-phase motors. The connected equipment will be subjected to damaging magnitude of currents, which can rise to 173% of normal current drawn by the motor and may go as high as 200% depending on power factor changes.
There are several instances that may result to secondary single-phasing. Generally, this is any open circuit in any phase anywhere between the secondary of the transformer and the load. These include, but not limited to the following:
· Open winding in one phase of transformer.
· Open fuse or open pole in switch or circuit breaker on main or motor branch circuit.
· Open cable or bus on secondary of transformer terminals.
· Open cable caused by overheated lug on secondary side connection to service.
· Open or poor connections in wiring such as in motor junction box or any pull box.
· Burned open overload relay from SLG fault on a three- or four-wire grounded system.
· Damaged motor starter contact (one pole open) is the most common cause of single-phasing.
· Open winding in motor.
Negative Effects to Motors
Single-phasing on transformer secondary will cause the current in the remaining two phases of a three-phase motor increase to 173% of normal drawn current. This is because the motor will try to deliver its rated horsepower until overload protective devices operate or until it burns out. Typically, the overload relays will safely clear the motor from the supply. Nonetheless, time-delay dual-element fuses that are properly sized for back-up overload protection could take the motor off from its power supply should the overload relays fail to do so.
It would be impractical to prevent or eliminate single-phasing, therefore, adequate equipment protection is vital. In other words, a protective device should sense and operate in the right length of time to protect the motor from damage against overcurrent conditions due to single-phasing.
Furthermore, in a lightly loaded three-phase motor (e.g. 65% of normal full-load current), the phase current will theoretically increase by √3 times under secondary single-phasing. Subsequently, the motor current drawn shall be around 112.6% of the nameplate full load current. If the protective devices were sized at 125% of the motor nameplate, circulating currents could still damage the motor. As a result, it is recommended that motor overload protection be based upon the actual operating current of the motor under its given loading, rather than the nameplate current rating. However, if the load varies, protection becomes complicated and difficult. Other devices such as current differential relays, phase failure relays and temperature sensors should be installed.