Motors, switchgear, transformers, buss-work, and in fact, all electrical equipment is vulnerable to serious failures due to single phasing. Single phasing is the loss or electrical breakdown of one phase of a three-phase circuit.

If undetected the simple failure grows and can cause a large loss of equipment. The degree of protection needed is the subject of some controversy. Economy is one factor influencing how much protection is justified.

Nevertheless, in a Property Loss Prevention Data Sheet, FM Global says more serious consideration is needed in view of the number of losses caused by single phasing. It says, protection against single phasing is dependent on more than one protective device.

Causes of Single Phasing

Causes of a single phase condition on three-phase circuit include a blown fuse, a broken utility line, a contact failure, a grounded conductor, or an open thermal element. Unless properly installed protective devices operate, the result can be overheating, arcing, fire, and burning out of windings, or a chain reaction of electrical failures.

An ‘‘open phase’’ on a polyphase system (i.e. motors on the system have available only a single-phase power) will lead to trouble. If one line (phase) to a threephase motor opens while it is running, the motor may continue to run, but as a single- phase unit. In this condition, the motor itself tends to maintain the voltage in all three phases, but current flow in the other two phases is higher than usual. This current will be greater than 1.73 times normal current, with maximum flow of current dependent upon motor efficiency and power factor. The poorer the power factor or lower the efficiency, the greater the current flow during a single phase condition.

A running motor may stall if carrying more than 35 percent of its rated load when single phasing occurs. More frequently it continues to run as a single phase motor with part of its winding drawing excessive current. This current, if measured by an overload relay, will trigger the relay to disconnect the motor before it is damaged.

Overload Relays

Many standard motor starters have overload relay protection in only two of the three lines (phases) since this arrangement usually will detect single phasing in most circumstances. There are conditions in which the excess current flows only in one line, and there is a 33 percent probability that this will be the unprotected leg.

An overload relay properly applied in each of the three phases would eliminate this probability.

If on the same feeder there is more than one connected load, the voltage generated by the transformer action in the open phase will supply power to these single phase loads. This results in heavier current flow through the connected two phases. It is only a matter of time until excessive overheating will cause burning out of the winding and damage to connected wiring and switchgear.

Since the ‘‘dead’’ single phase will always generate at a voltage close in potential to the supply voltage, any phase-failure device which is wholly dependent on voltage alone will not open the circuit.

Basic Fault Protection

Protection against single-phasing and related faults may start with first providing basic protective devices. Without the necessary basic protection, other protective devices cannot function satisfactorily to recognize faults in the system.

Fuses

The standard fuse: Lacking a built-in time lag, the standard fuse cannot handle a 700 percent starting current, and at the same time give the required running overload protection. If the requirement is only for short circuit protection, then the standard fuse is satisfactory.

The time-lag fuse: The time-lag fuse is used for both overload and short circuit protection. Careful selection of size is necessary for reliable protection against short circuit or ground. This is governed by the full load current rating of the motor. Fuses are in themselves one of the causes of single- phasing since a blown fuse can create single-phasing in a three-phase circuit. It is therefore important that other protective features be incorporated in the circuit. The dual-element fuse:

The dual-element fuse is a special fuse incorporating two elements one of which has the speed of response required for short circuit protection and the other will provide running over current protection for the motor windings with enough time delay to handle the motor starting current.

Thermal Trip Devices

Thermal overload relays are installed in the motor controller to provide the required running over-current protection for the motor. Where circuit breakers are used for protection of the motor branch circuit, they are equipped with thermal time delay trips to provide the required overload protection for the conductors and a magnetic trip for instantaneous tripping on short circuits.

Thermal trips are of the melting alloy or bimetallic type and are widely used as protection against single-phasing and motor overload. The development of heat due to current flow through a series sensing device will actuate a toggle and trip the controller circuit breaker.

If the thermal protection is limited to only two of the three phases of the controller, protection against single phasing is not complete since there is a 33 percent probability that the overload will occur on the unprotected leg. FSM

Source: “Protect Electrical Equipment from Single Phasing and Faults,” an FM Global Property Loss Prevention Data Sheet.