Single-phase protection

Adequate single-phase protection is provided on low-voltage ac motor starters by three overload relays, which are now standard. Rotor heating is not particularly a problem on smaller motors which have more thermal capacity, but it is important to protect the stator windings of these machines against burnout. 

A separate single-phasing protection device is available in two versions ... 

Unbalanced Voltages and Single Phasing Protection, M/s Minilee Controls Private Ltd. 

Overload devices in current use are typically thermal overload relays to BS 4941, motor starters for voltages up to and including 1000 V A.C. and 1200 V D.C. or BS 142 electrical protection relays. Relays to BS 4941 generally provide overload and single-phasing protection. Those complying with BS 142 are also frequently fitted with instantaneous earth fault and over-current trips. 

Freeze thermostat (low water temperature limit) Single-phase protection (preventing attempts to start with loss of one phase of a three-phase electrical supply) Time out (preventing too-frequent starting and motor burn). 

Fuses should not be used to protect secondary voltage feeders. The time current characteristics of fuses above 100 A will not coordinate with the groimd fault pickup currents and time delays of the main overcurrent protection (circuit breaker or fused disconnect switch) ground fault protection. A main load break disconnect switch can be equipped with current-limiting fuses to reduce the available short-circuit current from the utility and should have a three-phase voltage relay for single-phase protection. 

The purpose of negative phase sequence protection is the same as for high voltage motors. It is not normally needed for motor ratings less than approximately 22 kW. In the simpler designs of moulded case circuit breakers and relays the negative phase sequence detection is more in the form of single-phase protection, wherein a phase is completely lost. 

Neither this nor the sensitivity of the melting-alloy type to phase loss should be seen as true single-phasing protection. If single-phasing protection is a concern, complete protection may be provided by an electronic voltage monitor or solid-state overload relay. 

Manual and Magnetic Starters Manual motor starters are operated by hand. The simplest type of manual starter is a snap switch with no overload protection, used only for motors of 1,492 kW (2 hp) and smaller, usually single-phase motors with integral overload protection. 

Some overload-protection schemes measure motor-winding temperature directly various methods are used. Small single-phase motors are available with built-in overload protection, A thermostat built into the motor senses motor-winding temperature directly. 

Thus, besides voltage unbalance it can also detect an inter-turn fault, which leads to a current unbalance. A small amount of unbalance is already detected by the thermal element of the overcurrent protection but a severe unbalance, such as during a single phasing, would require quicker protection and hence, this protection. The relay may be set for an of around 3% or so. 

Protection of a domestic or an industrial single-phase system... 

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