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Protection against Faulty Conditions Caused by Neutral Phase Loss (Neutral Power Lead Breakage) in 3-Phase Distributor System of 220/380V

A neutral phase loss. i.e. a neutral lead disconnection from a transformer substation solidly earthed neutral at an inlet service feeder switchboard can occur in those cases:

its burning-out at an abnormal phase overload;
short-circuits;
a poor contact in a lead connection place;
its breakage in a result of an accident damage (wind, a fallen tree);
a neutral lead theft;
and old electric wiring;
attending personnel error or evil intent, and so forth.

If these cases take place in a 3-phase balanced system, the load will not be influenced. An illustrative example is a 3-phase induction motor that needs in operation no neutral, because a 3-phase induction motor winding is balanced and all phases are equally loaded. But in a single-phase distributor systems phase loads are never coinciding that brings about phase voltage imbalance. The more a phase load difference the more is the imbalance. To equalize the phase voltages one needs to ground the neutral. Then a circulating current will flow inducing a forced relative-to-neutral phase equalization.
A summing potential is developed in a common point of that 3-phase system in case of a neutral phase loss. The summing potential is determined by each phase load resistance. The neutral drift will cause a big phase imbalance. The phase voltage applied to the C-phase-powered loads will be considerably less than 220V while the phase voltage applied to the A&B-phase-powered loads will be considerably greater than 220V. At the same time the line voltage will remain unchanged. The most crisis situation will arise when one of the phases has no load at all (no load is carried). Then the voltage developed in this phase can be close to the line voltage (380V) while the voltage developed in the most loaded phase will be close to zero. If a short-circuit in one of the phases occur when a high voltage is applied the line voltage of 380V can be applied to the all loads that are other phase-powered.

For the protection from neutral phase loss consequences we recommend to use microprocessor-based relay devices that are developed and produced by NOVATEK-ECLECTRO company. Use the RN-101 and RN-111 voltage relays in 1-Phase systems: the relays will trip for the minimum or the maximum voltage settings. Use the RNPP-301 and the RNPP-311 voltage monitors/relays in 3-Phase systems because the relays will trip for phase imbalance settings.

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