TN-S systems are widely used but comparing with TN-C and IT they hide some traps. The use of 4-poles circuit breakers  in UPS bypasses could lead to critical situation to be considered at the plant design stage.

Aim of this paper is a non exhaustive focus on potential consequences for the loads not considering safety aspects.

Behaviors of the UPS and consequences

The plant neutral is usually distributed downstream the UPS via bypass. Depending on national prescriptions 4-poles circuit breakers in PDUs could be employed . This must be considered in case of TN-C neutral systems, since the standards do not allow any interruption of the earth conductor, but also for TN-S, since the plant portion downstream the UPS could get a temporary IT system. The situation is the same in case of 4-poles UPS bypass breaker or maintenance of UPSs without the proper external maintenance bypass.

Temporary IT

For temporary IT, the international standards do not require all the constrains requested to native IT distributions because of the hypothesis of limited time duration of such condition. It is advisable, by the way, to check possible national prescriptions.

As a native IT system, the Temporary one if not affected by a first failure since all the phases are floating till the moment of a failure.

Supposing a phase-to-ground short circuit the failed phase (FP) gets bind to ground whereas those healthy (EP) get a potential to ground equal to V_n. The main consequence are:

• a stress on EMC capacitors with a consequent potential life reduction;
• immediate destruction of overvoltage suppressors.

A second failure is considered improbable by the standards therefore it is not requested to face both the events.

Possible solutions

Bases on individual choice, it is possible to avoid the abovementioned risk in two ways:

1. by integrating an isolation transformer to recreated a new installation grounding;
2. by installing a neutral contactor downstream the UPS system which will ground the neutral only if the continuity plant grounding is lost.

The constraints of the two solutions are respectively:

1. high cost of the solution both for purchasing and operating;
2. allowance of local authorities must be requested since the grounding is made by an electronic device.

Despite constraints for both the solutions, when one must take place, the use of the insulation transformer remains preferable and it can be applied in two further ways:

1. using the bypass transformer provided by UPS producers;
2. installing the transformer downstream the UPS.

In this case the benefits of solution a are: turnkey solution without plan complication and a better overall efficiency since the bypass is passed through by the power only for very limited amount of time.

Tech Info – Sep/Oct  2010

TN-S NEUTRAL SYSTEM AND UPSS

by

MATTEO GRANZIERO

Technical Communication Specialist

SOCOMEC UPS