Q & A of the Day – Supply arrangements for a telecoms room

This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.

Question:

I have come across a 3 phase supply to a domestic board within a telecoms room. This supplies various Ccts including telecoms equipment, switch sockets, lighting etc. It appears to be supplied via a 1:1 ratio isolation transformer, there does not appear to be an earth, and the earth within the D.B. is linked to a D.C. telecoms earth bar. The neutral is also linked through the earth. There is no info available. So, my question is: Does the earth linking through the neutral to the DC Bar sound correct?

Thanks for any assistance

Answer:

ECA - Steve Martin

An isolation transformer, such as the one you describe, may have been specified by the designer of the comms room facility for a number of reasons. Such applications are usually of specialist nature, and it is essential that if considering any changes, modifications or even just testing/verifying that you seek clarifications from the original system designer.

As a guide, isolation transformers as you describe are usually incorporated in the electrical supply system, to deal with issues such as 'electrical noise' which certain electronic equipment can generate, usually in the order of higher octave harmonics. Such harmonic waveforms may degrade the performance of adjoining circuitry or other equipment fed from the same source or distribution board.

To ensure that the isolation transformer continues to fulfil this purpose, the designer should always address other parameters such as harmonic disturbance tolerances of critical equipment as well as the specification of the transformer itself, as well as its connection arrangement.

As you will see, it is impossible to give detailed design solutions on what is at present a generic type question.

For further information in the absence of the original designer's input, we would suggest that you have a look at:

BSEN61558:2005 +A1:2009 - which is still listed as current by BSi. This document is entitled 'Safety of Power Transformers, power supplies, reactors and similar products. General requirements & tests.'

This will give some idea of the various connection arrangements feasible in particular circumstances.