Q & A of the Day - For a retail unit, can the client insist on a TN-S or TN-C-S supply?

Question: In a multi-tenancy retail unit under the same metal roof, we have been supplied with a TT supply by the electricity utility company. These units are 15,000 square feet each in area and although the electricity supplier has a TN-C-S supply into the first unit, it will not give a earth connection to subsequent retail units that are looped from the first unit.

The supplier has insisted that we supply our own earth via rods, for which we have a Ze of 20 ohms on the rods only. When connected up to steel etc, we are getting a reading of 0.06 ohms.

Because we have a TT system, we have installed RCD of 300mA time-delayed on the mains as per client design. However, we are getting problems of intermittent tripping, which could be caused by heating or refrigeration, or simply by the lighting control gear.

The question is, can the client insist on a TN-S or TN-C-S supply, or is the client stuck with what supplier has given them? If so, we can only remove the RCD from the mains, covering individual circuits or groups of circuits with RCDs - plus all the associated expense.

Answer: Firstly, you cannot insist on a TN-S or TN-C-S supply, first eliminate any potentially defective equipment, which may be the cause of the tripping.

Secondly, if the electrical system has connected equipment with earth leakage currents which cause unwanted tripping, the solution may be to provide RCD protection for smaller groups of circuits, instead of a single main RCD.

To achieve total selectivity, delayed or selective RCDs must be installed with the tripping times of the devices connected in series being coordinated.

To see many more Q & A in Voltimum UK's Experts Area, please click on the link:

www.voltimum.co.uk/consult.php?universe=consult.index.questions