Problems of Metal Consumer Unit on TT Supply- Video

by Editor on April 22, 2015

Here’s a very informative video from John Ward about the problems of installing a metal consumer unit on a TT supply.

The problem

Should a fault occur between line and earth on the supply side of a TT system, due to the high earth loop impedance (resistance back to the supply transformer) it will not cause a high enough current flow to blow a 60 to 100 Amp supplier’s fuse. This would result in all of the metalwork within the installation becoming live, and remain so, with the obvious serious risk of electric shock.

To overcome this problem an RCD in an insulated (plastic) enclosure would be used to disconnect the supply if such a fault occurred. However due to fire risk concerns, BS7671 Amendment 3 now requires that consumer units and similar enclosures are metal (non-combustible) so plastic is no longer an option.

John clearly explains the characteristics of a TT supply and the problem that could arise where a metal enclosure is used without  an upstream RCD to protect it. He also explains the potential solution of fitting a plastic cable gland to the meter tails but points out that this wouldn’t absolutely prevent a fault to earth occurring inside the metal enclosure.

The Old Way

Prior to amendment 3 regulation 421.1.201 (the old way)- If your consumer unit was of metal construction then a 100mA RCD needed to be installed between it and the origin of the supply. The 100mA RCD device would protect the meter tails and give you discrimination between any 30mA RCDs protecting the sub-circuits of the installation. Discrimination meaning that the 30mA would trip before the 100mA and all power to the installation wouldn’t be lost (obviously a lot safer and less inconvenient for the consumer}.

The problem we have now is that, as explained by John in the video, the 100mA RCD would need to be mounted in a plastic enclosure which isn’t allowable under amendment 3.

More about Amendment 3 regulation 421.1.201

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{ 5 comments… read them below or add one }

W Riggs January 11, 2016 at 12:48

It would appear desirable to avoid the problem by always reqesting the supply authority to change the supply from TT to TN-C-S (PME), which may be available. Supply authorities used to do this free of charge. It is not unknown for electricians to renew installations without requesting this, and to continue with a TT supply.

BUD BARKLEY August 13, 2015 at 11:37


K J Petrie July 22, 2015 at 23:26

On reflecting overnight on this, I’ve come to the conclusion that, though the analysis is good as far as it goes, it doesn’t go far enough. Because the voltage is dropped across the distance between the customer’s and supplier’s earth rods, the effect is not just to raise the voltage of the electrical earth bondings to live, but to raise the voltage of the local earth to live. This includes the ground beneath the house, the masonry, any metal attached to the house whether bonded or not, and even the occupants of the house. In fact, within the property, the normal convention that Neutral is near earth potential and Live is 230v away has been locally reversed, so that Neutral rather than Live has become the dangerous conductor to touch. This has all sorts of implications for protective systems based on the assumption that Live is the line to be cut in the event of a fault, but a shock hazard for someone touching an earthed metal casing is not one of them, since everything that’s not specifically connected to a different potential will remain close to Live. Or to put it another way, the connection to an earth rod ensures everything stays at earth potential, whatever earth is actually connected to in terms of the supply.

For neighbouring properties, things are a little more complicated, since they are likely to be supplied by a different phase. For them, Neutral will also be about 230v away from earth (or perhaps a little less since they are further from the fault and on the path back to the substation) but their Live will be nearly 400v above Earth. This obviously increases the hazard for anyone touching either conductor. However, in normal use, people don’t touch conductors. The real danger is to electricians working on a circuit where the voltages might not be what they expect, especially if they believe removing a fuse or circuit breaker has removed any danger. The importance of relying on a double pole isolator rather than single pole techniques becomes apparent with this possible fault scenario, not just in the property being worked on, but in neighbouring properties too.

However, the danger to the general public is not as great as might first appear.

K J Petrie July 21, 2015 at 23:44

Maybe the manufacturers need to be persuaded to consider porcelain CUs, or even Pyrex ones. Certainly, porcelain is widely used for sanitary ware and insulators for HV distrbution, so it can be made robust and insulating. Electrical fittings were made of it once, so why not again?

p.dews July 14, 2015 at 22:08

If you have a TT supply surely the answer is to put the incoming rcd protection in a plastic box outside the property. The new regulation is to protect sleeping people from toxic nasties released by fire. If you live in the sticks and have a transformer on a telegraph pole in your field outside then a plastic box out there should be OK.

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