Do TT Installations Need Up Front RCD?
The 18th edition of the wiring regulations brought some serious changes to how installations need wiring up. Despite this, the problem of whether or not TT installations need up front RCD fitting actually predates the 18th edition! This problem actually stems back to the changes made to the 17th edition under Amendment 3 in 2015.
These changes required that all consumer units and similar switchgear in domestic installations need to be made from non-combustible materials (ie: metal). Now this brings up some issues as prior to this, metallic consumer units were something of a no-no on TT earthing arrangement systems.
This was due to the fact that the external loop impedance would generally be so low that any contact between a live meter tail and the metallic exterior of the consumer unit would leave the casing live. Not only that, but this would also generally leave anything connected to the earthing terminal within that consumer unit live too!
In fact, we’ve already covered fitting metallic consumer units to a TT earthing arrangement system on another guide, however this guide is intended to dive a little deeper into the question of whether or not a TT installation needs an up front RCD.
Providing Protection to Meter Tails on a TT System
So prior to these changes made in the 17th edition Amendment 3, it was generally standard practise to fit an ‘up front’ RCD to provide protection to the meter tails and busbar within the consumer unit.
This would have taken the guise of a time delayed (S type) RCD in another enclosure mounted ‘before’ the main consumer unit.
This was designed not to provide additional protection as is the case with a 30mA RCD, however it is designed to provide fault protection to the tails & busbar (and ultimately as another layer of fault protection to the final circuits).
However, this proves to be an issue in itself as the enclosure of the ‘up front’ RCD would also now need to be metallic, thus only having the same issue again as that of the main consumer unit. This being that any conductors which are ‘not protected’ by the RCD (ie: those coming into the RCD from the meter) would potentially be liable to come into contact with the metallic enclosure and thus create the self same problem.
As we discussed in our guide to fitting metal consumer units to a TT system, protecting the meter tails now boils down to more mechanical means rather than electrical as was the case previously.
Not Using an RCD on TT System Meter Tails?
Going against previous electrical practise, it is now common to omit RCD protection on meter tails within a TT earthing arrangement system.
The requirement to fit metallic switchgear enclosures means that wherever the RCD is fitted, there are always a set of meter tails which are unprotected and will potentially cause a risk of contact with the outer enclosure.
Modern practise is to ensure that the tails are sufficiently held in place such that there is little to no chance of them coming free from the main switch.
This can be achieved in a number of ways, although the main types used are compression glands over the meter tail entry and cable clamps holding the meter tails tight into the main switch.
Between these there should be little chance of the meter tails coming loose from the switch.
Another option is to use mechanical means to hold the meter tails tight outside of the enclosure. Examples of which would be cable clips, cleats or alternatively something like all-round band holding the cables.
So Do TT Installations Need Up Front RCD?
So to recap, Do TT Installations Need Up Front RCD?
Well, technically the answer now is no, there is no requirement to fit an ‘up front’ RCD to TT earthing arrangement systems.
However, this comes with a caveat, that other means must be taken to ensure that the chances of the meter tails coming loose. Assuming this is done, there is no need to fit an RCD before the main switch in TT earthing arrangement systems.