RCD in TN-C Earthing System

Using high-sensitivity RCD's may be a bit of overkill.

If you are seeing high current on your neutrals, you should consider finding the source of imbalance on the branches affected.

You don't have a TN-S...you have a combined earth and neutral conductor? Do you utilize TNC?

If you don't have a TT, what benefit are you looking for through the use of RCD's?

If you want to use RCD protection then each sub distribution board must be made TN-S.

You can't use TN-C with RCD's for sub circuits. The RCD will trip indiscriminately depending on circulating currents. The worse case scenario is the RCD won't trip when needed.

If you can't convert to TN-S then forget RCD protection, it just isn't going to work.

You need a major rethink of you're distribution system.

In the UK.

Only the incoming supply can be TN-C to the service head. After the intake the neutral conductor must be separate from the CPC (ground) conductor, making it effectively TN-S.

For once I'll echo PWSlack, consult IET BS7671 rev 2. There is a lot of information in there.

Please note that for RCD (RCCB - residual current circuit breakers) to work correctly, the earthing of neutral shall be only upstream of RCD/RCCB.

That means if insulation of neutral conductor of a particular circuit, after it has passed through the RCD, is damaged resulting in low insulation resistance to ground, RCD of particular circuit shall trip.

Similarly if common neutral is used for different circuits - RCD will trip.

A good practice is to

1. Ensure that earthing of Neutral is only on Neutral Bus before the RCD (Up stream of RCD0.

2. The line and neutral conductors run together to all loads connected in the circuit down stream of particular RCD.

3. Insulation of Neutral is not damaged or week or is passing through hot area (near furnace etc.) where the insulaton suddenly drops. Pl not insulations have negative temperature co-efficient of resistance.

4. If the length of circuit is long - resulting in high capacitive current to earth , run line and neutral in twisted pair (Trifoil profile for 3 Ph and N circuit with neutral in centre of phase conductors).

5. Use higher current settings for main RCD say 150mA or 300mA (where long lengths of cables/ wires are expected) and use 30mA RCD near the load (say at plug outlets). This will provide

5.a Back up protection against shock. And main protection against other risks of electrical leakage such as fires while at the same time minimize nuisance trips.

5. b Prevent nuisance tripping due to long length of cables/ wires.

5.c Ensure that personnel subject to risk of shock are well protected. as well as no chance of nuisance tripping due to long lengths of wires (due to capacitive currents).

Regards,

Ramesh Kapur

It has absolutely nothing to do with imbalance.

Check that there are no inadvertent connections between earth and neutral downstream of the circuit board (been there, done it, secondhand T-shirt now on eBay). It's easy enough to do with a multimeter while disconencting each circuit one-by-one at the distribution board. Anything with a low resistance from the earth conductor to the disconnected neutral conductor represents a safety-related fault that must be isolated and eliminated.

If in doubt, consult a qualified Electrician.

Hello,

I have noted your concern, however, it seems unlikely that whole building is really in TN-C system (including for example socket-outlet circuits). If this is really the case, you cannot use 30 mA RCD for protection against electric shocks : the reason is very simple. In TN-C system, the Neutral conductor and the PE conductor are the same conductor; therefore in case of residual current to ground, the current will leak to ground, but ground is also the neutral conductor. As a consequence, the residual current will also flow in the RCD. In this case the RCD will see no residual current, and will not be able to protect against electric shocks.

Jean-François