Failed RCD in TN-S system

[steviewonder]

Greetings all,

I have just moved into a new house. The electrical installation seems fairly new. the CU is a Wylex split load. The main switch is a 100 amp/100 ma time delay RCD. The sockets and immersion heater are protected by a 80A/30ma RCD. The earthing system appears to be TN-S ( the earth comes from the sheath of the incoming power cable).

The 30ma RCD trips when the test button is pressed. The main switch does not trip when the test button is pressed, but does turn the power off when it is switched off.

From my research in this forum and other sites it seems that it is not necessary to have an RCD on the main switch for a TN-S installation. A replacement for this time delay RCD is very expensive. Is it really necessary? Could I leave it as it is or replace it with something cheaper?

I would be really grateful for some advice

Steve

 

[davy_owen_88]

From my research in this forum and other sites it seems that it is not necessary to have an RCD on the main switch for a TN-S installation.

Correct. You don't need an RCD main switch for a TN-S earthing arrangement. You can either replace it with a standard 100A DP switch or leave it as it is so long as the switch is still functional.

Remember that replacing the RCD will mean the main fuse will need to be removed (unless you already have an isolation switch fitted after the meter?) which can only be done by your DNO

 

[JohnD]

it sounds like it is not necesary, but it would be best to have an electrician in to test the quality of the earth. a photo will help.

ask your electricity supplier if they can give you a PME (TNC-S) which might be be cheaper than a new Time Delayed RCD and is a better solution.

btw RCDs are prone to stick if they are not tested from time to time. once a month is good. once a year is not often enough.

 

[Adam_151]

Could have been fitted due to a high Ze? Though it would have been more sensible to have the DNO deal with the issue

 

[scousespark]

As stated before the Earth may have been poor on the TNS system so there may be an Earth Rod somewhere in your property. As adam said the better option would have been to get in touch with the DNO.

Given the incomer RCD doesn't trip I suggest you get an electrician to come and test it. Pressing the button is not really sufficient in your case.

 

[Lectrician]

Fitting an RCD due to a poor TN-S earth loop would be shocking, and the spark kicked up the arse. This is not a cure nor a work around. It would be a bodge.



Also. JD.....



Asking for a PME connection where there is already a satisfactory TN-S earth connection is madness.




I would first check the earth loop of the TN-S supply, and if this is ok, I would swap out the RCD for a main switch. I would also take the opportunity to shift the Immersion to the non-rcd side.




Some photos would help, just to confirm your earthing arrangements.

 

[JohnD]

Asking for a PME connection where there is already a satisfactory TN-S earth connection is madness.

I don't know if it is satisfactory, do you? there are hints that it might not be. PME is better and more reliable than the twisted wire or pipe-clamps that are often seen.

 

[Lectrician]

Asking for a PME connection where there is already a satisfactory TN-S earth connection is madness.

I don't know if it is satisfactory, do you? there are hints that it might not be. PME is better and more reliable than the twisted wire or pipe-clamps that are often seen.

No we dont, but to jump in on a job and assume that as it has an RCD as a main incommer means the main earthing is poor is a jump to far.....

We cannot comment further without knowing the result of the Ze on the TN-S supply. Suggesting a PME conversion prior to this is a little premature.

IMO anyway!

 

[JohnD]

it sounds like it is not necesary, but it would be best to have an electrician in to test the quality of the earth. a photo will help.

ask ....

 

[securespark]

It might even be a sub-terranean TT supply...

 

[scousespark]

Fitting an RCD due to a poor TN-S earth loop would be shocking, and the spark kicked up the a**e. This is not a cure nor a work around. It would be a bodge.

Sorry for not being clear that this would be unacceptable.
A spark on the ground should be able to sort this out.