Up front Rcd question

[Dirk boddington]

After talking to someone who's Rcd failed them recently during a regular test it got me thinking.

I have a TT system and regularly test my rcd however if it failed to operate during a phase to earth fault then there is no backup.

I considered upgrading the consumer unit and adding rcbos but that's well beyond my budget right now. (they may fit in mine now but it will be very tight)

My second thought was adding a rcd in line with my existing consumer unit to act as a backup. All of my circuits are fed from this single rcd at present as its not a split load unit.

I've spoke to several electricians about it and I've had varying answers so thought I would ask a wider audience.

Can this be done with 2 30ma rcd or would this create nuisance tripping or would one have to be a 100ma?

 

[Ian1981]

After talking to someone who's Rcd failed them recently during a regular test it got me thinking.

I have a TT system and regularly test my rcd however if it failed to operate during a phase to earth fault then there is no backup.

I considered upgrading the consumer unit and adding rcbos but that's well beyond my budget right now. (they may fit in mine now but it will be very tight)

My second thought was adding a rcd in line with my existing consumer unit to act as a backup. All of my circuits are fed from this single rcd at present as its not a split load unit.

I've spoke to several electricians about it and I've had varying answers so thought I would ask a wider audience.

Can this be done with 2 30ma rcd or would this create nuisance tripping or would one have to be a 100ma?

An expectable method which may or may not be in your budget is a dual 30mA rcd db with 100mA time delay main switch

 

[westward10]

Do you mean put another separate rcd in parallel with the one already in the board.

 

[Dirk boddington]

No in series. This would act as a backup so if one failed during a fault condition the other would likely trip. The chances of two failing at the same time would be unlikely

 

[westward10]

If they both protect the same circuits it would serve no purpose.

 

[Octopus]

I would recommend that you get a spark to test your RCD.....

 

[Ian1981]

There no requirement to have a upstream rcd protecting A downstream rcd of the same nature for back up as you call it.
You may want to fit just a 100mA time delay as an upstream device tho it won't offer additional protection when required and may not give you your 0.2 second disconnection time for circuits upto 32amp.

 

[Dirk boddington]

The purpose is a backup, belt and braces so to speak. My rcd works fine, I'm talking about a remote chance that it fails, could be from some crap getting in it over the years, contact welding themselves.. Anything really.

Maybe it's my paranoia but let's say a fault behind a socket puts live to earth, my rcd fails so I have a short circuit current of 20-30 amps flowing on earth as it's a TT system. What would happen to someone touching an exposed part which is bonded, or someone in the shower? It's hypothetical and I actually don't know the answer which is why I'm asking

 

[westward10]

Some things are not worrying over, test your rcd frequently and if it fails consult an electrician.

 

[Octopus]

My rcd works fine,

How do you know this for certain?

 

[Dirk boddington]

It's been tested, that's not really the issue here

 

[richy3333]

Surely the first principle is to ensure earthing and bonding is all correct and the earth rod/mat etc gives the lowest Ra possible so any touch voltages are as low as can be? Or am I missing something?

As an aside the only RCDs I've ever known to fail testing were MK and Wylex ones, so maybe ensure you have good quality equipment?

 

[Octopus]

It's been tested, that's not really the issue here

By pushing the test button or by a calibrated tester....

FWIW I wouldn't have 2 x 30mA RCD's in series..

 

[Dirk boddington]

calibrated test equipment but were really going off track here...

Why wouldnt you have two in series?

 

[Toneyz]

The main switch RCD would need to be time delayed 100mA as should you have a line to earth fault of a negligible impedance there would be no discrimination of which RCD would trip first. You need the final circuit RCD/RCBO to trip first.

 

[Dirk boddington]

Thanks for the reply Anthony. For the purposes of what im looking for, do I need to have discrimination between the two as essentially I wont care which goes as long as one does ? (please dont think im questioning your answer, I genuinely dont know the answer to this myself)

 

[Toneyz]

Thanks for the reply Anthony. For the purposes of what im looking for, do I need to have discrimination between the two as essentially I wont care which goes as long as one does ? (please dont think im questioning your answer, I genuinely dont know the answer to this myself)

well if you don't want to loose all power eg. lights heating etc. if say you plug in a faulty appliance to the R.F.C. you would want that circuit to trip out rather than main switch RCD.DISCRIMINATION. The time delayed RCD would in this scenario be a back up if it fails.

 

[Ian1981]

well if you don't want to loose all power eg. lights heating etc. if say you plug in a faulty appliance to the R.F.C. you would want that circuit to trip out rather than main switch RCD.DISCRIMINATION. The time delayed RCD would in this scenario be a back up if it fails.

However a time delay can trip between 500-130 seconds so if you indeed had a faulty 30mA rcd and the time delay when tested trips at 430ms then as it's tt upto a 32amp circuit , your over the 0.2 max disconnection time allowed

 

[Toneyz]

However a time delay can trip between 500-130 seconds so if you indeed had a faulty 30mA rcd and the time delay when tested trips at 430ms then as it's tt upto a 32amp circuit , your over the 0.2 max disconnection time allowed

That's why RCD's/RCBO's need testing regularly.

 

[Dirk boddington]

Anthony, sorry I see what you mean. As my board is a 8 way fed by a single rcd then it wouldn't matter I assume as I'm going to lose all power either way