RCD's in series

[La Poste]

I was having a chat with my electrical boss today about this and that and we discussed wiring a radial in a garden supplying various things like a greenhouse and a garden shed.

He was of the opinion that things like greenhouses should be TT'd with an earth spike and RCD protected.

The thing is if the radial supplying the garden is taken from a ring main in the house then the cable will already be RCD protected at 30ma.

This 30 ma RCD will effectively protect the ring main in the house and the garden radial, the radial will then be split from the main earthing in the house and TT'd.

What my question is is would it be acceptable to run two 30 ma RCD's in series?
I know this would not satisfy discrimination but it would give you a second layer of protection in case the RCD in the house failed.

If both RCD's tripped due to an earth fault then so what, no big deal just reset them both but should the house RCD fail then at least you have a second layer to protect you and also of course should the RCD in the garden fail then you would have the RCD in the house as a second layer of protection.

People talk about 100ma S type RCD's to offer discrimination but what is really wrong with putting two 30 ma RCD's in series, the only thing I can see would be a little inconvenience in that you would have to reset them both but weigh this against having a second layer of protection, especially if the garden is TT'd and disconnection in the event of an earth fault would be 100 % reliant on an RCD then I think it is a reasonable trade off.

Does anyone have any opinions on the matter?

Thanks.

 

[netblindpaul]

One should always trip, there are even reasons for redundant protection.
However, in this application as E54 says, just do it right.
If you don't need redundant protection don't bother.

 

[1shortcircuit]

Just a question but why can't you configure your circuits so that the greenhouse/shed/outbuiling or whatever it is runs from an rcbo on an unprotected way?

RCD's will remain happy and so will outbuilding and discrimination will not exist

Sometimes the perfect solution is not always the easiest to achieve but it does exist one way or another Worst case, stick a two way consumer unit (main switch and rcbo) in before the consumer unit) :thumbsup

 

[La Poste]

It's just impossible to get a new cable from the CU.

The guy has had all new floors put down and the CU is in a sort of concrete bunker (and so is the main incomer) in the middle of the house.
He's had all sorts of extensions added in an outward direction from this concrete bunker and sealed all the floors with wood laminate.

I'm going to really take my time on this one but as it looks at the moment a new cable is nigh on impossible.

Supply is underground TN-S.

 

[1shortcircuit]

Surface mount trunking right the way through the property " Gert Lush" lmao

No such thing as impossible, just means that a sacrifice or two may be required

:thumbsup

 

[La Poste]

I will go and do some more testing tomorrow but right now I gotta crash.

Thanks for your help everyone.

I won't be starting this job until after my assessment that's assuming I pass but I will test as much as I can in the meantime.

 

[SPARTYKUS]

Always a way La Poste!

 

[richy3333]

Always a way. If you can't do he job pro

 

[richy3333]

There's always a way. If you can't do the job properly don't do it. A good reputation is hard earned and easily lost.

Do you have any photos of this concrete bunker?

 

[mydigitalhome]

I have been doing some testing work on a Nursing Home (new client) which had two 30ma RCDs in series (1 x DB supplying 5 x DB's approx 40m appart) and while testing it was tripping at both RCDs 1 x on one and 5 times on the other really annoying as made testing awkward ! Not ideal.

Back to original scenario of outside supply I always put the outside supply on the non RCD side of the board supplied via a RCBO 100ma and 30ma at shed CU this gives full discrimination for the outside circuit. How do the rest of you do it?

 

[1shortcircuit]

I always put the outside supply on the non RCD side of the board supplied via a RCBO 100ma and 30ma at shed CU this gives full discrimination for the outside circuit.

Are you sure about that?

What are you doing when you're testing at x5 on RCD test?

You would need a time delayed RCD as I understand it from reading posts on the forum?

 

[mydigitalhome]

The RCD testing was not consistant sometime tripped only on one rcd other times took both out. Yes you are correct time delayed 100ma sorry took that for granted.

 

[Octopus]

Back to the OP, if the circuit serving the outbuilding is protected by a 30mA RCD, why bother with a 2nd RCD?

 

[Edd]

I've had discrimination with two 30ma RDC's where neither tripped on a fault, so i would not take the chance.

Im really intrested in this, anyone got a technical reason or idea why this could happen?

 

[SPARTYKUS]

Im really intrested in this, anyone got a technical reason or idea why this could happen?

Faulty RCDs? Cannot see any other reason theres no magic secret with them as far as Im aware. Needasparks what do you mean you had discrim. with two 30mA RCD's?

 

[Deleted member 26818]

The fault didn't produce a high enough fault current.