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.

 

[richy3333]

What do the Regs say about RCDs and discrimination?

The 100mA RCD would be on the ring (in your example) and thus n

 

[richy3333]

...and thus not necessarily compliant?

 

[needasparks]

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

 

[La Poste]

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

Wow, why do you think that happened?
Neither of them tripping.

Richy I am talking about two 30ma RCD's in series, forget about the 100 ma RCD.

 

[needasparks]

Wow, why do you think that happened?
Neither of them tripping.

Richy I am talking about two 30ma RCD's in series, forget about the 100 ma RCD.

Exactly, I took one out and worked fine after.

 

[MarkieSparkie]

There will be no discrimination between two 30mA RCDs, it will be a lottery if one or both RCDs trip under fault conditions. "Sods Law" dictates that the most inconvenient RCD will trip.

 

[La Poste]

Exactly, I took one out and worked fine after.

So it ends up being more dangerous.
I'd love to know the reason why, the technical explanation if there is one.
Thanks for the reply.

The idea was to make it safer, if one 30 ma failed you would always have the other to fall back on, if both tripped then not really a big deal.

 

[needasparks]

I think that is the big question, perhaps i just had a flukey one out of the two, but no matter what mA i tested at they would not trip so took one out of line and the problem went. All testing was fine after. I was of the belief also that at least one would work but as proved its not the case.

 

[La Poste]

I think I might try an experiment and see what happens, we have a 30ma RCD in this house protecting a cable that supplies the garden shed, I will stick a 30ma RCD on at the shed end, test it and see what happens.

 

[trev]

Mate your result will be unpredictable and random. With a set up like this it's luck of the draw.

 

[Knobhead]

You have to look at cumulative E/L over the two circuits.
RCD 1 is protecting the house
RCD 2 is protecting the garden Via RCD 1
The house in it’s self will have an inherent leakage current.

Lets assume both RCD’s have an equal threshold trip of 20mA. The inherent leakage in the house is say 10mA. That leaves us with a 10mA headroom. Add 10mA leakage from the garden then the house RCD trips, the garden RCD is quite happy it can only see 10mA.

You or your boss needs to have a rethink.

 

[netblindpaul]

I regularly fit 2 30mA RCD's in series to provide backup protection in certain applications.
Client is aware of the lack of discrimination and is happy.
End user has this explained to them in detail and they are happy once this is done.

If the downstream RCD only allows 10mA of leakage before tripping as per the example from Tony then in my application it can only be a good thing!!!

 

[La Poste]

Thanks for the replies, great debate.

The garden radial will be supplied from the house ring whatever happens, it is impossible to get to the fuse board to lay a new circuit so this leakage current will be the same whatever circuit protection we use.

I am stuck with the 30 ma RCD in the house as per regulations, it is a ring main and supplies sockets for general purpose, however if the garden is TT'd with let's say an EFLI of 150 Ohms then if the house RCD were to fail and an earth fault in the garden were to happen I will be left with a live frame to a greenhouse if everything is bonded as it should be.

I am trying to eliminate this risk by adding another layer of protection and this is one idea.
The other being to rod the ground so well that I get an EFLI low enough to trip a 13 Amp fuse not an easy thing to do.

 

[Engineer54]

Might be an idea to do the installations to these out buildings and greenhouse correctly, instead of spending time and money trying to get around things.

With 2 RCDs in series you will never know what is going to happen, both tripping together, not tripping at all, upstream tripping, down stream tripping. All depends on several things coming together at that particular point in time.