RCDs in series

There's a thing called "discrimination" please familiarise yourself with this.

 

its a reg breaker sadly hence rccob's (tho i often wondered why if you could get a 20ms trip rcd for an out building then use a 30ms trip rcd in the building supplying it) there for when running a sub distuburation system to out buildings from a main building (i tend to run a double pole isoloator to feed the out building. (each 1 on its own) with an rcd in the out building. Then if the rcd trips its in the outbuilding where it should be easy to acess to reset.) rather than having to dig at the back of a cupboard to re set an rcd

 

To answer your question
No it's not dangerous, but on a fault any of them could trip, possibly more than one of them too, so in the dark it may well be classed as significant nuisance.
The trip time would not change as they all act independently of each other.

 

also take it you mean wired in rcds (as in consumer unit rcds) rather than rcd sockets plugs or plug through rcds as i dont know what the regs are on the use of those in series.

 

Is it not only a "reg breaker" if by multiple devices tripping it creates a danger?

 

Only way round this is an "s" type RCD to give discrimination with devices downstream. .

Two 30mA devices, the one near the origin of the supply will open first on a fault. .

 

It could be depending on the circumstances. 3, 30mA RCDs on one circuit is poor design. Discrimination is the key here.

 

Any spark would know that and not install.

Regs are the same for All RCD's be it plug in and/or DB installed

 

Someone posted up here a while back that it's actually safer to have two rcd's in series in case one of them fails. Also, I don't think there is a reg per se, unless some one can quote it? I have always thought discrimination was for breakers rather than rcd's....

 

531.2.9 refers.

 

You answered your own question. Whomever said two RCDs installed is safer in case one fails was talking rubbish.

 

You need to get all the facts before you give advice. A 30mA time delayed RCD may not provide the required protection in some circumstances.

 

Where in the regs does it say that you cant use 2 identical rcd's in series.......(certainly not in 531.2.9)
(tho not really general practice)
Statistically...2 in series will be more safe than 1....all other things being equal.
Also 2 30mA rcd's will have different tripping characteristics....so will be a toss up which one in series might go first.
RS

 

I beg to differ. 531.2.9 states - "Where, for compliance with the requirements of the regulation for fault protection or otherwise to prevent danger, two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger, the characteristics of the devices shall be such that the intended discrimination is achieved."

Furthermore, 531.2.4 states - "An RCD shall be so selected and the electrical circuits so subdivided that any protective conductor current which may be expected to occur during normal operation of the connected load(s) will be unlikely to cause unnecessary tripping of the device."

 

531.2.4 does not apply to 2 in series.
531.2.9....
...."two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger...."
What danger will be caused with 2 in series for an average cct ?
RS

 

If the RCD is not NECESSARY then why would someone install it?

Possible injury to the little old lady searching for the RCD in the dark that has tripped in two places for no apparent reason.

 

I agree with RS as I questioned above, but also suggest 531.2.4 relates to putting for example a significant number of PC's on one circuit which due to their natural leakage trips the device. The circuits should be split to cope with this. It is not referring to series RCD's

 

Try to concentrate ...just for a few moments.
We are talking about TWO similar rated RCD's in SERIES.(NOT 1, not MCB's, not the price of kippers)
So, in general, there is no regulation against it.
It may not be usual or best practice.
In certain installations it may be inadvisable.
In some cases it might be down right wrong thing to do.
....
RS

 

I would like a link to your 30mA RCD time delayed device. . ?????

I am referring to the minimum rating for an S type that being 100mA letting the 30mA 'G' type open first

Thought you knew this. .?