Discuss RCD discrimination in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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SparkusMaximus

Hello everybody.

I would like to understand how discrimination works for rcd's if say for say a garage supply you would put in a 100mA rcd, while the main board would be protected by a regular 30mA.

So this ensures that a fault in the garage will not disturb the point of origin but I don't quite understand how that happens.

why if we just put another 30mA would it trip both or just the origin. I have a vague idea of why but want to be sure, as I like to understand why we do what we do not just do what we do, if you get me.

Thanks very much for your time.
 
They recommend a value of 3x between tripping values, to achieve discrimination, but i've yet to see it work!! ...lol!! The only real way of achieving discrimination between say RCD devices of 30 mA and 100 mA is via a selective RCD ie one with a time delay function to a trip value....

So if you want RCD protection for your shed example, either have the RCD at the supply or at the shed position, but in general terms NOT at both!!! Not unless your going to feed the shed CU with a say a 100 mA selective RCD....
 
I think you have it back to front, ...putting the 100mA at the load end and the 30mA at the supply/feed end ...lol!!!
 
The only way to achieve reliable discrimination is to use an S type time delay device, usually a 100 or 300mA at the in coming supply end, which gives over-current protection and then 30mA devices elsewhere to limit touch voltages. In a TT system it can be easiest to use different earth spikes for different locations in detached buildings. Be warned S type devices are more expensive though.
 
The only way to achieve reliable discrimination is to use an S type time delay device, usually a 100 or 300mA at the in coming supply end, which gives over-current protection and then 30mA devices elsewhere to limit touch voltages. In a TT system it can be easiest to use different earth spikes for different locations in detached buildings. Be warned S type devices are more expensive though.

Think you have got a bit confused yourself. A 100mA or 300mA S type RCD does not offer overcurrent protection.
 
The only way to achieve reliable discrimination is to use an S type time delay device, usually a 100 or 300mA at the in coming supply end, which gives over-current protection and then 30mA devices elsewhere to limit touch voltages. In a TT system it can be easiest to use different earth spikes for different locations in detached buildings. Be warned S type devices are more expensive though.

How are we onto 'different rods in different buildings' on a simple rcd discrimination question?
 
Sorry it was getting late. Perhaps what I should have said was that the higher current time delay RCD then provides earth leakage protection at a higher earth leakage current for the distribution circuit and allows for some cumulative earth leakage in other circuits which are provided with 30mA RCD's for personal protection? The time delay allows the predictable operation of the relevant RCD. They all limit the touch voltage at the various points.
Apologies. I'll try to be more careful and less complicated.
 
Well thanks for trying to explain it to me, however I don't feel I still fully understand but guess this is something I need to talk to somebody about rather than have it explained via a forum.

Thanks.
 
Discrimination can work with overload devices eg fuses, but it does not work with ordinary RCDs. If you have 100mA and a 30mA RCDs in series you can not predict which will operate first in the event of a large earth fault.
If you use an S type rcd with a time delay on it that will operate after an ordinary RCD - the S type would not be suitable for additional protection.
 
Sorry it was getting late. Perhaps what I should have said was that the higher current time delay RCD then provides earth leakage protection at a higher earth leakage current for the distribution circuit and allows for some cumulative earth leakage in other circuits which are provided with 30mA RCD's for personal protection? The time delay allows the predictable operation of the relevant RCD. They all limit the touch voltage at the various points.
Apologies. I'll try to be more careful and less complicated.

Think it would be more correct to say they limit the size of residual/leakage currents or the duration for which they flow.:)
 
Ok thanks pushrod that clears things up a bit for me. But leaves me wondering what is the correct device to put in if your adding a separate C/U.

Can you just put a 30mA rcd in the second board if I you put the cable supplying the sub main on an RCBO?

Sorry but I have seen different things done and heard different things I just would like to understand what is the correct way.
 
Take a seperate garage/shed/workshop installation. and say it is 15 metres from the house. If you run a sub main from the CU with say your RCBO and it's a 32amp 30mA kind. Then at the outbuilding you put a little 2 way board with say a 30mA double pole RCD and then a couple of MCBs for your lights and sockets.

If you have a fault in the outbuilding the chances are it will trip the RCBO in the main CU and your RCD in the outhouse. This is because the RCBO and the RCD both give you 30mA on the earth fault protection, the RCD will though not give you overload protection as the RCBO, which is why when we use RCD protection we fit MCBs

Now say you decide right to give me discrimination I'll fit a 100mA RCBO in the main CU and then a 30mA RCD in the local outbuilding board as that will give me discrimination. No that is not right if you go to appendix 3 pg 243 in the BRB you will see a normal 10mA, 30mA, 100mA, 300mA and the daddy 500mA will all trip in 40mS at 5x I delta N what is the difference is the mA that is needed to trip it in that time so 30mA is 150mA and and 100mA is 500ma. So under fault conditions the amps in a circuit the fault current can rise to many 100's of amps, so within milliseconds it will rise well over the 500mA it takes to trip the 100mA and so will take both RCDs out.

An S type works differently as you can see in appendix 3. A 100mA RCD though still will take 500mA to trip it at I delta N, but the times have altered to 40mS min max 150mS, so a manufacterer will increase the time to perhaps 90mS, so in this case the 30mA will trip before the S type will. That is your discrimination
 
Ok thanks pushrod that clears things up a bit for me. But leaves me wondering what is the correct device to put in if your adding a separate C/U.

Can you just put a 30mA rcd in the second board if I you put the cable supplying the sub main on an RCBO?

Sorry but I have seen different things done and heard different things I just would like to understand what is the correct way.

No - well certainly not the best thing to do as you have effectively 2 RCDs in series.

Best to supply the submain from a non rcd protected way in the main CU using SWA (it does not need rcd protection) then just have an rcd at your submain.

or you could(if the submain is say at the bottom of the garden) separate the earths of the 2 consumer units and use an RCD at the shed but earth it via an earth electrode (TT system). Then the supply cable is being protected by an RCD at the original CU and the submain is being protected by its own RCD. As the 2 systems are now on separate earthing systems only one RCD can trip :)


edit : another option, you could supply the submain from an rcbo at the main CU and have no rcd at the submain. The drawback is that in the event of a fault, or when testing, you have to keep going back to the main CU to reset it.
 
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