RCD discrimination

[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.

 

[Engineer54]

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....

 

[Engineer54]

I think you have it back to front, ...putting the 100mA at the load end and the 30mA at the supply/feed end ...lol!!!

 

[RecycleElectron]

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.

 

[Deleted member 26818]

Overload protection is provided by MCBs, RCBOs or fuses.
RCDs provide earth fault protection.

 

[Hawk81]

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.

 

[IQ Electrical]

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?

 

[Deleted member 26818]

Perhaps he made them for his back?

 

[IQ Electrical]

Perhaps he made them for his back?

Oh dear, it's getting late!

 

[RecycleElectron]

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.

 

[SparkusMaximus]

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.

 

[pushrod]

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.

 

[fredthetester]

i know its more expensive but do away with the rcd's and go rcbo's any circuit goes ---- up you still got the rest!!

 

[pushrod]

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.

 

[SparkusMaximus]

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.