RCD vs GFCI

[yankey-sparky]

I assumed, when I first started learning about British electrical work, that a UK Residual Current Device and an American Ground Fault Current Interrupter were virtually identical. However, in the recent "kitchen ring" post in the UK electric forum, I found the following two statements:

This 80A RCD (question 1 answered!) means it can 'monitor' a max of 80A of current at any one time. This is probably quite adequate but remember that an RCD does not protect against over current so a 32A MCB should be installed in conjunction with this. This RCD can be used along with an additional MCB to protect a new Ring.

knowledge on the difference between RCDs and MCBs, and where in the hierarchy of protection they sit, and what kind of protection they provide....

This casts doubt on my assumption. A GFCI provides both overcurrent protection AND ground fault protection in one package. Am I to understand that an RCD provides ground fault protection only, and that a circuit breaker must be installed in series with it?

 

[breezer]

yes you are correct in what you are saying, you have also enlightend me, i always tought it the same of your gfi, not that i doubt you, (that much) but are you SURE that a gfi provides overload protection? i was lead to believe gfi stands for ground fault interruptor, just that, not ground fault interruptor and overlaod protection

 

[yankey-sparky]

I should clarify:

The GFI Protected Outlet provides only ground fault protection. The circuit breaker that is powering the outlet provides the necessary overcurrent protection.

The GFI Circuit Breaker, which you would put in the circuit breaker panel just like any other breaker, provides both ground fault protection and overcurrent protection for everything hooked up to it.

 

[breezer]

you learn something new every day. thanks for that

 

[BR]

So, a gfi is equivalent to our rcd
gfci = MCB/RCD combi
Am I correct?

 

[yankey-sparky]

Sorry for the confusion, BR. The terms GFI and GFCI are interchangeable in the US. We sometimes drop "current" from the name as redundant, obvious, and unnecessary.

I don't think there is an exact equivalent to the RCD in the US. The difference I was pointing out above was simply the difference between a socket with ground fault protection and a breaker with ground fault protection.

What we don't have is a stand-alone unit that does nothing except provide ground fault protection. You either get it packaged with an outlet (not providing overcurrent protection) or with a circuit breaker (which does provide overcurrent protection). Click the links I posted above to see pictures of the two units to further clarify.

 

[ban-all-sheds]

...The GFI Circuit Breaker, which you would put in the circuit breaker panel just like any other breaker, provides both ground fault protection and overcurrent protection for everything hooked up to it.

We call that an RCBO.

...What we don't have is a stand-alone unit that does nothing except provide ground fault protection. You either get it packaged with an outlet (not providing overcurrent protection) or with a circuit breaker (which does provide overcurrent protection)...

That's what most of ours are, they either protect the entire installation, (or part of it with a split load board), but they provide no overcurrent protection and circuit breakers sit immediately downstream.

RCBOs and individually RCD protected outlets are an expensive solution over here.

 

[yankey-sparky]

That's what most of ours are, they either protect the entire installation, (or part of it with a split load board), but they provide no overcurrent protection and circuit breakers sit immediately downstream.

Is the CB required to be downstream from the RCD, or is it just generally put there?

*Edit* Actually, now that I think about it, because the RCD is protecting multiple breakers it would have to be upstream from them...

I am not aware of a unit in the US that's intended to provide ground fault protection for multiple circuits like your 80A RCDs. Usually even a GFI breaker is only protecting a handful of outlets.

RCBOs and individually RCD protected outlets are an expensive solution over here.

What does RCBO stand for?

 

[ban-all-sheds]

"Residual Current Breaker with Over-current protection ".

As for RCD incomers, they usually protect all the circuits in an installation with the exception of a few - lights, mains powered smoke alarms and possibly fridge freezer.

In a split load CU you'd typically see a 100A main incomer and an 80A RCD, which shows you the expected ratio of RCD:non, and as you know from reading the UK forum, when you allow for diversity there could be 10s of circuits on the RCD side, and for ring or radial socket circuits there is no regulatory limit to the number of sockets that they can have.

 

[Big_Spark]

I hate to pour water on this discussion and possibly confuse yankey, but an RCD DOES give overload protection at the same time and providing Earth fault protection. see this