What does provide discrimination?

[Carlos Arruda]

Hi all.

I am finishing my level 3 in electrical installations and as such, I have tried to explain this to a friend.

So much so, that now he's got me doubting myself.

He's powering his garage which is attached to the main house.

He's also providing the circuit supplying the garage with RCD protection.

At the garage consumer unit, he's got a standard 40A RCD 30mA protecting a 32A type B and 6A type B MCBs.

My advise for him was in terms of the given protection to the both circuits in the garage and the circuit from the main consumer unit to the garage. He's not providing discrimination as both RCDs are rated at 30mA.

Now, he's adamant that discrimination has been given as one RCD is 63A and the other is 40A.

Am I wrong when I say that discrimination hasn't been given.

Kind regards.

Albert

 

[telectrix]

he's talking out of his arse. the 63A and 40A ratings are just the max. operating load of the RCDs. it's the 30mA rasting that does not give discrimination. you're right.

 

[stevethesparks]

You are right, he is wrong - he is also a plonker

 

[Pete999]

Two RCDs on The same supply, not a good idea, no discrimination unless the one in the house is a time delay, or a different mA rating ie CU house 100mA and Garage 30mA

 

[telectrix]

get him to perform a RCD test in the garage and see which RCD trips.

 

[Leesparkykent]

No discrimination as both are 30ma. What cable is supplying the garage? And what's the installation method? The RCD doesn't provide over current protection that's what the MCB is for.

 

[Richard Burns]

Two non time delayed RCDs in series will not have discrimination.
Certainly not if they are the same rating.
Even different rated (30/100/300/500) RCDs will not discriminate in most cases, remember that the RCD is coping with a potential fault current of at least 100's of amps so a difference of 0.47A is not going to make a difference.
Time delayed is the only way to ensure discrimination.

 

[Octopus]

If sheds stopped just stocking "garage" CU's which all have RCD's in, this confusion would end.

People think the garage needs a RCD irrespective of whether or not the house has one too!

 

[Carlos Arruda]

Two RCDs on The same supply, not a good idea, no discrimination unless the one in the house is a time delay, or a different mA rating ie CU house 100mA and Garage 30mA

Hi Pete999.

There has been several discussions on forums about the use of two RCDs on the same supply.

I even took this subject to college and, unless I was sleeping throughout the lesson, it seems that it's the regulations?

But still, to my understanding, the RCD on the main consumer unit provides additional protection to the cable supplying the garage and the RCD in the garage is providing additional protection to the garage circuits i.e light and sockets.

On another note to all, is any RCD rated at above 30mA considered to be additional protection? I was under the impression that additional protection is only given at 30mA! I might be wrong.

I have also been looking online today for 30mA time delayed RCD and couldn't find any at this rating.

Many thanks for clearing my main question.

He won't be happy.

Cheers,
Albert

 

[Carlos Arruda]

get him to perform a RCD test in the garage and see which RCD trips.

I did tell him that if he does, he will trip both of them.