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.

 

[Octopus]

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

wrong!

 

[Carlos Arruda]

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!

I understand what you are saying.

However, people wouldn't be too happy to, in case of tripping, having to leave the garage to switch the RCD back on.

In my understanding (still studying) I'd rather have RCD at the garage.

Thanks.

Albert

 

[Octopus]

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

No and No

So you would remove the 30mA protection from the house? I don't think so.

 

[Carlos Arruda]

wrong!

Apologies but if no discrimination was given!?

In this case and, in your experience, which one would trip?

Many thanks.

Albert

 

[Leesparkykent]

Why do you want a 30ma time delay RCD? The 30ma rcd in the house will provide the protection to the sub cable and the circuits on the garage board.

 

[Octopus]

Apologies but if no discrimination was given!?

In this case and, in your experience, which one would trip?

Many thanks.

Albert

99.9% of the time the one in the house. You need to consider what exactly the RCD is doing.

i.e. the house could have a cumulative leakage of 18mA and the garage almost none - introduce a fault at the garage and as most RCD's trip at around 24mA, the one in the house will trip first

 

[Carlos Arruda]

No and No

So you would remove the 30mA protection from the house? I don't think so.

Murdoch;

That is the issue when so many say two RCDs in the same circuit is not a good idea.

I don't see this as two RCDs in the same circuit.

I wouldn't remove the one in the house main consumer unit as this RCD is the one giving additional protection to the circuit supplying the garage.

And it's my understanding that the other RCD in the garage is giving additional protection to the garage circuits.

I think I see where the issue is with these so called garage C.U. The lack of a dual pool isolating switch. This would then separate both RCDs.

But please, do correct me if I am wrong.

One is always learning and I would just like to give my friend the best correct advice.

Thank you.

Albert

 

[Carlos Arruda]

99.9% of the time the one in the house. You need to consider what exactly the RCD is doing.

i.e. the house could have a cumulative leakage of 18mA and the garage almost none - introduce a fault at the garage and as most RCD's trip at around 24mA, the one in the house will trip first

Makes sense.

Thank you.

Always learning.

 

[Carlos Arruda]

Why do you want a 30ma time delay RCD? The 30ma rcd in the house will provide the protection to the sub cable and the circuits on the garage board.

Because the garage is also protected with a 30mA RCD thus giving no discrimination?!

Still, please do correct me if I am wrong.

Many thanks.

Albert

 

[Leesparkykent]

Murdoch;

That is the issue when so many say two RCDs in the same circuit is not a good idea.

I don't see this as two RCDs in the same circuit.

I wouldn't remove the one in the house main consumer unit as this RCD is the one giving additional protection to the circuit supplying the garage.

And it's my understanding that the other RCD in the garage is giving additional protection to the garage circuits.

I think I see where the issue is with these so called garage C.U. The lack of a dual pool isolating switch. This would then separate both RCDs.

But please, do correct me if I am wrong.

One is always learning and I would just like to give my friend the best correct advice.

Thank you.

Albert

Where two, or more, RCDs are connected in series, discrimination must be provided, if necessary, to prevent danger (Regulation 531-02-09 refers). During a fault, discrimination will be achieved when the device electrically nearest to the fault operates and does not affect other upstream devices.
Discrimination will be achieved when ‘S’ (Selective) types are used in conjunction with downstream general type RCDs. The ‘S’ type has a built-in time delay and provides discrimination by simply ignoring the fault for a set period of time allowing more sensitive downstream devices to operate and remove the fault.
For example, when two RCDs are connected in series, to provide discrimination, the first RCD should be an ‘S’ type. RCDs with built in time delays should not be used to provide personal protection.