RCD descrimination

[studentspark]

Could someone please explain why you are not to have two RDC's in series unless discrimination is applied.

Eg.
Garage had a small CU with a 30mA RCCB.

House CU has had been upgraded to a dual 30mA RCD type.

Power to the garage is now RCD protected from the house CU, and still has origional RCCB

So the garage now has two RCDs in series, why is this a problem.

I understand the upstream device should operate first, with out taking out the primary device. But if I were to remove the garage RCCB, it would still take out the primary RCD.

Earthing for the house is TT.

Is there any danger from having two RCDs or is it a matter of issues caused by tripping and losing power, which may be inconvinient

Thanks

 

[holmslaw]

Its not dangerous, its inconvenient, circuit protection should be designed to cause the least inconvenience when tripped. Thats why split cu's with two main RCD's do not comply with the regs. But are deemed acceptable by the scheme providers and are in common use.

 

[davelx]

I understand the upstream device should operate first, with out taking out the primary device. But if I were to remove the garage RCCB, it would still take out the primary RCD.


Thanks

Why do you think this is the case - have you actually thought about how this would work? If the same current imbalance is seen by both RCD's, what makes you think you can state that one would trip in preference to the other (assuming that they both have exactly the same characteristics)?

 

[studentspark]

Davelx, not sure what you mean,


So two senarios

One RCD. would take out all on that RCD if tripped

Two RCDs. I presume two 30mA would trip about the same time, but either one could be more sensitive than the other, so downstream might go first.

So in both senarios the circuits connected would all be interupted.

This is for a TT system, but as far as I can see, the 100mA RCD would not comply as circuits have to be protected by a 30mA RCD, and time delayed RCDs would also bo a no go.


I read about discrimination, and the books talk about to 'prevent danger'
but never say what that danger could be.
I was just wondering what the dangers could be i.e. if two RCDs could some how cancel each other out or some such senario.

 

[davelx]

Two RCDs. I presume two 30mA would trip about the same time, but either one could be more sensitive than the other, so downstream might go first.

Exactly - BTW, the upstream RCD is the one in the house CU, the downstream is the one in the Garage. (Upstream = nearer the source).

So imagine what might happen, on a dark winter's night, if you are working in the garage, and your wife/mother/sister etc is working in the kitchen, standing on a stool to reach a high cupboard, and something you do in the garage causes a fault, the RCD in the house trips (instead or as well as the one in the garage) and all the lights go out. Oh, and also there is a pan of hot oil on the stove.

 

[ban-all-sheds]

That scenario would mean that there was more wrong with the design than just 2 RCDs in series.

 

[davelx]

That scenario would mean that there was more wrong with the design than just 2 RCDs in series.

Yes, I know. I'm trying to get the OP to realise that there are more dangers than direct electrical dangers to be considered when designing an electrical installation.

 

[studentspark]

I do understand. But I guess that could be the same for anything on the same RCD.

I could be in the living room and plug in a lamp, causes the RCD to trip and Mrs has a pan of hot oil up a ladder in the kitchen.

The only solution is seems is to RCBO everything, because you will have different circuits on same RCD, Considered planning of what goes on which side is important also.

Thanks for replys

 

[ban-all-sheds]

I do understand. But I guess that could be the same for anything on the same RCD.

Yes.

I could be in the living room and plug in a lamp, causes the RCD to trip and Mrs has a pan of hot oil up a ladder in the kitchen.

Yes.

That's why even without the garage CU & RCD in the picture what you have is wrong, and if done after 01/01/2005 arguably illegal.

The only solution is seems is to RCBO everything, because you will have different circuits on same RCD, Considered planning of what goes on which side is important also.

No - there are other things you can do. You can have multiple RCDs to reduce the risks, and emergency lighting to mitigate them, for example.

 

[studentspark]

Hang on.
I started by saying the board had been changed to a dual rcd.
I was concerned about the rcd in the garage in series with the one in the house
So I have the installation split over two rcds, what have I done wrong or illegal?

 

[Spark123]

You could argue that section 314 of BS7671 hasn't been fulfilled but I wouldn't wory about it too much.
In an ideal world on a TT system you would have used a supply from a 100mA TD device to supply the garage CU which has a 30mA device.

 

[ban-all-sheds]

I started by saying the board had been changed to a dual rcd.

Yes, you did - I overlooked that - sorry.

So I have the installation split over two rcds, what have I done wrong or illegal?

I didn't say you had done anything illegal - I thought you'd already got a CU with a single RCD incomer.

 

[flameport]

The only solution is seems is to RCBO everything

Not really, since a total power failure could occur at any time. This means the street lighting won't be working either, so it will be very dark indeed.

 

[ban-all-sheds]

1) Find lights that you like enough to use in key areas of the house - kitchen, basement, hall/landing/stairs that use fluorescent lamps with the ballasts in the luminaires.

2) Replace the supplied ballasts with emergency ones.

 

[RF Lighting]

I've got little 1W LED emergency lights at home. They are really good.