RCD descrimination

[crunchynut]

This has been an interesting post to read. I would like to add that generally it is an individual MCB that is most likely to 'pop' and then require resetting. RCD's, RCBO's, RCCB's are less susceptible to tripping in a typical domestic set-up than good ole MCB's, unless you have a dodgy ole fridge and 30 PC's in there!

I understand the OP's point about discrimination; ideally a more sensitive RCD would be utilised in the garage, this would hopefully minimise inconvenient tripping in the house. You are possibly more likely to get nuisance earth faults in a damper outbuilding (??!). I would 'Henley' block the tails and have a front end RCD feeding the garage via SWA (if necessary). Effectively turning the dual board to a three-way split board. Or a 5/5/3 (High integrity board would do it).

 

[niceguy235uk]

Thats why split cu's with two main RCD's do not comply with the regs.

Any reg number for that?

 

[ColinJacobson]

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.

I see the makers sell pre-configured CUs with two RCD's giving RCD protection on all the CU. Are you saying this is illegal?

 

[ban-all-sheds]

Thats why split cu's with two main RCD's do not comply with the regs.

Any reg number for that?

Only one in which nobody else would see that prohibition.

 

[JohnD]

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

can you show us?

 

[holmslaw]

Thats why split cu's with two main RCD's do not comply with the regs.

Any reg number for that?

Only one in which nobody else would see that prohibition.

See 314 'Division of installation'

 

[holmslaw]

bump

Who's going to contradict me?

 

[RF Lighting]

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

can you show us?

This is one of mine. Thats the charge LED thats lit at the moment.

It seems they're actually 3W. They fit in a 32mm cutout, and you don't even notice they are there. I've got one at the top of the stairs, and it lights the landing and the entire staircase perfectly well.

I've also got one more in the hallway.

The only link I could find was this one but I got mine for a lot less than that.

http://www.lightingcentre.com/emergency-lighting/led-emergency/3-watt-led-compact-downlight.html

 

[JohnD]

£90.00

(Not inc. VAT)

 

[AndyPRK]

much tider than the ones you see in public buildings

 

[RF Lighting]

£90.00

(Not inc. VAT)

I got mine for a lot less than that

 

[ban-all-sheds]

See 314 'Division of installation'

I have done.

It neither says nor implies that a CU with 2 RCDs is not allowed.

bump

Who's going to contradict me?

I am.

 

[ericmark]

I do think that to just fit a duel RCD consumer unit without doing a risk assessment is wrong. It says:-

314.1 (iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit.

Unless you do a risk assessment how can one take account of danger?

However one can't have it both ways and if with a caravan you can have a 30ma @ 40ms RCD in both site and caravan in series then of course you can do the same anywhere else. But the norm is to go up and down by a factor of three so a 500ma RCD feeds large board with in turn feeds 3 or 4 boards with 300ma RCD which in turn feed cabins with 30ma RCD's and the power for outside socket goes through 10ma RCD.

Although this may discriminate where the leakage slowly raises when a worker puts a nail through cable to hang his coat on it takes all the RCD's out including the one set to 1A and 5 seconds delay. And although the theory says the 40ms one will trip before type S in the real world they both trip together.

As to emergency lights yes fitted to my house but I question if we are to "(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation." then the RCD should not trip so there is no difference in having a general power cut to having a local RCD trip. Except that it may have tripped due to something you have done like knocking a nail through cable and getting a shock is bad enough without also being plunged into darkness.

However multi RCD's do not help. Often when there is a fault both my RCD's will trip together. Why I don't know but been fitted for around 18 years and I know often when they trip both go together must cause spikes on line when they trip. So using RBCO's will not help and if there is a danger when lights fail emergency lights are only option.

All RCD's are not equal and there are those with built in RCM's and auto re-setting types. And the different types A, and AC do trip differently when looking at spikes for example. And I have needed to replace the AC type when using an inverter. Even types which should be the same do have varying resistance to nuisance tripping.

But consumer units are type tested so we are only allowed to use the types (and make) recommended by the manufacture. With distribution units we are permitted to mix and match but these are only allowed where there are electricians on payroll. I suppose we could use them at home and we also unless metal in walls don't need to use RCD's but when we came to sell the house to non electrician there would be a problem.

So to original question if a combined RCD/RCM was used in the house then I see nothing wrong in using a RCD of same rating in the garage since the combined RCD/RCM is designed to trip at between 90% and 100% of the 30ma where a standard RCD is designed to trip at between 75% and 100% of the 30ma. However one would need a ramp up tester to confirm the garage RCD will trip first. However how do you test an RCD if two are in series and the second one trips at a higher leakage to first? Do we need to carry an inverter so we can test the RCD even though we know it will never trip in real life. I would say no. Where one RCD feeds another we regard the second as an isolator rather than RCD. Others may not agree with this.

 

[holmslaw]

See 314 'Division of installation'

I have done.

It neither says nor implies that a CU with 2 RCDs is not allowed.

bump

Who's going to contradict me?

I am.

314.1 (i) Every installation shall be divided into circuits, as necessary, to avoid hazards and minimize inconvenience in the event of a fault

314.2 Separate circuits shall be provided for parts of the installation which need to be separately controlled, in such a way that those circuits are not affected by the failure of other circuits..........

I suppose you would also argue that today is Monday the 25 October, talk about stand on your head.

 

[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.

I see the makers sell pre-configured CUs with two RCD's giving RCD protection on all the CU. Are you saying this is illegal?

Definitely.