RCD before CU

[ColinJacobson]

A friend asked me to look at his lights. The cheap dimmers had conked. He got a home repair man in who made it worse and walked away charging £110 for doing nothing.

The dimmers were already bought and ready to fit. I traced all wiring and fixed the dimmers eventually. The wiring was a disgrace with wires running to strange places with one hall light switch box being a sort of junction box. Well now sorted after much figuring out.

I noticed that the RCD was before the CU and main breaker in the CU. The CU is an old Wylex with plug in mcbs, about 25-30 years old. The mcbs are the sort that converts fuses. It is a flat. Outside the flat there is a distribution box with a cluster of main fuses for around 10 flats. To work on the RCD the main fuse would need pulling.

In ready made CUs with RCD inside they are all after the main breaker. Is is right to put an RCD before the CU and main breaker?

Please only replies relating to the topic. No dross posts thank you, which unfortunately are far too common on this forum. Will the Mod please delete off-topic posts.

 

[ericmark]

To put a 100ma RCD before the main isolator in the CU is normal on a TT system. However to put a 30ma RCD before the CU would likely cause problems with tripping. However saying that my house has two 30ma RCD's feeding two Wylex CU with the fuses replaced with MCB's and there is no problem. Only if tripping is regular and it can cause danger would it not comply and since there are emergency lights on stair well and garage it causes no danger so although done in 1992 it still complies with BS7671:2008.

But do check type of RCD fitted many before the CU are voltage type which need replacing.

 

[ebee]

Please could you elaborate on these two?

"one hall light switch box being a sort of junction box"



"I noticed that the RCD was before the CU and main breaker in the CU."

Thank you

 

[riveralt]

A friend asked me to look at his lights.

*****

The wiring was a disgrace with wires running to strange places with one hall light switch box being a sort of junction box. Well now sorted after much figuring out. .

I assume that is an amateur's view - because all switches are junction boxes. - ****

I noticed that the RCD was before the CU and main breaker in the CU. The CU is an old Wylex with plug in mcbs, about 25-30 years old. The mcbs are the sort that converts fuses. It is a flat. Outside the flat there is a distribution box with a cluster of main fuses for around 10 flats. To work on the RCD the main fuse would need pulling..

You are not allowed to touch the fuses - call the DNO if you wish to work on the consumer unit.

In ready made CUs with RCD inside they are all after the main breaker. Is is right to put an RCD before the CU and main breaker? .

Can be and is a common arrangement - but since you haven't explained what the earthing arrangements are it is difficult to comment further.

 

[holmslaw]

..

 

[ban-all-sheds]

The wiring was a disgrace with wires running to strange places with one hall light switch box being a sort of junction box. Well now sorted after much figuring out.

I hope your sorting wasn't extensive enough for it to have generated a requirement for you to do something about the RCD protection...

To work on the RCD the main fuse would need pulling.

No different to working on any CU.

In ready made CUs with RCD inside they are all after the main breaker.

Maybe they are now, but up until the 17th split load CUs in TT installations had a 100mA time delayed RCD as the main switch.

Is is right to put an RCD before the CU and main breaker?

It was probably done at the same time the fuses were swapped out for plug-in MCBs, and for the same reason - to avoid replacing the entire CU. It was an imperfect solution and arguably did not comply with 314-01-01, but it was better than no RCD at all if it's a TT supply.

 

[ricicle]

main breaker

 

[ColinJacobson]

Please could you elaborate on these two?

"one hall light switch box being a sort of junction box"

"I noticed that the RCD was before the CU and main breaker in the CU."

With pleasure.

A L & N from the mcb was taken to the hall box. terminal strips was used to feed a wire from here to the living room. A right F up. It is electrically sound, just complex. The renovators did it the hard way.

Meter.
Then RCD in a box.
Then the CU.

Ericmark and others had no problem understanding it.

 

[ColinJacobson]

But do check type of RCD fitted many before the CU are voltage type which need replacing.

Ho do you tell? If it is the wrong type then it should be replaced.

 

[ban-all-sheds]

A live from the mcb was taken to thje hall box. terminal strips was used to feed a wire from here to the ,liveing room. A right F up.

Is it?

Or is it looped through the switch(es), an increasingly common practice?

Where did the living room light get its neutral?

 

[ColinJacobson]

The earth is attached to the copper sheathing of the mineral insulated cable from the distribution board to the flat.

 

[ban-all-sheds]

The earth is attached to the copper sheathing of the mineral insulated cable from the distribution board to the flat.

Yes, but where does the main earth originate?

Is the supply TT or not?

 

[JohnD]

Not sure if that is a fair question, because he is not an electrician so can't be expected to understand. Unfortunately he won't say if he is an enthusiastic handyman or a trainee.

 

[ban-all-sheds]

It's not difficult for him to gain an understanding:

//www.diynot.com/wiki/electrics:earthing-arrangements

But I'm wondering if he's even seen the question. He's not responded to anything, and I'm trying to find out from Admin/Mods if I'm on his ignore list, if they're allowed to tell me.

If I am, fair enough - there are people on mine - I'll just stop asking him questions.

If he is reading them but just not bothering to reply that doesn't seem to sit well with his desire for topics to be conducted in a proper manner...

 

[ebee]

"A L & N from the mcb was taken to the hall box. terminal strips was used to feed a wire from here to the living room. A right F up. It is electrically sound, just complex. The renovators did it the hard way. "

It might be OK ,it is fairly common practice, not the way I like to do it but hard to fault purely from your description, that's why I asked you to elaborate.

When you say "Main Breaker" are you referring to an RCD mainswitch?

"Ericmark and others had no problem understanding it." What relevance has that?