I think I need a second trip switch

bludger · 23 Jun 2009

Hi,

We are all electric in our place. There are two consumer units, (both with MCB's), one for the seven storage radiators and one for everything else in the house.

The 'everything else' one is connected to a 63amp, 100ma trip, RCB. I've just noticed, when decorating the cubby, that the storage radiator one is not connected to a trip at all. Am I right in thinking that it should be?

If I am, what size and rating trip should I get, please? Both consumer units are earthed to a common block, by the way.

ban-all-sheds · 23 Jun 2009

bludger said:
Hi,

We are all electric in our place. There are two consumer units, (both with MCB's), one for the seven storage radiators

Do you have a two-tariff supply, one for peak, one for off-peak?

The 'everything else' one is connected to a 63amp, 100ma trip, RCB.

Where is it?

I've just noticed, when decorating the cubby, that the storage radiator one is not connected to a trip at all.

Well what is it supplied by?

If I am, what size and rating trip should I get, please? Both consumer units are earthed to a common block, by the way.

You should read this - //www.diynot.com/wiki/electrics:part-p.

Work on CUs, new circuits etc, is notifiable, and non-trivial - you might like to get an electrician rather than CU components.

bludger · 23 Jun 2009

Hello ban-all-sheds,

Yes, we have peak and off-peak supplies.

The mains supply and meter are in a box on an outside wall. The box has two fuses from which two heavy supplying cables enter the house to the consumer units.

A single, heavy cable for the 'everything else' consumer unit first enters the top of the RCB. Two lesser cables leave the bottom of the RCB and serve the unit.

A single heavy cable enters the bottom of the storage rad consumer unit and seven individual cables leave the top of the unit to serve the rads. An RCB is not involved.

The consumer units are within a meter of each other in a sort of cubby. I could learn to post a picture, I think, if this isn't clear.

I will read the 'electric part

' if you tell me that the storage rad unit needs its own separate RCB.

I appreciate your willingness to help.

ColJack · 23 Jun 2009

yeah, do us a picture..

depending on the installation, you didn't need an RCD when it went in, and because it's already installed, you don't really need and RCD on it now..

define "industrial" cable?

AndyPRK · 23 Jun 2009

I think I have read on here that is it generally perfered not to put storage heaters on RCD's as you get false trips.

So I would leave it alone.

sparkiedave · 23 Jun 2009

Sounds like you may also need to look at getting the 100mA "RCB" swapped for a 30mA RCD, what type of system do you have? Is the main earth terminal connected to a spike in the ground? This would be a TT system which would explain the 100mA, but that would normally protect the whole of the installation in that instance

AndyPRK · 23 Jun 2009

Off topic, so sorry.
But why is 100mA RCD used in a TT system ?

Lectrician · 23 Jun 2009

WHY would you want to change it for a 30mA for gods sake?

If it is a TT system, there would be two RCD's. You can't use one if you have a dual tarriff meter.

I agree that with the 17th edition of the regs, 30mA protection is required for sockets and buried cables, but changing the 100mA at the origin to a 30mA does not comply.

securespark · 23 Jun 2009

100mA S type ot time delay RCD's used to be fitted to TT supplies under the 16th Ed.

Because of the high impedance of the earth electrode (up to 200 ohms), the 100mA RCD would be fitted to limit the flow of fault current.

It would be TD so as to allow discrimination between itself & the 30mA unit protecting socket outlets etc...

Lectrician · 23 Jun 2009

securespark said:
the 100mA RCD would be fitted to limit the flow of fault current.

Wording a little backwards?!

The high impedance will limit the fault current, which is why you need the RCD to be able to clear earth faults.

bludger · 23 Jun 2009

Oh dear! I've downloaded a couple of pictures of the consumer units to Picasa, my photo program. I've also created an Album in Diynot.com. The trouble is that I have to select a 'browse' button to start putting the snaps into the album but I can't find the button. Anyone know where it is?

The main earth terminal seems to be a brass connection square in the mains fixture outside. There are five earth wires going into that brass terminal - one from a small box labelled P.M.E. system, one from the meter, one each from the on and off peak switches, and one from the consumer units themselves. There certainly isn't an earth spike anywhere.

I vaguely remember when all this was done that the guy said something about PMEs were better than earth spikes. But I'm probably wrong as it was years ago and a complete nightmare. It started because I wanted our overhead supply cable buried underground. I had to have the meter moved outside and the new cable buried 18 inches deep. It took five different contractors to get the job done and one had to come back a couple of times because our storage radiators weren't sufficiently well fused.

Poster AndyPRK has reminded me that when we first came here 20 years ago, we did get a lot of false trips at night. I think a private electrician I eventually brought in took something away to fix this - would this be the original RCB I wonder.....

Anyway, I'm very grateful for your replies and hope you can help.

sparkiedave · 23 Jun 2009

he doesn't mention anything about having two RCD's, so I'm assuming there's only one, we don't yet know if it's TT, but if it isn't, and the 100mA is protecting the main board (ie the normal tarrif board) then I would at least change the protection to 30mA, prefferably to a RCBO loaded board or split with 2 RCD's, my point was, that if the OP was concerned about not having RCD protection on the E7 board, he should be more concerned that the protection he has got is 100mA instead of 30mA

securespark · 23 Jun 2009

Lectrician said:
securespark said:

the 100mA RCD would be fitted to limit the flow of fault current.

Click to expand...

Wording a little backwards?!

The high impedance will limit the fault current, which is why you need the RCD to be able to clear earth faults.

Absolutey right, old beanm!

I'm sorry. I hardly drink and tonight I've had a shot of Lagavulin. It's gone straight to my grey cellds...

Lectrician · 23 Jun 2009

sparkiedave said:
he doesn't mention anything about having two RCD's, so I'm assuming there's only one, we don't yet know if it's TT, but if it isn't, and the 100mA is protecting the main board (ie the normal tarrif board) then I would at least change the protection to 30mA, prefferably to a RCBO loaded board or split with 2 RCD's, my point was, that if the OP was concerned about not having RCD protection on the E7 board, he should be more concerned that the protection he has got is 100mA instead of 30mA

You can't just wack the whole lot on a 30mA though. It needs a dual split or some RCBO's as you say.

The supply sounds to be PME, so no 100mA main RCD required.

30mA RCD protection would not have been required under the 16th edition for the storage rads, but with the 17th edition now, it would. (Assuming cables supplying storage rads are buried.....blah blah blah.)

sparkiedave · 23 Jun 2009

From what you say, it's not TT, the E7 board doesn't have to have an RCD provided that none of the wiring for that board is buried in plaster less than 50mm from the surface, and to know that you'd have to know the layout of your property very well, as I said before, I'd be more concerned about the 100mA RCB, you should really have 30mA

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I think I need a second trip switch

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