second rcd

[johnad]

i am putting a new consumer unit in and i have an rcd between the meter and the old unit
i wish to use a split load unit. must i have the old rcd
(there is no main earth connection on the service cable)
if i use both will the new one trip for its own curcuits
any advice please

 

[andy]

yes, you can use a split load board, however, the first RCD (existing) will have to be a time delay so that if there is a fault on the other RCD circuit then that will trip before the main RCD so you dont lose power to everything

 

[johnad]

thanks for the info.
is the rcd in place because there is no main earth, ?
or am i better off without it using only a split load unit.

 

[andy]

what rating is the current RCD? if you have a TT supply (no earth from REC) then youll need everything to be RCD protected. i would recommend wither a split load board or use RCBO's, so that you still have light if there is a fault on the ring main etc.

 

[johnad]

the rcd has no trip rating on it it just says 60A,
its a big black single switch with a small yellow test button on it.
whats the advantage of rcbos.
can you also tell me what size earth wire i should use on main water and gas with this system.

 

[andy]

the rcd has no trip rating on it it just says 60A,
its a big black single switch with a small yellow test button on it.
whats the advantage of rcbos.
can you also tell me what size earth wire i should use on main water and gas with this system.

RCBOs are MCB's with an RCD built in. altho youd have to have 1 RCBO per circuit unless you have external RCD protection. there should be a trip level for the RCD. look a bit harder

for a TT system you can use 6mm earth to bond the water and gas, use 10 for earthing arrangements.

 

[theshogun]

From your description you may have an old voltage trip[
quote]ts a big black single switch with a small yellow test button on it.

Can you post a picture?
Do any earth cables go to the device?
Does it operate if you push the yellow button?
If it is a voltage tip it will need to be replaced with a suitable RCD.

 

[johnad]

im new to this site so im not sure how to post a picture
but yes operates when you push the yellow button
and it has two small earth wires running in the bottom
is this in place because there is no main earth ?

Picture

 

[mapj1]

Posting a picture is described in the 'for reference' stuff at the top of the forum.
It sounds like you have 'TT' installation (see the pics in for reference for what that means, and see if that is true or not)
If so you have an outside earth rod only (and no company earth) and this will have a resistace of 10 ohms to perhaps a few hundred depending on the dampness of recent weather and how corroded it is.
Because a 100 ohms to earth will not blow a fuse or fire an MCB, but just make the rod live, and give you a large electricity bill, it is a requirement ot have something that positively disconnects the supply if a fault to earth occurs.
The old way was to put a trip that measured the current going to the earth rod, but as this does not detect faults where the fault current flows through you into the garden pond or the plumbing, this method has been depracated for many years, in favour of the RCD, which compares live (outbound) and neutral (return) currents, and if not exactly equal and opposite, cuts the juice (as any fault where current goes out the live and doesn't come back down the neutral is a bad thing....).
Such a device has 4 terminals, live and neutral in and out, but no earth connection is needed. Nowadays one of these in a plastic box should be the first thing after the meter on a TT installation.
If it is not just one 30mA trip for the whole lot (which can give nuiscence tripping), then a 100ma slow acting ('time delay') for the whole lot, and then a 30mA type for all socket circuits that might serve outdoors.
So a T.D. in a plastic box of its own, then a split load board is a good possibility.
The old trip should go, and I'd suggest looking at the earth rod, and seeing if a new one might be in order - it should be hammered in somewhere wet, and installed in a proper 'pit'. See the TLC website for pictures. Installing the new RCD in place of the old trip is probably not a DIY job unless very confident, as the main fuses will have to be pulled and the electricity board (DNO) will want to be involved in that.
Nothing to stop you doing as much of the enabling work as possible though.

regards M.

 

[DIYnot]

Picture added ^^

 

[mapj1]

Yup, change that one when work is next being done on the system, the protection is inadequate to deal with many fault conditions (any in fact any fault where the earth current path is not via this trip will not trip it).
Its not worse than useless, but it is certainly fairly useless, and should be replaced with a 100mA time delay RCD type trip as soon as practicable.. If the earth rod is the same vintage, change that too, and the wiring to it needs to be fatter to meet curretn regs..
Once again we see the meter being changed recently, but no effort made to correct any other defects.

M.

 

[HDRW]

Describing an RCD...

Such a device has 4 terminals, live and neutral in and out, but no earth connection is needed.

Out of interest, why do RCBOs have a (tiny) Earth wire? As far as I can see they perform the same function as an Incomer RCD, but designed to protect a single circuit rather than a number of them (and also designed to fit in a single unit space), so why is an Earth needed?

Cheers,

Howard

 

[mapj1]

There is sometimes a functional (not proective) earth wire, whose role is to form one side of transient suppression components usually capacitors or VDRs, these are present to reduce nuiscence firing when a spiky (but not really earth leaky) load is connected. Think of it as analgous to a connection to an interference suppressor
electronic RCDs are more likely to have this than the more traditional electro-magneto-mechanical models.
(but whats in the photo is definitely the undesirable old 'volt operated' or 'earth sensing' type.)
Ask again if more info needed.
M.

 

[ban-all-sheds]

Nowadays one of these in a plastic box should be the first thing after the meter on a TT installation.

Or the main incomer on a split-load board...

If it is not just one 30mA trip for the whole lot (which can give nuiscence tripping), then a 100ma slow acting ('time delay') for the whole lot, and then a 30mA type for all socket circuits that might serve outdoors.
So a T.D. in a plastic box of its own, then a split load board is a good possibility.

But the trouble is that with either approach, if a fault on a non-socket circuit takes out the time delayed one, whether it's separate or the main incomer, then everything goes.

RCBOs would be an answer, but rather an expensive one. I think the split-load that Hager* do is neat - it's two completely separate buses, so it has a single incomer, then 2 RCDs, one 100mA-S, and one 30mA:

*And for all I know, other makers...

Or Plan B - just use 2 separate small CUs, one with a 100mA time-delayed incomer, and one with a normal 30mA. Split the tails in a henley block and Bob's your uncle.

Installing the new RCD in place of the old trip is probably not a DIY job unless very confident, as the main fuses will have to be pulled and the electricity board (DNO) will want to be involved in that.

I very much doubt that they will these days, but anyway - isn't that a built in isolation switch I see on the meter?

 

[johnad]

thanks for all the help
so i need to out the exsisting rcd
then i need a 100ma rcd across the whole lot, are all 100ma time delay
if i then use a 30ma for all sockets and this will trip in the event of a fault
leaving other curcuits working
i was intending to use mk or wylex whats best ?
is it best to do this all in the same enclosure or put up a separate 100ma rcd then a split load unit ?
(i cant find an earth rod anywhere)
the earting looks like its on water, gas, bath, and sink only ?