Confirming I've got this right...

[DBGlos]

Why cant I have a 100mA Time delayed RCD incomer with single pole RCBO's (for socket outlets) and MCB's for lighting circuits all on the same busbar?

Huh? Huh?

It seems that if I have split load busbar then SP RCBO's are fine for the unprotected side so why not for an RCD protected busbar?

 

[mapj1]

It would be fine, and electrically equivalent in terms of quality of protection. It is probably worth checking the cost of going down that route -RCBOs are quite a lot more than ordinary MCBs.

 

[DBGlos]

Already have the RCBO's in place for the Ring circuits... dont fancy having them all on one RCD and then having the whole house go down when Im away!

So can I confirm..

100mA time delayed RCD Incomer
MCB's for lights and radial circuits to rmote CU's with their own RCD's
RCBO's for Ring circuits
Single load Busbar and intact link on the neutral bar

Or stuff the lot and have 30mA RCBO's for the each and every circuit except the radials to the remote CU's...

And are single pole RCBO's fine in either of these cases?

Or... I could just fit a 100mA time delayed RCD between the meter and the CU and leave the CU as it is! Sorted? Hopefully cos Ive read just about enough books so far today!

 

[ban-all-sheds]

I havent started this remedial work yet so I still have time to notify them... my queries and understanding that Imight have it wrong only started today!

Doh - I've done a mapj, and not read your OP properly - I thought you'd only just done the original rewiring...

Why do the RCBO's need to be DP? What are the indications for SP RCBO's then? Why are SP RCBO's suitable in options 3 when a 2nd Incomer is used and not on the same busbar as a 100mA TD RCD?

A single pole RCBO will not clear a N-E fault. The fault will trip the RCBO, and not any other circuits, but the fault will remain so almost immediately afterwards the time-delayed RCD upstream will trip and take out all the circuits.

The reason they are OK in option 3 is that the two halves of the board are completely separate. On a normal split the incomer feeds some non-RCD MCBs and the RCD which then feeds the RCD circuits. When you replace the main incomer with an RCD the existing one is downstream of it, so a fault will/could cause either or both to trip, so a time-delayed one is used as the main incomer, giving the downstream RCD (which is always a DP one) time to clear the fault.

A board as described in Option 3 looks like this:

In that example, the 30mA RCD on the left is not downstream of the 100mA one on the right, so the 100mA one does not need to be time delayed, and if you wanted to use RCBOs for the 30mA circuits you would replace the 30mA RCD with a regular switch, and then the RCBOs would not be downstream of another RCD device and would therefore not need to be DP ones.

What would you suggest then seeing as I already have in place the 32A RCBO's so dont wanna have to throw em out useless! It sounds to me that the 3rd option might be best!

Ah - but does the maker of your existing board, MCBs and RCBOs offer one like that? If not you can't go that route as you can't (strictly speaking 'may not be able to') put your devices into another make of CU. (the DIN standard covers the size and shape of the rail, not the size or shape of the devices themselves, nor what the terminal/busbar arrangements are).

 

[ban-all-sheds]

Err - no. You should have notified them before you started work. Thinking about it, I'm not sure why we mentioned Part P, as there is now nothing you can sensibly do to get back within the law...

Rubbish.

(Ha!! bad luck B-A-S got my chance and jumped!!)

Yup - 15:15. I didn't read his post properly and I thought he'd done the rewire this year.

Mind you - I did say "nothing you can sensibly do". If he had done the rewire this year owning up to BC and risking them asking for it all to be exposed for a full end-to-end inspection wouldn't be sensible...

 

[ban-all-sheds]

Why cant I have a 100mA Time delayed RCD incomer with single pole RCBO's (for socket outlets) and MCB's for lighting circuits all on the same busbar?

Huh? Huh?

Because in the event of a N-E fault, the fault will remain after the single pole RCBO has broken the live. So the upstream RCD will still see the fault, and when its time delay expires it will trip.

It seems that if I have split load busbar then SP RCBO's are fine for the unprotected side so why not for an RCD protected busbar?

Because upstream of the unprotected side of a split load CU there is an ordinary incomer, which doesn't give a hoot about N-E faults.

 

[ban-all-sheds]

Already have the RCBO's in place for the Ring circuits... dont fancy having them all on one RCD and then having the whole house go down when Im away!

Why? If it's purely because of the F/F why not out that on its own mini-CU and free it from any possibility of being lost because of faults on other circuits.

So can I confirm..

100mA time delayed RCD Incomer
MCB's for lights and radial circuits to rmote CU's with their own RCD's
RCBO's for Ring circuits
Single load Busbar and intact link on the neutral bar

But your RCBOs are single pole, so if there is a fault you could also have the entire house go down..

Or stuff the lot and have 30mA RCBO's for the each and every circuit except the radials to the remote CU's...

And are single pole RCBO's fine in either of these cases?

You could do that, and SP RCBOs would be fine, as they would all be in parallel.

Or... I could just fit a 100mA time delayed RCD between the meter and the CU and leave the CU as it is! Sorted? Hopefully cos Ive read just about enough books so far today!

No, because then the SP RCBOs in your CU would be downstream of the external RCD, and if an RCBO leaves a N-E fault then at the end of its time delay the external RCD will trip....

 

[HDRW]

Why cant I have a 100mA Time delayed RCD incomer with single pole RCBO's (for socket outlets) and MCB's for lighting circuits all on the same busbar?

Huh? Huh?

Because in the event of a N-E fault, the fault will remain after the single pole RCBO has broken the live. So the upstream RCD will still see the fault, and when its time delay expires it will trip.

Hang on though - if the RCBO has disconnected the Live, where does the current come from to trip the time-delay RCD?

Also, if there are several RCBOs on the same Neutral bus, wouldn't a N-E fault trip all of them?

Cheers,

Howard

 

[ban-all-sheds]

Hang on though - if the RCBO has disconnected the Live, where does the current come from to trip the time-delay RCD?

From any of the other circuits still live, as there is now a path for some current to flow from the neutral bar, through the still-connected neutral of the RCBO to earth.

Also, if there are several RCBOs on the same Neutral bus, wouldn't a N-E fault trip all of them?

No, because they won't see it. Some of the current flowing through them will be leaking to earth instead of returning along the supply neutral to the substation, but each RCBOs live and neutral current will balance.

 

[ban-all-sheds]

OOI:

No electric shower - nor will there be one - ever!

Why not?

 

[DBGlos]

I geddit now, the N-E fault will still be present on a SP downstream RCBO so the upstream TD RCD will 'wait a bit' then trip! Just what I wanted to avoid!

But answer me this... and rest assured your wisdom is being soaked up like a sponge and greatly appreciated...

If an RCD detects a N-E fault then that is a good thing.... yes? I am assuming a N-E fault is also a dangerous situation... so why is it OK for SP RCBO's with no upstream RCD to disconnect the Line and leave the N-E fault intact just because they are in parallel? Is it because there is no path for current to leak to the circuit and the fact that the RCBO has tripped brings your attention to the fault and as it's line has been cleared there is no risk from the circuit? Hang on, that doesnt make sense... does it? Surely there is still a path for current to flow the wrong way down the RCBO Neutral and to the fault?

Have I got it? Haven't I got it? <puts hands together and prays to an unknown God>

And Ill never have an electric shower because I have fitted a mains pressure hot water system and its better than any electric shower I have ever tried! Simple as that...

 

[Damocles]

A N-E fault in the grand scheme of things is unlikely to be dangerous.The voltage on the N line should not be dangerous, though it could perhaps be so should some other fault also occur. There is a posibility that a large current might flow through a thin earth wire shorted to N, which caused serious heating of the thin E. This is most probably also unlikely, because any such wire would most probably be relatively long and hence a relatively high resistance path. Might have a relatively high resistance short which could heat at the join.

The fact that a RCD trips on N-E shorts is secondary effect from the way it works. Its intended protection is to detect any current from L escaping to anywhere else.

A common problem when RCD are fitted for the first time is that they trip because of existing undetected faults......such as a N-E short. Undetected because previously there was no consequence of the fault.

You still have the option of keeping the power circuits on rcbo with just one rcd for all lighting. Have some table lamps around the place? This would still be a better arrangement than most TT. Use the consumer unit in split load configuration, but in reverse- the lights on the RCD and RCBO etc etc on the unprotected side.

Now I am a little concerned about the idea of cables running off mcb unprotected by RCD before they get to the garage or whatever. What happens when you put a nail through it?

 

[HDRW]

Hang on though - if the RCBO has disconnected the Live, where does the current come from to trip the time-delay RCD?

From any of the other circuits still live, as there is now a path for some current to flow from the neutral bar, through the still-connected neutral of the RCBO to earth.

Ah, got it now - I was thinking of a N-E fault upstream of the CU, for some reason.

However, isn't this a tad overcautious, given that an MCB or fuse would have precisely the same situation anyway? Or are we trying to protect against a disconnected N that is shorted to E and the Earth impedance is high enough to allow a dangerous voltage to develop on it?

Cheers,

Howard

 

[HDRW]

Now I am a little concerned about the idea of cables running off mcb unprotected by RCD before they get to the garage or whatever. What happens when you put a nail through it?

Given that it's SWA, doesn't the short to the armour trip the MCB supplying it?

Cheers,

Howard

 

[ban-all-sheds]

However, isn't this a tad overcautious, given that an MCB or fuse would have precisely the same situation anyway? Or are we trying to protect against a disconnected N that is shorted to E and the Earth impedance is high enough to allow a dangerous voltage to develop on it?

No - it's just an issue of inconvenience and discrimination.

Start at the beginning - a basic split-load board has an incomer that will isolate all the circuits, and then part way along a 30mA RCD that will isolate some circuits and will also trip if any of those circuits develop an earth fault. When it trips, it leaves all the circuits upstream of it working.

In order to minimise inconvenience, i.e. losing all of the circuits downstream of the RCD when just one of them develops a fault, RCBOs are used, and then only the faulty circuit gets disconnected.

In a TT installation, everything has to be RCD protected, so the simple solution is to replace the main incomer with an RCD. In theory the RCD should be specced with a trip current such that the touch voltage in a fault doesn't rise above 50V, but in practice people bung in a 100mA device. As a time-delayed one is used, everything remains the same when there's a fault on the 30mA circuits - the RCD disconnects, all of those circuits are lost, but the rest of the installation carries on.

But if downstream there isn't an RCD, but a bunch of RCBOs, and they are single-pole ones, you're actually worse off, because the fault may not be cleared by the RCBO which means that you'll go on to lose not only all the circuits that you would have lost with a traditional split-load board, but the entire installation.

If you want to use RCBOs for the 30mA circuits in a TT environment, you have to do one of these:

1) A non-split CU with RCBOs on every circuit, not just the 30mA ones
2) A completely split CU like the one shown earlier
3) Two completely separate CUs
4) A normal split-load board using DP RCBOs