RCBO required?

[sparkz]

Hi........iv just completed a re-wire which is going to be signed off by another electrician because im only a 3rd year apprentice. I installed a 11 way split load consumer unit with the RCD and a main switch. The other electrician has told me the consumer unit needs a RCBO. Could someone explain why its needed.

Cheeers

 

[simonatlondon]

Did he say it was for a particular circuit? sockets downstairs/upstairs? lights...

 

[sparkz]

Yeh he said it was for upstairs lights and the fire alarm system.

 

[simonatlondon]

Are the upstairs lights a fire alarm system on the same circuit from the consumer unit?

 

[sparkz]

Yes on the same side on the board

 

[Shakey]

Yes on the same side on the board

well have you done the work to the 16th or 17th?

Reg 314.1 calls for seperation of circuits and avoidance of danger caused by failure of a single circuit such as lighting. (among other things)

the classic scenario is a problem on a downstairs socket, the main RCD trips, and the little old lady comes tumbling downstairs because all of the lights have gone off.

This is why we use twin RCCB's (with say upstairs lights and downstairs sockets on one, and vice versa on the other,or use one RCCB and then RCBO's.

Dont know about fire alarms, but certainly with smoke detectors, they should preferably not be on an RCD, but if they are, they should not be on the same RCD as socket outlets, so again an RCBO would be needed

hope that answers your question

 

[sparkz]

Sorry shakey, im confused now.

 

[simonatlondon]

sparkz,

if there was a fault that tripped the RCD, then obviously all the circuits protected by that RCD would cease to be live. If you add an RCBO on a different phase bar within the same consumer unit, that circuit fed from that RCBO would not be affected and thus stay live whilst the rest of the installation would have been cut off by the RCD. The obvious benefit in this case would then be that upstairs lights would still be operational as well as the fire alarm system when there is a fault on the RCD side.

It could just be a company policy that upstairs lights and fire alarm systems are protected seperatley?

 

[stevie h]

Did you fit a duel RCD board( new 17th) or a older 16th half RCD half none RCD , or does it have 1 RCD covering all circuits ?

 

[tony.towa]

If you have your upstairs lighting circuit on an mcb and change it for an RCBO switch one of the upstairs lights on and then switch your landing light on and then the downstairs hall light on. If the upstairs lights trip the RCBO when you do either of these you are sharing either a neutral or live feed between upstairs and downstairs. The most common is a sharing of the neutral on the landing light from the upstairs circuit with the live being on the downstairs circuit.

Hopefully this will not be the case but I've posted this just to save a lot of head scratching if you do have the circuit trip out.

 

[Shakey]

sparkz,

if there was a fault that tripped the RCD, then obviously all the circuits protected by that RCD would cease to be live. If you add an RCBO on a different phase bar within the same consumer unit, that circuit fed from that RCBO would not be affected and thus stay live whilst the rest of the installation would have been cut off by the RCD. The obvious benefit in this case would then be that upstairs lights would still be operational as well as the fire alarm system when there is a fault on the RCD side.

It could just be a company policy that upstairs lights and fire alarm systems are protected seperatley?

Thanks Simon, saved me the trouble of writing it

as a side issue, went to a gaff the other day, he had a 16th edition split board. (it was a 4 bed detatched with one ring for the whole house!)

anyway the RCCB was spuriously tripping regularly. An old sparks with 30 years experience had 'upgraded it the 17th edition' for him.......by putting the ring on an RCBO, which was fed off the RCCB protected bar...... (both 30mA before you ask)

took two minutes to fix.......but heres a question for the more...ahem....'seasoned' thinkers out there......why did this cause the main RCCB to blow occasionally, never the RCBO mind, always the RCCB........

by the way, the RCBO protected ring was the ONLY circuit fed off the RCCB.....c'mon chaps, lets 'ave your theories then

 

[sparkyork]

mmm, to be honest i dont really know but will do some typing out loud!!

socket circuit fed from rcbo fed from rccb...

so we got an rccb which is just an rcd and and rcbo but the fault prefers to trip the rccb? is it cos the rccb is monitoring the rcbo as well and most times does a better job of tripping??

 

[Shakey]

mmm, to be honest i dont really know but will do some typing out loud!!

socket circuit fed from rcbo fed from rccb...

so we got an rccb which is just an rcd and and rcbo but the fault prefers to trip the rccb? is it cos the rccb is monitoring the rcbo as well and most times does a better job of tripping??

aah but Rich, there wasnt actually a fault there (since i have moved the RCBO neither has tripped)......interesting aint it?

 

[sparkyork]

chuckle!

erm, the rccb is monitoring the line and neutral beinused by the rcbo the rcbo also has a torroidal coil inside it monitoring the socket ....could it be the energy used to energiize the rcbo's coil is enough to trip the rccb??

good en this shakes!!

 

[tony.towa]

First thought
Residual current devices, rated at 30mA, in order to comply with regulations must trip at between more than 50% of rated current and rated current (i.e. 16mA to 30mA).
With two residual current devices in series RCCB then RCBO the actual tripping current of the RCCB was less than that of the RCBO effectively giving negative earth fault discrimination.
With one final ring circuit for the whole house the total earth leakage then occasionally exceeded the tripping currect of the RCCB but not the RCBO. (washing machine, kettle, iron etc all on at once)

Am I close?

 

[sparkyork]

think your a lot closer than me tony!!

 

[tony.towa]

Must learn to type faster. Two more threads have been added since I started typing my answer!!!

 

[sparkyork]

i used to work in a solicitors so can type about 143,000 words a minute!! all -------s tho ussualy, i like your above answer it actually makes sense. i was on the series thinking but not in the way i should of been like your post!!

 

[tony.towa]

Perhaps a daft question Shakey, but where did he take the neutral feed from for the RCBO?

 

[sparkyork]

think he's gonna let us stew over it, like an eastenders style cliffhanger!...nah nar nah nar nah nah nar!

 

[simonatlondon]

Ok shakin stevens, I reckon the neutrals of that ring circuit were connected into the rcbo, the rcbo neutral connected to the neutral bar like normal. but to keep that circuit in balance, volts, you took a fly lead from the rcbo to the rccb so that the phase and neutral would be of equal voltage within the rccb?

Shakin, I really do mean the '?'

 

[Shakey]

First thought
Residual current devices, rated at 30mA, in order to comply with regulations must trip at between more than 50% of rated current and rated current (i.e. 16mA to 30mA).
With two residual current devices in series RCCB then RCBO the actual tripping current of the RCCB was less than that of the RCBO effectively giving negative earth fault discrimination.
With one final ring circuit for the whole house the total earth leakage then occasionally exceeded the tripping currect of the RCCB but not the RCBO. (washing machine, kettle, iron etc all on at once)

Am I close?

good try Tony, and my first thoughts were to do with culamative leakage currents, but.........

if that was the case, and the RCCB was tripping at say, 20mA culamtive, then the presence of the RCBO would have no effect either way on that.....and it hasnt happened since i moved the RCBO.......

Ok shakin stevens, I reckon the neutrals of that ring circuit were connected into the rcbo, the rcbo neutral connected to the neutral bar like normal. but to keep that circuit in balance, volts, you took a fly lead from the rcbo to the rccb so that the phase and neutral would be of equal voltage within the rccb?

Shakin, I really do mean the '?'

Ha Ha Simon, it long has been estabilshed on the forum that my username has absolutely nowt to do with Shakin' Stevens

erm.....no

It was wired 'conventionally' - line conductor from switched side of main isolator to RCCB input, neutral from neutral bar to RCCB input. RCCB neutral output feeding RCCB neutral bar.

Line output busbar from RCCB feeding RCBO

Ring line conductors and ring neutral conductors connected to RCBO, RCBO neutral lead connected to RCCB protected neutral bar.

The ring, through the RCBO, was the only circuit connected off the RCCB

So theoretically then, ANY current flowing in the ring would have to flow in and out of the RCCB - so there should be no imbalance, so why was it tripping

 

[sparkz]

sparkz,

if there was a fault that tripped the RCD, then obviously all the circuits protected by that RCD would cease to be live. If you add an RCBO on a different phase bar within the same consumer unit, that circuit fed from that RCBO would not be affected and thus stay live whilst the rest of the installation would have been cut off by the RCD. The obvious benefit in this case would then be that upstairs lights would still be operational as well as the fire alarm system when there is a fault on the RCD side.

It could just be a company policy that upstairs lights and fire alarm systems are protected seperatley?

Thank you everyone....appreciate all your help especially "simonatlondon" o you a drink falla . Which reg's does this come under? so i can have a read.

 

[Shakey]

Thank you everyone....appreciate all your help especially "simonatlondon" o you a drink falla . Which reg's does this come under? so i can have a read.

314.1 'Division of Installation'

 

[Spudmiester]

Gonna take a wild stab in the dark here. We have an RCCB feeding an RCBO. The RCCB as a device has no connection to earth, where as an RCBO does through that little white wire.(am I getting warmer?) I actually dont know why this is, I always thought that an RCBO was an MCB and RCD all in one. Neither an RCD or MCB has an earth wire, so why does an RCBO ? There must be a reason. Whetever that reason, this is what the problem is. If I am wrong, I am open to being shot down in flames, but its all I can think of at the moment !

Just had another thought.........................................................................Er, no thats not it either !

It may have only taken two minutes to fix, but how long did it take you to suss out Shake ?

 

[Shakey]

Gonna take a wild stab in the dark here. We have an RCCB feeding an RCBO. The RCCB as a device has no connection to earth, where as an RCBO does through that little white wire.(am I getting warmer?) I actually dont know why this is, I always thought that an RCBO was an MCB and RCD all in one. Neither an RCD or MCB has an earth wire, so why does an RCBO ? There must be a reason. Whetever that reason, this is what the problem is. If I am wrong, I am open to being shot down in flames, but its all I can think of at the moment !

Just had another thought.........................................................................Er, no thats not it either !

It may have only taken two minutes to fix, but how long did it take you to suss out Shake ?

Hi Spud

how long to suss out, well about 4 or 5 seconds to be honest.

CU was in garage, opened the garage door, looked up at the CU, saw RCBO sitting next to the RCCB which was on the left of the CU with main isol on the right.

New INSTANTLY that the RCBO had to be coming off the RCCB, and thought very quickly, "well i bet that aint helping......."

Oh and your theory, Spud

*dut dut dut dah dah* (that was a fanfare of trumpets)

*fireworks go off*

*everybody hugs*

you got it matey!

the little flylead on the RCBO is a functional earth, and the RCBO NEEDS it to be there to function/operate.

Therefore, some of the line current coming through the RCCB must be going through the line input of the RCBO and then being directed to earth through the flylead.

So the RCCB sees an imbalance. Why doesnt the RCBO trip?, well because the 'leakage' is from the RCBO input to earth, the RCBO has the same current flowing through the output as it has flowing back through the neutral, so it is happy

Well done matey, its certainly one to watch out for aint it?

ok mine was obvious, but with the 17th, you could well be getting RCBO's in sub-CU's coming off RCCB protected circuits

*Shakey claps the Spudmaster* (deliberate spelling mistake)

 

[Spudmiester]

Spuds head is now as big as Jupiter !


Seriously though, it is one to remember. Awkward bu**er !

 

[tony.towa]

Have just got in from work and was just about to post that answer - Honest.

Seriously though, another anomolie to remember. What is it they say, don't fix it if it ain't broken.

Cheers Shakey you got me thinking late into the night with that one.

 

[sparkyork]

yeah it got me thinking that i know bugger all!!

 

[London Luke]

it really makes sense to get this right !!

 

[Shakey]

Hi guys,

I enjoy this sort of thing - its easy to get around the 'what size bonding' type stuff, but its good to get the old cogs turning now and again

I remeber driving back from the job thinking "well i am sure i have sorted it, but i am buggered if i know how or why"

and of course chaps, we will all remember it............

 

[simonatlondon]

Shakey keep it coming, like you say, it keeps your cogs turning and puts mine into motion. Seriously though, its great to be pushed like that. I absaloutley love it....... apart from the last bit where I end up in a mental home depressed to hell, realising I know smoke all.

Thanks.