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?