Best practice for submain circuit protection

[GPack]

I am currently changing an old consumer unit for a new RCD protected split board. One of the circuits is a 6mm submain circuit protected by a 32A MCB that exits the house, crosses the court yard via a catenary wire, and enters the garage 4 way RCD protected board. Is it ok to put this circuit into an unprotected way in the new consumer unit. The cable is visable throughout it's length. Thankyou

 

[Guest123]

Yes...that should be fine.

 

[Jason Smith]

Ah, Lenny, can you help me on this one, I see in the post that it says that the cable to the garageboard will be from an unprotected way, i.e an MCB before the RCDS on the board. What would protect the cable from the board to the Garage board? lets say someone cut the cable with a hedgetrimmer, the cable is out of the equipotentai zone and would be unprotected. I cant see the garage board protecting it, as its not at source, I know the housewil likely trip if there a garage fault as well if fed from a 17th board, its alwaysbeen my practice to fit an RCBO on feeds to garages, independant of the other two on the board.

 

[Guest123]

Well unless it's a TT system where the loop impedances are too high for an MCB to achieve the required disconnection times then it will.

The sub-main circuit itself including the installation method in this instance do not require additional protection by RCD so I would not give it.

There is an RCD protecting sub-circuits in the garage (assumed 30mA) to put another 30mA up front of this gives no discrimination between the 2.

 

[megthemaddog]

I would have a 100mA time delay RCD protecting the sub main cable from where it leaves the house so there is discrimination between the RCD's whilst still having RCD protection

 

[Guest123]

I would have a 100mA time delay RCD protecting the sub main cable from where it leaves the house so there is discrimination between the RCD's whilst still having RCD protection

The op is asking if it's ok by regs to put this circuit on the un-protected ways in the CU....which it is.

Personal preferences shouldn't cloud the issue.

 

[Jason Smith]

Thanks you two, Depending on where it hits in the wave, decides which Trips, garage board or RCBO in the main board, or both, either way the main cable is protected. A 30 ma RCBO is miles cheaper than a delayed RCD. so for me, at the moment, There doent seem to much of an advantage to the customer doing anything different, apart from potentially having to reset two RCDS. Can you run past me what you mean by Descrimination please, need to brush up here. My questions are asking for advice and not at all argumentative.

Thanks you two, can you explain why we need to descriminate when one or both RCDS will trip depending on where the fault hits on the wave. A 100 ma delayed will do the job well but is expensive compared to RCBOS. The only down side to my system is that the customer may have to reset two RDC devices.
Can you tear me apart on this one please as I need to do more learning in this area.

 

[Guest123]

No one's going to tear you apart mate.

Asking questions is a good thing. You have essentially answered your own querey in that there is no telling which RCD would operate first in the event of an earth fault hence you must discriminate between them....i.e not have 2 x 30mA RCD's in series.

A 100mA TD RCD would be the obvious choice, but in this instance technically no RCD protection is required to that sub-main as it is surface run in it's entireity and has the 30mA RCD protection at the garage CU.

An unprotected T+E outside is not brilliant TBH I would offer some additional protection i.e plastic conduit, change to armoured etc.

As the op is merely upgrading the CU he is dealing with an existing install so has no effect on it, other than the correct protection of the circuit and ensuring it is safe for continued service..

 

[Jason Smith]

Thanks lenny.Sorting out that reg as its one i havent used yet. The brb is a big document that doesnt waste words on meaningless text, I find the best way to get more and more from it is to refer to it when something new comes up. Thanks for the heads up on this one.
What I need, as being heavily influenced in my apprenticeship and training by safety, perhaps over the top sometimes, was your opinion on the fact that (if it is a fact) that even if the garage board trips, then there is still nothing protecting the cable to it, as it will remain back live to the source, all that will be stood there is an MCB.A RCBO or a 100ma delayed would then protect the cable itself. Would you say that installation of either device at the source, is better than the minimum alowed by the regs(just an MCB OR FUSE ) as a matter of good practice, in the real world.

 

[Guest123]

Well an MCB to BS EN 60898 with a loop impedance within limits will offer the required protection to the circuit without the need for additional protection by RCD.

Fitting a 30mA RCBO on the unprotected side of the main CU to protect the sub-main is totally different to having an MCB for it on one of the 2 other RCD's as it is still only protecting the shed circuit, and will not have any bearing on any other circuits so I suppose it's not such an issue.

That said if I did install a 30mA RCBO for the sub-main in the main CU on a new install I would then use a garage CU with a standard main switch in it. As this unit is already there then I'd probably leave it as it is.

 

[UNG]

I'm with Lenny on this one in my college days (14th Ed) I was taught that with the correct protection MCB / HRC fuses etc there should be no need for an ELCB as they were then

 

[GPack]

Thanks for the help and explaination. Much appreciated.