TT installation

[i=p/u]

Any useful help
Or similar expieriences accepted.

TT installation with two fuse boards as normal fitted with rcbo's.

First fuse board is fitted for the house installation and second fuse board is fitted for the shed with one 40A rcbo.

They had new tarmac put down which the owner said they damaged the cable going to shed.

The shed rcbo trips when they turn on the shower on the house installation board, but works when the shower is off.

Anyone seen similar fault before ??

I will go and test cable myself next time I'm down that way and try work it out

 

[Leesparkykent]

There is a neutral earth fault on the circuit that's tripping. The high current of the shower is passing enough through the fault to trip the RCBO.

 

[i=p/u]

Even tho different tails from isco's to each board .

 

[Leesparkykent]

Yeah the normal use of the circuit generates below 30mA to earth but with the high current of the shower passes/proportionally passes through the fault its enough to trip the RCBO.

 

[Leesparkykent]

Hope @Richard Burns doesn't mind but here is one of his superb drawings which helps to explain it. Slightly different as you have an RCBO board and the high current load is the shower not the cooker but the theory is the same.

View attachment 35260

 

[i=p/u]

Im thinking the shower current wouldn't flow through the neutral of another board,

 

[Leesparkykent]

As Richards drawing explains at the common point ( the isco's in your case) of the neutrals the current splits according to the resistance of the fault vs the resistance of the circuit as resistors in parallel.

 

[Octopus]

2 years ago I tracked a fault on a lighting / smoke circuit (on its own RCBO) - it tripped every time they switched on the oven (on a RCD).

Leakage from the oven to earth "pushed" the RCBO over the 23mA trip......

It does happen

 

[peterdaniels]

How are the Cu's supplied? Tails into the house board and off the live side of main switch to second? or henley blocks? Or a circuit from house board supplying shed board???

 

[Leesparkykent]

How are the Cu's supplied? Tails into the house board and off the live side of main switch to second? or henley blocks? Or a circuit from house board supplying shed board???

Henley blocks as the OP says isco's which are the same.

 

[i=p/u]

Isco= Henley block?

 

[Toneyz]

N TO E on garage supply ,what is this fed from

 

[i=p/u]

If I change the rcbo to mcb of the armoured going to the shed and protect circuits at shed with rcd mains switch.

But then the armoured isn't protected for fault current as probably high ze reading

Which = replace cable with another route or rig up tarmac and repair if he knows where was damaged

 

[jackhammerJIM]

double pole rcbos ?

 

[Toneyz]

do istallation restistance on supply cable to shed find out what has been damaged what cable is to the shed ?

 

[i=p/u]

N-E fault on the armoured going to shed, which is supplied from its own rcbo and separate enclosure which is fed from iscos

Il go have a good nosey if he willing to pay for it

 

[Leesparkykent]

If I change the rcbo to mcb of the armoured going to the shed and protect circuits at shed with rcd mains switch.

But then the armoured isn't protected for fault current as probably high ze reading

Which = replace cable with another route or rig up tarmac and repair if he knows where was damaged

Murray loop/wheatstone bridge.

 

[Octopus]

Test the shed cable properly first

 

[Pete999]

How was the SWA damaged? just interested that's all.