Shed Supply

[beginnerspark]

Have just had a neighbour knock on my door and ask me to go round and look at a shed power supply he has attempted to install.

Layout is as follows:

House 32A MCB (63A RCD Protected) protecting downstairs ring - 13A Fused Switched Connection Unit (off downstairs ring) - 40A RCD 2 way Garage consumer unit

He has made all the connections from the fused connection unit off the downstairs ring, and connected those to the garage consumer unit into the RCD incomer

When he puts the RCD incomer switch on, the RCD on the house consumer unit trips.

I have checked Insulation resistance on cable between spur and garage is >299Mohm, between L-N, L-E and N-E.

No trip when the FCU is turned on and garage incomer is off, as soon as garage incomer is turned on, the RCD trips inside the house.

He has gone so far as to replace the RCD on the garage board with a main switch, and the problem is still there - he suspected the RCD's on both ends might be interacting with each other.

Please could someone kindly advise what I can look for as I am new to the job, having just completed my diplomas.

Thank you to all for their help in advance.

See below for image of the garage consumer unit he has fitted.

 

[Sintra]

If it was me I would be looking for the way out Seriously if it was me I wouldn't touch it unless he let me strip it out and start again.

 

[beginnerspark]

I have told him this may be the case - how would be the correct way to run this?

Bear in mind there is no easy route from fuse board to garage with SWA.

Thanks for the quick reply Sintra!

 

[Gazthesparky]

If the house rcd stays on with the spur on and the shed rcd off I would guess there is a fault on the shed circuits.

At a guess there is a light and socket circuit. So check these for faults.

Also the socket mcb needs swapping for a 16/20 amp mcb as its a radial circuit as the 32amp mcb is too big for the single 2.5 t&e

Edit
Saying that the supply is fused at 13amp by the spur unit off the house ring so the 32 amp mcb isn't too much of a problem

 

[sinewove]

I would disconnect all outgoing ccts from the garage unit and reconnect each in turn until the rcd in house trips and thats your faulty circuit.
Regards,Sw

 

[beginnerspark]

Hi All

Thank you for your responses so far!

So, I have removed ALL the circuits connected to the garage consumer unit apart from the incoming supply, and the RCD in the house still trips when the garage incomer is switched on.

From what I can see, the spur is wired correctly, as is the incoming supply into the garage consumer unit.

I can verify 230v incoming into the garage consumer unit between L-N.

When the garage supply is isolated (i.e. the 13A fused spur in the house is off), I have checked across L-N, L-E and N-E for continuity in the garage consumer unit - I get nothing across L-N, and L-E, but there is continuity across N-E.

 

[Marvo]

So, I have removed ALL the circuits connected to the garage consumer unit apart from the incoming supply, and the RCD in the house still trips when the garage incomer is switched on.

Did you remove the neutrals as well?
Just start from scratch testing each circuit as per GN3.

 

[Retrosparks]

Looking at the picture it would appear that the busbar is disconnected :uhoh2:

 

[Dave OCD]

And I can't see the main neutral to the bar connected at the bottom of the RCD - may just be out of view with the angle of the picture though ?

 

[beginnerspark]

And I can't see the main neutral to the bar connected at the bottom of the RCD - may just be out of view with the angle of the picture though ?

Hi Dave,

The main neutral is connected - sorry, bad picture taking skills!

 

[telectrix]

Hi All

Thank you for your responses so far!

So, I have removed ALL the circuits connected to the garage consumer unit apart from the incoming supply, and the RCD in the house still trips when the garage incomer is switched on.

From what I can see, the spur is wired correctly, as is the incoming supply into the garage consumer unit.

I can verify 230v incoming into the garage consumer unit between L-N.

When the garage supply is isolated (i.e. the 13A fused spur in the house is off), I have checked across L-N, L-E and N-E for continuity in the garage consumer unit - I get nothing across L-N, and L-E, but there is continuity across N-E.

from that i'd suspect incorrect wiring at the FCU.

 

[Richard Burns]

from that i'd suspect incorrect wiring at the FCU.

The FCU could be single pole.

 

[Richard Burns]

The bus bar that is removed has four connections, this wasn't originally connected to the four available terminals was it?

 

[micknew]

The FCU could be single pole.

Why should it be single pole?

 

[Dave OCD]

And if it was why would that cause any problems if wired correctly ?

 

[micknew]

And if it was why would that cause any problems if wired correctly ?

It wouldn't, but it would not isolate a N-E fault which the fault obviously is.

 

[Swcfisher]

And if it was why would that cause any problems if wired correctly ?

Think about it, 2 mcbs 1 rcd.

misesed a reply I d read as you where asking why busbar would be a problem

 

[Dave OCD]

It wouldn't, but it would not isolate a N-E fault which the fault obviously is.

Of course but the OP said insulation tests gave very good readings ?

 

[Dave OCD]

Think about it, 2 mcbs 1 rcd.

misesed a reply I d read as you where asking why busbar would be a problem

Ah I did wonder what you were on about then :teeth_smile: Surely the busbar wasn't connected across both RCD outgoing terminals though ?? :uhoh2:

 

[Swcfisher]

Ah I did wonder what you were on about then :teeth_smile: Surely the busbar wasn't connected across both RCD outgoing terminals though ?? :uhoh2:

Looking a the state of That CU i wouldn't be surprised