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.

 

[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

 

[Dave OCD]

Yeah it is a bit rough lol. :teeth_smile:

 

[uksparks]

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

i bet it was!

 

[Dave OCD]

That would be about as bad as one I saw where a bloke changed a one way switch for a 2 way [decorative plate] and connected the cpc to the 'spare', terminal - went bang every time he switched the light off lol.

 

[micknew]

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.

View attachment 29234

In the photo the live outgoing bus-bar is not connected lol.

 

[Richard Burns]

Why should it be single pole?

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

I was posting in response to Telectrix post about the low N-E continuity result indicating a fault in the FCU wiring.
It could indicate a fault in the FCU wiring or the FCU could be single pole in which case N-E continuity would be low even when the FCU is switched off.
(and a continuity check could cause an RCD to trip depending on the type of meter and the loading of the installation)

 

[beginnerspark]

Hi everyone

Thanks to all for your replies.

To clear up the busbar confusion - It does have four terminals, one was connected to the L out terminal on the RCD, one connected to each MCB and one spare. The remaining terminal on the RCD was connected to the Neutral you see on the right hand side - my picture taking skills aren't great!

I removed it myself and can verify that it was connected in this way.

I haven't had time to check the FCU wiring, but what should I be looking for in light of the above?

 

[NJR]

Sometimes having 2 RCDs in line can cause one to trip and as you say its only when you turn the shed RCD main switch on. I would test with a normal main switch and see what happens.

 

[beginnerspark]

Sometimes having 2 RCDs in line can cause one to trip and as you say its only when you turn the shed RCD main switch on. I would test with a normal main switch and see what happens.

Hi NJR

Thanks for the response.

This has already been tried, with both a main switch and RCD, the house RCD trips as soon as the garage consumer unit is turned on.

 

[NJR]

Have you tested it with every circuit disconnected. I saw in a post you get 230 out to the shed so I guess something has a poor connection in sockets or lighting

 

[Dave OCD]

Hi everyone

Thanks to all for your replies.

To clear up the busbar confusion - It does have four terminals, one was connected to the L out terminal on the RCD, one connected to each MCB and one spare. The remaining terminal on the RCD was connected to the Neutral you see on the right hand side - my picture taking skills aren't great!

I removed it myself and can verify that it was connected in this way.

I haven't had time to check the FCU wiring, but what should I be looking for in light of the above?

Check that all the connections are sound and in the correct terminals in the FCU and that no cables are damaged [maybe a neutral caught by a plate screw or something simple like that], also make sure you isolate the FCU properly before checking it out.