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

 

[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.

 

[Richard Burns]

With 30mA RCD at garage or (changed to) main switch at garage OFF the FCU in the house can be switched on and then in either case turning on the 30mA RCD or main switch in the garage trips the house 30mA RCD.

With all circuits disconnected (you will have to clear this one up on how these were disconnected) the same thing happens.

This is only possible if both the RCD and the main switch had faults in them (to earth! as a L-N fault would not necessarily trip an RCD and would trip an MCB.)

Therefore there is something missing in our information.

When you disconnected all circuits were all conductors removed from their connections or were the MCBs just switched off?

If you just switched off the MCBs or removed the live connections from the MCBs (or removed the bus bar) but left the neutral and earth connections in place then the RCD would be tripping in the house because there is a neutral earth fault on the garage circuits.

Leave the bus bar off, remove the RCD neutral cable from the neutral busbar and perform an insulation resistance test (if there are sensitive items wired into the garage circuits test at 250V) from neutral bus bar to earth bar (i would expect a low reading here if I am correct).

If this test is low then disconnect the neutral and earth for each circuit and test insulation resistance between them.

The low reading indicates a fault on that circuit, then check that circuit, say by disconnecting at the mid point of the circuit and checking back to the consumer for insulation resistance and forward to the end of the circuit for insulation resistance, continue checking on the low reading side in the same way until you have reached the point of the fault.
Resolve the neutral earth interconnection that you find and reinstate all circuits, test the global test for insulation resistance as you did for the first test to ensure the fault is gone. Reconstruct the cu connections and all should work.

From your description any other fault seems unlikely in the extreme.

 

[Dave OCD]

Being as it's only a shed there's probably only a couple of sockets and a light anyway.:smile5:

 

[beginnerspark]

With 30mA RCD at garage or (changed to) main switch at garage OFF the FCU in the house can be switched on and then in either case turning on the 30mA RCD or main switch in the garage trips the house 30mA RCD.

With all circuits disconnected (you will have to clear this one up on how these were disconnected) the same thing happens.

This is only possible if both the RCD and the main switch had faults in them (to earth! as a L-N fault would not necessarily trip an RCD and would trip an MCB.)

Therefore there is something missing in our information.

When you disconnected all circuits were all conductors removed from their connections or were the MCBs just switched off?

If you just switched off the MCBs or removed the live connections from the MCBs (or removed the bus bar) but left the neutral and earth connections in place then the RCD would be tripping in the house because there is a neutral earth fault on the garage circuits.

Leave the bus bar off, remove the RCD neutral cable from the neutral busbar and perform an insulation resistance test (if there are sensitive items wired into the garage circuits test at 250V) from neutral bus bar to earth bar (i would expect a low reading here if I am correct).

If this test is low then disconnect the neutral and earth for each circuit and test insulation resistance between them.

The low reading indicates a fault on that circuit, then check that circuit, say by disconnecting at the mid point of the circuit and checking back to the consumer for insulation resistance and forward to the end of the circuit for insulation resistance, continue checking on the low reading side in the same way until you have reached the point of the fault.
Resolve the neutral earth interconnection that you find and reinstate all circuits, test the global test for insulation resistance as you did for the first test to ensure the fault is gone. Reconstruct the cu connections and all should work.

From your description any other fault seems unlikely in the extreme.

Hi Richard

Found the problem! I did as you suggested and tested each circuit and found it was the radial socket that had a dead short between N-E, so I opened the faceplate and couldn't see anything obvious. On closer inspection the neutral cable was stripped back and left too long so exposed conductor was touching the earthed back box's screw terminal resulting in a dead short. Re-terminated ensuring no cables were pinched, and re-tested across all conductors, no shorts. Happy days!

Thank you so much for your help everybody, a new guy like me needs all the help he can get!!!

 

[DPG]

Thanks for letting us know, nice one. Daz

 

[Richard Burns]

Logical, sequential testing will find most faults. It is some times easy to jump to a point where you think the fault is but you may well have missed the actual fault. Obviously burnt and melted cables would be worth going for but an inobvious fault needs a logical approach.

Glad you found the fault and resolved it, always nice to hear back.
Another skill in your fault finding portfolio!

 

[Dave OCD]

Hi Richard

Found the problem! I did as you suggested and tested each circuit and found it was the radial socket that had a dead short between N-E, so I opened the faceplate and couldn't see anything obvious. On closer inspection the neutral cable was stripped back and left too long so exposed conductor was touching the earthed back box's screw terminal resulting in a dead short. Re-terminated ensuring no cables were pinched, and re-tested across all conductors, no shorts. Happy days!

Thank you so much for your help everybody, a new guy like me needs all the help he can get!!!

,

Well done , it's a nice feeling when you find and clear a fault.

 

[telectrix]

it's an even nicer feeling when you call in the pub on the way home and exchange some of your call-out fee for beer.

 

[Dave OCD]

I'm usually too knackered by then. :smile5: