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.

 

[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: