Garage Conundrum - Confused.com

[oldhousescot]

Hi All
It's been a while since posting and many thanks for help in the past.

I was hoping to run an extension from one socket in my garage to give me another socket close to where I want to site my compressor.

The garage has been wired from house consumer unit to another consumer unit in garage. This consumer unit has 5 MCBs, 2 are for 2 banks of two flouresent tubes and he other 3 are for sockets. 1 has one socket running freezer. the other two ran 2* double sockets.

I switched off the MCB relating to the sockets and started to run the new extension. Wired into socket with no issue however when I cut the twin and earth at the required length it tripped the RCD back in the house.

I reset the RCD and got very confused, I ran the voltage pen along the cable and there was no light. went to complete the cut and it tripped again.

Am i right to be confused/worried?

Thanks guys

 

[rebuke]

That is entirely normal - basically when you turn off the breaker you only isolate the live wire, the neutral and CPC (earth) are still connected, which isn't normally a problem, except where an RCD is involved.

In simple terms an RCD works by sensing a difference between the current going out on the live wire and coming back on the neutral - if these differ by more than the threshold (usually 30mA) it will trip.

When you cut the cable you likely temporarily shorted neutral to earth - this creates an alternative path for current to flow back to earth, and thus the RCD senses an imbalance and trips...

 

[ericmark]

Normal the neutral is still connected so you have an earth neutral fault. Should isolate before working that need a double pole switch not a MCB.

 

[oldhousescot]

Gents

I had suspected that but I suppose that I hadn't figured that the RCD was so sensitive. Time to stop twitching!

thanks guys
Alan

 

[jc69817]

Hi guys
This setup sounds just like mine. If there is a fault both rcd trip house and shed. Would it be a big job to make just the rcd in the shed trip and not the house one ??

Many thanks and sorry for the hijack

 

[PrenticeBoyofDerry]

Having two RCD (of the same rating) on the same circuit is not really advisable.
How is the cable to the shed distributed, is it directly from the CU or spurred via an outlet within the house?
And how is the cable routed to the shed and what type of cable is it?
I ask these questions, because if the cable is mechanically protected or even surface mounted/within conduit and directly distributed from the CU, there maybe a case that RCD protection can be removed within the house CU and only offered at the shed CU.
If it's a spur considering putting a more sensitive RCD within the shed CU could resolve your issue.

 

[jc69817]

thanks for the reply
shed 1 is fed from CU with RCD
shed 2 is fed from shed 1 through conduit with t&e to a garage CU.
is it difficult to change the RCD in the CU or is it a job for a pro ? I am a confident diyer and can do most things but dont like to take too many risks.

thanks for your help

 

[PrenticeBoyofDerry]

What method of cabling is used between the house CU and shed 1?
This would ideally need to be either SWA if buried in ground or surface mounted cable within conduit if externally run.
Also are any cables if not SWA, buried in the walls less than 50mm between house CU and shed 1?
As there are requirements were RCDs can be omitted, but these need to be conformed to.

is it difficult to change the RCD in the CU or is it a job for a pro ? I am a confident diyer and can do most things but dont like to take too many risks.

With regards to the removal or RCD protection to this part of the circuit, this would depend on the board type and the way it is configured. There could be a chance that you can fit a MCB via live busbar to the main isolator, a split neutral connection is required and moving over the other components within in the CU may need to be done. A picture of the CU with both cover on and off would be helpful, as these provision may already be there. (please isolate the CU, before removing cover)
The other option would be to have the protective device for this circuit, located outside the CU, this would require a small CU with isolator and a connection from the main supply and earth.

Your you are right by not taking any risks, and that being said, I would suggest that although you are a confident DIYer, the above work be best left to a competent electrician.