Wiring up my shed - power there but nothing works - why?

[staithes14]

I ran a cable out to my shed at the end of the garden. It's a heavy duty armoured cable, 50m run or thereabouts, buried in the garden for most of its length.

At the houuse end, it terminates at a junction box and ordinary 2.5mm T&E cable runs to the consumer unit, where it's wired into a spare way left over when I ditched the electric cooker. That's an MCB and has a RCCB protecting it (30mA).

At the shed end, the cable terminates in mini consumer unit (sort of). It goes to another 30mA RCCB, that feeds three MCBs. From these, I took off the live feeds to the sockets, two lights and a pond pump. All the returns join into a connector block and back to the RCCD and out.

It all seemed straightforward but when I powered up, nothing works. The lights don't come on when I close the switch. Ditto with the pond pump, and the sockets don't power up anything plugged into them.

I checked the connections and everything seems OK. None of the breakers tripped, and there's live feed into the switches, light fittings and sockets, and to the return neutral connection on the RCCB, when any of the circuits is active.

The other peculiarity is that the RCCB at the shed end won't trip when the test button is pressed.

I'm completely baffled by it. Can anyone advise what might be the problem?

Incidentally, the switches and circuit breakers etc are mostly reused, having been salvaged from a lab strip-out where I work.

I've done a sort of sketch of the wiring below:

 

[streetlighter]

The 2.5mm cable from the house CU should not be protected by a 32 amp MCB also and RCD may not be reqd at the house end if the whole run back to the CU had been done in SWA.
Have you informed the BCO of this work and had it tested ?
From the information you have given i would say there is no neutral to check this you must use proper voltage testers.

 

[electronicsuk]

I ran a cable out to my shed at the end of the garden. It's a heavy duty armoured cable, 50m run or thereabouts, buried in the garden for most of its length.

Oh dear, where do we even start? Are you aware that this is (or, should I say, was) notifiable work. Carrying it out without notifying local building control first is in contravention of part P of the building regulations. You might not see this is as a problem, but it will make things very difficult for you now if it ever comes to selling the house.

At the houuse end, it terminates at a junction box and ordinary 2.5mm TEE cable runs to the consumer unit, where it's wired into a spare way left over when I ditched the electric cooker. That's an MCB and has a RCCB protecting it (30mA).

2.5mm Twin & Earth is not suitable for use with a 30A breaker. Not to mention that if the SWA is 2.5mm as well, the volt drop down a 50m run will likely be more than 20 Volts! As you have lighting circuits, the maximum V.D. permitted under the 17th edition is 3% of the supply, therefore you can see this is well in excess of the 6.9 Volts permitted!

At the shed end, the cable terminates in mini consumer unit (sort of). It goes to another 30mA RCCB, that feeds three MCBs. From these, I took off the live feeds to the sockets, two lights and a pond pump. All the returns join into a connector block and back to the RCCD and out.

What do you mean by "sort of"? It either is or it isn't. I'm starting to suspect this is an enclosure fitted with RCD and MCBs with no busbars or N/E bars, hence everything connected in terminal blocks.

It all seemed straightforward but when I powered up, nothing works. The lights don't come on when I close the switch. Ditto with the pond pump, and the sockets don't power up anything plugged into them.

That's because it isn't straightforward. I don't mean to come across as all high and mighty, but you have to understand what you have done may be EXTREMELY dangerous. You haven't even taken the most basic factors of electrical installation into account, and there could be who knows how many problems that are causing the circuit not to work. Volt drop is too high, EFLI is almost certainly going to be too high, protective devices wrong rating, protective devices may not operate either on account of EFLI, earthing arrangements unknown... need I go on?

Needless to say, as much of an annoyance as it is for someone like me who works in an industrial environment not being able to carry out work in his own home without notification, the procedures are there for a reason. Had this work been notified, it would never have passed LABC inspection. Suggest you undo everything you have done and contact a qualified electrician who is a member of a self certification scheme.

 

[NamelessRob]

If i understand your diagram - you've taken a live supply into the first socket, then from the neutral of the first to to the live of the second, then taken the neutral of the second back to the rcd?

You haven't shown any earths - is this just for the sake of clarity?

 

[flameport]

Can you post some pictures of this installation?

 

[david_uk]

There may be a problem with the earthing arrangements in any case. The shed may need to have it's own earth bearing in mind the distance from the house, plus if it is a metal shed you might need to bond structural steelworks.

 

[staithes14]

I just missed out the earths for clarity. Everything is earthed and the shed is wooden.

Sigh. I didn't think taking power to a shed was notifiable work. Obviously it is from what you've said. At the moment it's disconnected so not a danger.

"What do you mean by "sort of"? It either is or it isn't. I'm starting to suspect this is an enclosure fitted with RCD and MCBs with no busbars or N/E bars, hence everything connected in terminal blocks. "

Er, yep. That's it. Other than neatness though, what's the difference?

Looks like I should remove it all except the cable and get a sparky in.

 

[sparkyspike]

Looks like I should remove it all except the cable and get a sparky in.

Definitely. It would be possible to explain how this should be done, but as you have a floating neutral, outlets wired in series, undersized SWA, suspect earthing and are working outside on a notifiable job, this is way too dangerous a job to persevere with!

10/10 for trying

0/10 for the results. Seriously though, someone could get killed...

 

[staithes14]

OK I'll take it all out and get an estimate from a pro. As the main cable is already in from the house to the shed, hopefully the rest won't be too expensive.

 

[staithes14]

If i understand your diagram - you've taken a live supply into the first socket, then from the neutral of the first to to the live of the second, then taken the neutral of the second back to the rcd?

No it's live1 to live2, neutral1 to neutral2 . My diagram's not very good.

 

[breezer]

OK I'll take it all out and get an estimate from a pro. As the main cable is already in from the house to the shed, hopefully the rest won't be too expensive.

I suggest you read it again, you seem to have missed this bit

2.5mm Twin & Earth is not suitable for use with a 30A breaker. Not to mention that if the SWA is 2.5mm as well, the volt drop down a 50m run will likely be more than 20 Volts! As you have lighting circuits, the maximum V.D. permitted under the 17th edition is 3% of the supply, therefore you can see this is well in excess of the 6.9 Volts permitted!

 

[staithes14]

If I remember right, the armoured cable is 4mm. Wouldn't that be OK? It would be pretty much impossible to dig up again. If it was no good, I'd have to bury a new cable on a different line.

 

[bongos]

If the cable run is 50m then according to voltage drop calculator you will be ok for lighting circuits (just).

So at the least the cable may be ok to stay...

 

[staithes14]

It's just less than 50m (about 45). It's intended to power 2 lights (one a ceiling striplight, the other a bayonet fitting above a bench), a pond pump (60W constant operation) and provide power to 2 sockets, which will be used occasionally to plug such things as hedge trimmers, lawnmowers or power tools into (obviously not all at once).

According to the calculator above:

Cable Type SWA 3 core PVC
Application underground
Circuit Type non-lighting
Ambient Temperature 20°C
Number of Circuits 3
Power 800W
Voltage 230 Volts
Length 45 metres

Required Cable Size 1.5 mm
Voltage Drop 3.91 Volts.
Percentage Drop 1.7%
Current Load 3.00 Amps
Max Cable Load* 15.3 Amps

If I get a sparky in, is it likely he would be prepared to certify the installation, given that the cable is already buried for most of its length? As I said, I can't dig it up again, (there's a stream over most of it!).

As you have probaby guessed, I'm not very knowledgeable about wiring, and the most complicated thing I've done before was install a socket on a spur. I ought to know better, doing what I do for a living.

 

[Lectrician]

As all the items are second hand, it suggests a faulty RCD to me.