Electricity supply to garden sheds seems very unsafe

Discussion in 'Electricians' Talk' started by HaveAGoButNoHero, Dec 17, 2013.

  1. Walter System

    Walter System Member

    Thanks. If at the house end there is no RCD and there is a RCD in the shed CU which end is it best to connect the metal outer part of the cable?

    Also, there is a lot of steel metal wires around the cable. How is this earthed? using a few of the wires to an earth bar or gathering all and using a brass clamp - gather them all together and have a 2.5mm wire to the earth bar?
     
    Last edited: Dec 21, 2013
  2. Walter System

    Walter System Member

    I looked at the link it does not say that the armoured cable must be to earth. It does give another ref. I assume that ref would say the cable must be earthed.
     
  3. My system to sheds is fed from a 16 amp MCB on the CU! Can i simply replace that with a RCBO for now? 20A 30mA? I have fitted two fused protection units in the first shed now, one protecting the lighting (3A)? the other 13A for shed 2. The 2.5mm twin & e is linked to the armoured cable in shed 1. There is an RCD in shed 2. Many thanks for further advice.
     
  4. Walter System

    Walter System Member

    You cannot have two RCDs in series. An RCBO is an RCD and a mcb combined. If you fit an RCBO in the house to give full protection, then remove the RCD in the shed and replace with probably an incomer isolator switch, which is a straight swap.
     
  5. Okay, thank you Walter System. Sound like you have the knowledge which, coupled with advice from this forum, will get you solving your shed problem quicker than I will!
     
  6. seneca

    seneca Screwfix Select

    There's nothing to say you cannot have 2 rcd's/rcbo's in series, it's just that if they both have the same tripping current there's no telling which one will trip first.
     
  7. Walter System

    Walter System Member

    I only have the cable earthing outstanding. ;)
     
  8. Walter System

    Walter System Member

    You are right but having one at the house protects the lot and is less problematic. This is all assuming the devices work and work properly. Two RCDs rated the same in series is not desirable.

    An incomer is about £9 on S/fix and usually a straight swap. The neutral bars on the CU have to be linked if removing the RCD - assuming it has two of course.
     
  9. Walter System

    Walter System Member

    I would have a separate CU in the house with its own tails from the meter via henley blocks. Have a RCD or RCBO in this CU - RCDs are cheaper. This separates the installations. You will then have technically "two" installations off the same main fuse. Now you divide and rule. Then have underground cable to a 2-way CU (none RCD) in the 1st shed and continue the armoured cable to the second shed and have another 2-way CU (non RCD) there. (you already have an underground 2.5mm cable)

    This underground cable may need a 20A mcb at the house CU, as each shed CU would have a 16A (power) and a 6A (lights) mcb each (you could go down to a 10A mcb for power). The intention of the 20A mcb at the house is to protect the 2.5mm underground cable (which is already there) which is rated to 27A max. You could go up to 40A at the house and have a larger cable underground, which means the two sheds could be drawing a full 22A each if you want - but this is more expense and appear to be overkill. The appliance usage in the two sheds is unlikely to warrant such a size or high current usage. A couple of lights will draw a tiny fraction of the 6A mcb rating.

    The two shed's CUs will protect all cable downstream from them in the sheds. In my suggestion keeping the existing underground armoured cable, you could have each shed drawing 12A each (say a 3kW electric kettle in each) and their local mcb's will not trip, but the house mcb will as the 20A mcb in the hosue protecting the underground cable has exceeded its current rating.

    ...and of course have the armoured cable earthed at the house end. :)

    In general. In most domestic installations if all circuits are loaded to just under 1A of the mcb rating the suppliers main fuse will blow. Few domestic installations will ever run at anywhere near the 60A or 100A rating of the main fuse. The 60A or 100A main fuse is to protect the meter tails and the supply cable from the street. The meter tails must be rated over what the main fuse is. (some suppliers put in 60A main fuse from a 100A if they see the meter tails are not up to it. Few have a main current rated breaker after the main fuse to protect the whole system. A domestic installation is based on probabilities that all circuits will never be drawing maximum current at the same time.
     
    Last edited: Dec 24, 2013
  10. DIYDave.

    DIYDave. Screwfix Select

    Walter,
    The earthing connection to the armoured cable is made by using "armoured cable glands" . Just use the right size gland for the size (mm) cable. The glands are a great bit of engineering, look a bit complicated but come with instructions or look on you tube for a tutorial. They will provide an earth connection, act as a cable clamp and come with a waterproof boot.
    Often the glands are not used directly at the CU but are fitted to a separate enclosure close to the CU. Terminate the armoured cores inside the enclosure and connect to the CU using correct sized T & E
    I'm just a diy'er so hopefully a pro will correct anything I have got wrong
     
  11. This is all useful stuff, thank you folks, I will try to make sense of it!
     
  12. DIYDave.

    DIYDave. Screwfix Select

    One thing I find strange with the installation is in the picture of the shed, the conduit running up the shed has been installed nice and neat and tidy, the saddles are evenly spaced and it looks true and vertical
    I realise we cant see the connections at the house CU or how the cable has been run underground but, the bit up the shed looks good

    From then onwards, all the wiring in the shed looks like total pants !

    The installation may be safe and working (to a degree) but you can tell a good workman, someone who takes pride in the appearance of their work by looking at the cosmetic angle of the installation
    What professional would stand back after wiring up the shed/s and say " that's a bloody nice job there"

    I wonder if the supply to the shed was run by a sparkie whilst everything inside was carried out by someone on a horse who wears chaps, leather boots and spurs ?

    Just wondering like
     
  13. Hi DIYDave. The previous owner once worked at the local nuclear power station! He was a model maker too, all very good as far as I know from other folk locally. Had a very well-equipped workshop. The electrics inside of the shed with the conduit outside are neat and tidy. The bad stuff is in shed 1. The 30A fuse there is clearly older than most of the system, I wonder if that was here long before him. He certainly had shed 2 installed. There's second CU there - see original pics posted. Does the RCD afford sufficient protection when using power tools in there? (I plug them in to a portable socket RCD just in case they don't)

    Here's a pic of the cable leaving the house? IT HAS BEEN FED THROUGH A SHORT LENGTH OF COPPER PIPE; then the twin and earth exposed outside, before entering more copper to go underground - I dread to think what you're gonna say about that! I'm amazed that rain/damp hasn't got through. It exits from the back of a double socket (where I found burn marks yesterday), and as far as I can tell from switching off the MCBs, it is alone on that circuit fuse - except that it then goes on to the two sheds!

    Having been in this house for a while now, it is clear that many aspects of his DIY were of a poor standard. I have receipts for various materials, from electrical items to copper and plastic plumbing, oak flooring, and remnants left over, clear evidence that he did things himself. Oak flooring buckling in one room, I suspect it's damp beneath. Junction boxes screwed to bookshelves as cable exits wall then re-enters... Discovered more burn marks in the back of a double socket unit yesterday - I think this place was close to burning down on at least two occasions. I should be careful what I say - he died in the house 3 years ago!! Anyway, he's left me with lots to do each weekend!
     

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    Last edited: Jan 9, 2014
  14. Walter System

    Walter System Member

    Thanks. I assume these glands are available in Screwfix.
     

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