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PostPosted: Mon Mar 11, 2019 3:40 pm 
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I have installed a small garage consumer RCD box to my shed and run 2 double sockets and a light from this, as you would do a house via the 2 trips (1 for lights 1 for mains)
From the house I have a double socket in my front room that I have spured off from using armoured cable to the shed consumer unit RCD

The problem I have is that about once a day the downstairs sockets trip randomly ?
The wiring seems fine inside the garage...

I have read that I could just spur off the living room plug to an fuse spur in the garage and run my plugs and then fuse spur for my light from this ? Am I getting the problem because I’ve put a garage RCD unit in there from the plug rather than from the board in the house ( this is a long way away from the shed)

Anyways any suggest would be great full

Many thanks


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PostPosted: Mon Mar 11, 2019 3:56 pm 
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You have over simplified what is required, as the cable gets longer so losses increase so we split our supply into circuits however for some reason I fail to understand the RCD split is not considered as a circuit, to my mind there should be a limit to the length of cable connected to a single RCD, however there is not.

However moving items in the consumer unit can help, best option is a RCBO for the ring final so that is on it's own RCD, the maths is easy allowed 30 mA per RCD so fit two then 60 mA and so on, so a CU with all RCBO's can work with more leakage to one with just two. But first thing is measure, be it a clamp on meter, or insulation tester you need to test.


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PostPosted: Mon Mar 11, 2019 4:43 pm 
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The armoured cable is max 8m in length away from house so not far at all
The garage consumer unit cane with two ready one 5amp for lights and 1 32amp for mains ?

I’m not sure I understand why you mean could you break it down to more simpler terms please thanks


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PostPosted: Mon Mar 11, 2019 5:53 pm 
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Stevebb wrote:
From the house I have a double socket in my front room that I have spured off from using armoured cable to the shed consumer unit RCD

To spur off a ring the supply needs a fuse or MCB limiting the power to 13A. If you can measure the distance of the two legs of the ring, and the spur point is some where near the centre may be you could draw 22A however it would need careful measuring.

But as far as earth leakage goes, the more length of cable you have, the more it will leak, there are two reasons for power to leak to earth, one is a fault, the other is AC will transfer with inductance and capacitance wire to wire, so there will never be zero leakage.

In the main there are two ways to test, one uses 500 volt DC so will not show inductance and capacitance leakage, we expect to see 20 Mohms although 1 Mohm is a pass, simple maths 230/1000000 = 0.23 mA and your allowed 15 mA however measured as an AC current using a clamp on ammeter you could see over the 15 mA limit as using AC and the insulation tester uses DC.

Of course the RCD could be faulty, with a 30 mA version it should not trip at 15 mA it should trip at 30 mA and at 150 mA it should trip within 40 mS. All is tested both on pos and neg half cycles.

With any new installation we should inspect and test, and so we should know if any of the limits are exceeded, however with DIY it is a trust to the lord, and very little testing is done.

So three meters are standard when testing an installation.
1) RCD tester rather expensive expect over £100.
2) Loop impedance tester again expensive expect over £140.
3) An insulation tester often also a low ohm meter, I bought one for less than £40

So here is an advert for one at just over £30 and seen them for a little less. Although the plug in tester EZ150 does not really test well enough, at just over £40 it is likely good enough so a DIY person keeps within the limits.

So your looking at least £80 for test gear even doing it on the cheap, if your not willing to pay that out, answer is simple, don't DIY.

Finding the fault may not cure the fault, and the more you split the supply up, the less likely it is that it will trip and the easier it is to find a fault, a RCD is around £30 and a RCBO is around £12 so replace the RCD with an isolator and fit 5 RCBO's and likely it will trip less and if it does trip easier to find the cause. But that means £60 and you still don't know if the fault will go, as you have not tested.


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PostPosted: Mon Mar 11, 2019 7:30 pm 
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Stevebb, you have made errors and installed something you believe "should work" but you do not know what it is you have done wrong.
With your lack of knowledge, I suggest you disconnect what you have done and get an electrician to check and test it for you, as he may also find something potentially lethal that you have done.

"Just because it works" does not mean it is safe.

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PostPosted: Tue Mar 12, 2019 10:54 pm 
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ericmark wrote:
To spur off a ring the supply needs a fuse or MCB limiting the power to 13A. If you can measure the distance of the two legs of the ring, and the spur point is some where near the centre may be you could draw 22A however it would need careful measuring.


That info is as out of date at as the 14th edition regs.


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PostPosted: Wed Mar 13, 2019 6:36 am 
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Thank you everyone


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