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PostPosted: Sun Jul 02, 2017 10:41 pm 
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Hello

I have an existing steel wired armoured cable in my garden that was previously connected to my old garage that is already dismantled. The SWA cable was buried directly into the ground without any conduit. Due to some tree removals and ground excavation, I've had to dig up the cable and will need to bury it again. I'm planning to re-use this cable to power a light and a double socket to a shed that I haven't built yet. I'm in the middle of renovating my garden, so I'm planning to do as much prep work as possible on my own before I hire an electrician to install the light and socket when the shed is ready. So the only thing I'll be doing is bury the cable into the ground and have the end point of the cable next to my new shed or inside it.

I have a couple of questions:
- Is it a legal requirement to have the SWA cable in a conduit before burying it into the ground? Or can I just bury the SWA cable as it is into the ground like how the previous owner had done it? I don't want to bury it without conduit only for the hired electrician to reject the light/socket installation due to this issue. I don't want to lift the cable back up after having paving slabs installed around the shed.
- Is there some legal requirement as how deep I need to bury the cable?

I appreciate your responses.


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PostPosted: Sun Jul 02, 2017 11:05 pm 
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There are regulations, it does not need conduit there are ways from simple tape to tiles to protect it, however the electrician will need to inspect the cable before it is back filled, so it does not really matter, you need to get an electrician first, he could tell you bury it 1.5 meters deep I will trust you, or he could say tell me when the trench is dug, it is his call.


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PostPosted: Mon Jul 03, 2017 7:30 am 
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I dont think there is a depth quoted in regs, but understand usually 450mm is ok.

Swa cable is designed to be used on its own, no conduit needed.

You need your electrician to advise whether the cable is the right size and he should also check your earthing arrangement in case you need a 4 core cable.

Im not an electrician so above may be wrong!


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PostPosted: Mon Jul 03, 2017 8:35 pm 
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The Regulations state that the cable should be buried "at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground". (Reg 522.8.10)

They should also be marked with suitable marking tape or cable covers (so if someone does dig down and find it they know it's a mains cable).

It doesn't need conduit as it is armoured cable and designed to be buried.

I've heard 50cm deep as being usual but if you don't "reasonably foresee" anyone ever digging or spiking the area greater than 20cm deep then the regs state that's ok.

As others have said, you need to get your electrician to have a look at the cable and the earthing system in your house. The shed will have to have RCD protection, either at the shed consumer unit or from your house consumer unit. The SWA has to be able to carry enough earth current to allow the RCD to operate correctly.

I'm about to replace the SWA currently supplying my garage which is 18 metres from the house CCU and I'm upgrading to 6mm 3-core cable to do the job properly.


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PostPosted: Mon Jul 03, 2017 8:46 pm 
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Stooby2 wrote:
The SWA has to be able to carry enough earth current to allow the RCD to operate correctly.


Sorry, that statement is in error.

Hapless Henry pokes a screwdriver into a RCD protected socket, he then "gets a zap" (technical term that is) but death is prevented because the RCD tripped (as it should) the earth cable or its size or its resistance had nothing to do with the RCD tripping.
The RCD tripped because of the imbalance.

When hapless henry touched the live, current flowed through him to earth.
Normally the current flows from live through what ever is connected and then back on the neutral.
As it does this what is flowing up one side is equal to what is flowing back down the other.
This was not the case when henry touched the live since the current flowed through him and not the neutral, so there was an imbalance, so the RCD tripped.

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PostPosted: Mon Jul 03, 2017 9:12 pm 
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If an appliance of Mr Henry's develops a defect where the casing becomes live, the earth system should allow enough current to flow to operate the RCD, without him having to touch it to cause the imbalance. If that defect is a small current leakage - say 500ma, then the steel armour might not be enough, depending on the distance of the shed to the house CCU. He shouldn't have to touch it and "get a zap" in order to make the RCD work.


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PostPosted: Mon Jul 03, 2017 9:19 pm 
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What you are saying now has nothing to do with an RCD. If a metal device becomes live the MCB should trip, in which case your statement about earth conductor size would be true.

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PostPosted: Mon Jul 03, 2017 9:21 pm 
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I do not see the point of arguing, which is where this could go. I believe you are in error, you believe i am wrong.
Let us agree to disagree.

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Which is correct? Metre or Meter? Click the link. to find out more.

No such thing as "Thou shalt put this wire here, Thou shalt put that wire there" .............Take a picture BEFORE you do the job.

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PostPosted: Mon Jul 03, 2017 9:22 pm 
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Fair enough - his own electrician will be making the call at the end of the day.


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PostPosted: Tue Jul 04, 2017 1:04 pm 
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Thanks for the responses everyone. :cheers:


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PostPosted: Tue Jul 04, 2017 9:41 pm 
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Stooby2 wrote:
I'm about to replace the SWA currently supplying my garage which is 18 metres from the house CCU and I'm upgrading to 6mm 3-core cable to do the job properly.


Would it surprise you to know that 6mm 2 core SWA using the armourings as the earth would in 99.9% of cases also do the job just as well and fulfill all the regs?


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PostPosted: Tue Jul 04, 2017 9:44 pm 
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Stooby2 wrote:
If an appliance of Mr Henry's develops a defect where the casing becomes live, the earth system should allow enough current to flow to operate the RCD, without him having to touch it to cause the imbalance. If that defect is a small current leakage - say 500ma, then the steel armour might not be enough, depending on the distance of the shed to the house CCU. He shouldn't have to touch it and "get a zap" in order to make the RCD work.


If you have an earth current flowing of 500mA than the 30mA RCD should trip. Or have I misunderstood the type of fault you are describing?


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