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Discuss Must be feeling better........ in the Electrical Forum area at ElectriciansForums.net

Baddegg

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....am now bored stupid but still not fit for work, with that in mind I’m going to use the time and sort out a few bits in doors,starting with this...
TN-s supply to meter into 100a-100ma s type rcd then into 25mm swa and into the CU under the stairs about 5/6 mtrs away under upstairs floor through joists ( I strengthened the joists when I moved in as I was not happy with a cable that size going through!....I want to change the rcd for a 100a dp isolator for discrimination purposes but with the installation method of the swa (under floor boards) i’m concerned with protection should someone ever hit the cable? Not likely I know but.....Or am I over thinking as it’s armoured? Cable sheath not earthed either....this was installed before my time and before I had any interest in electrical work.
 
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buzzlightyear

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i’m concerned with protection should someone ever hit the cable
hitting a swa cable under the floor boards ,let me rip up those floor boards ,wow swa let me have a crack at that ,more chances of being kissed on the lips by a giraffe ,LOL.or him
 

KEV 1 N

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The armouring of SWA must be earthed.......otherwise this may happen!
667450BD-C8B4-4DEE-8908-E3AE79324658.jpeg
 
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Deleted member 26818

RCD protection is not an acceptable method of providing additional protection for cables below floors or above ceilings.
Whereas using a cable with an earthed metallic sheath is acceptable.
Installing a cable through joists 50mm from the surface of the floor or ceiling is another acceptable method.
Protection from penetration by screws or nails wherever the cable passes through a joist is another acceptable method.
 
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TJ Anderson

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Earth the steel armour of that cable and fit your DP isolator.

As you have TN-S arrangement, need to check what is your Ze and what rating the cut out fuse is. Install method of SWA doesn't appear to need a derating applied so can likely avoid fusing down.

With yours, You can go up to 0.42 ohms Zdb and achieve ADS with 100A Cut Out Fuse. That's worse case, you may have an 80A in there.
 

Richard Burns

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If you are going 5-6m from the supply fuse then the distribution cable should be fused independently of the suppliers fuse, the SWA should not need RCD protection if the armour is earthed.
Therefore I would install a switch fuse after the meter and earth the SWA armour at the switch fuse. All done.
Take it gently and do not cause yourself to go backward on the healing.
 

TJ Anderson

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If you are going 5-6m from the supply fuse then the distribution cable should be fused independently of the suppliers fuse, the SWA should not need RCD protection if the armour is earthed.
Therefore I would install a switch fuse after the meter and earth the SWA armour at the switch fuse. All done.
Take it gently and do not cause yourself to go backward on the healing.
I know you are referring to the the DNO 3M max rule. His DP switch will be within 3M of Meter, SWA CCC is ok and he will likely still acheive ADS (will need to verify) protected by cut out fuse.

Is there a specific rule prohibiting that and stating you must fuse down?
 

Baddegg

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Hi - listen to RB :) .
Is the 100mA S already in place?
Yep already there wilko, wasn’t untill I started learning that I realised it wasn’t ideal, now I got some time I’m going to use it to make good
Thanks as always
 

Baddegg

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If you are going 5-6m from the supply fuse then the distribution cable should be fused independently of the suppliers fuse, the SWA should not need RCD protection if the armour is earthed.
Therefore I would install a switch fuse after the meter and earth the SWA armour at the switch fuse. All done.
Take it gently and do not cause yourself to go backward on the healing.
Thanks Richard,
 

Richard Burns

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I know you are referring to the the DNO 3M max rule. His DP switch will be within 3M of Meter, SWA CCC is ok and he will likely still acheive ADS (will need to verify) protected by cut out fuse.

Is there a specific rule prohibiting that and stating you must fuse down?
Chapter 43 gives guidance on when overload and fault current can be omitted and states that if the supplier agrees you can use their protection this is OK; however in general the supplier probably only allows this for up to 3m away from the supply fuse (not the meter). The wiring regs also state that a protective device can be installed no more than 3m away from the point at which overload protection is required so long as the intervening section of cable has no branches and is installed so as to minimise risk of shock and fire.
In practice this is often taken to mean that meter tails must be fused down within 3m of the supply fuse.
Generally this is most conveniently done just after the meter.
I do not believe there is a need to provide selectivity from the supply fuse but for convenience this would be useful; practically it would significantly limit the property supply capability so perhaps going down one size might be OK, or have two of the same fuse in series and hope your accessible one goes first!
 

Baddegg

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Thanks Tj, I’ll take a ze over the next day or so and check the fuse
 
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