Discuss Protection for cables suppling Consumer Unit, due to distance greater than 3 Metres. in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

R

Rik

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Hi Folks,
I'm currently doing a full re-wire to the Mother-in-Law's bungalow which she has just bought. The bungalow was built around 1960's so needs a complete over-haul with both electrics and other building works.

The earthing system is TT.

Currently, the supply meter is indoors in the same cupboard with the Old Consumer Unit. The supply meter is being re-located to a flush mounted external meter box.

Due to the meter being re-located outdoors the distance between the Meter and the new Consumer Unit is now aroung 7 Metres. My question is - what will be the best practice to protect the the cables supplying the consumer unit? Should I install a Switched Fused as in in link below to protect the cables from overload or is both a Time Delay 100mA RCD (as per second link) and Switch Fuse required?

100A Mains Switched Fused c/w 80A & 100A fuses

100 Amp 100mA Time Delayed RCD 2 Pole - Wylex

Much Appreciated for your help Guy's.
 
T

thekingiam

  • Thread Starter Thread Starter
  • #2
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

is there a regulation dealing with distance from meter to c/u?
 

Des 56

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Arms
Esteemed
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

You have a TT system,that being one thing that will govern the choice of protection for your supply cable
What cable will you use?
What method of installation will be used ?

You need to provide this information initially
 
R

Rik

  • Thread Starter Thread Starter
  • #5
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

The consumer unit will be approx 7 metres from the main cut-out.

The installation method will be, up through the wall cavity using 25mm SWA. The SWA will be glanded into Galvanished Adaptable boxes at both ends, one dappy box in the meter unit enclosure with tails onto the new switchgear. One dappy box in the loft, with tails onto the new consumer. Dappy boxes will earthed.
 

Des 56

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Arms
Esteemed
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

The 100 m/amp time delayed is a must have (unless of course you can squeeze a PME out of them when they do the alterations)
The Switch fuse is only necessary if the supplier impose distance restrictions,most will state from 3 to 4 meters, personally to save any possible refusal to connect, or having to get this info from them somehow,I would go with both your suggestions

The other thing to consider is using the meter box for this equipment,many have had problems because they may not permit
I have never had a problem using the cabinet
 
R

Rik

  • Thread Starter Thread Starter
  • #7
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

Thanks Des.

I've tried asking for the DNO to supply an earth but got refused, don't know why this would be such a big deal as there are 5 cables running along posts on the front road. Obviously, these being the 3 phases, neutral earth. I have spoken to the the DNO installer who says just and isolator needs fitted for them to terminate into but think this needs more protection that just an isolator.

Given that, do you think I should just install the 100mA time delay RCD, or still run with both?
 

Des 56

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Arms
Esteemed
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

Can you consider using a 1-way cons unit with the s type rcd as the main isolator and mcb for overcurrent
 
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

If your doing this via the BC you might get there hackles up a little with a cable going down the cavity, technically I would be arguing they are tails but you never know ......................

Also it as it sounds it's an external cavity wall remember there maybe now or in the future insulation in there, which will seriously affect your CCC

And let's not forget 522.8.4
 
K

Knobhead

  • Thread Starter Thread Starter
  • #10
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

I've tried asking for the DNO to supply an earth but got refused, don't know why this would be such a big deal as there are 5 cables running along posts on the front road. Obviously, these being the 3 phases, neutral earth.
The road I first bought a house on had five O/H lines. The three phases, neutral and street lighting. Things mat not be as they appear.
 
R

Rik

  • Thread Starter Thread Starter
  • #11
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

Hi Des 56,
I like your solution, however, I'm unable to find a 1 Way Metal Clad D/B that has a 100A RCD 100mA Time Delayed as the incomer, with an outgoing 100A MCB.

Is there a manufacturer doing such a board? Or, even if it can be ordered as individual items?

Cheers.
 

Des 56

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Arms
Esteemed
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

You may have to construct something yourself
Here are a few items I looked at for you to start your search,note the mcb seems to be available only as C type

High Current 10KA Single Pole MCB


100A 100MA TWO POLE TIME DELAY RCD - Control Gear Direct - AllAboutElectrics.co.uk

4 Module Metal Enclosure - AllAboutElectrics.co.uk
 
R

Rik

  • Thread Starter Thread Starter
  • #13
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

Thanks for your help Des. A good solution.
 
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

The road I first bought a house on had five O/H lines. The three phases, neutral and street lighting. Things mat not be as they appear.
Your dead right tony, In our area the Fifth line, which is usually smaller is a swithed lighting feed for the street lights running back to a call point, in the old days this was where the lighting man switched on the street/area, these days there is usually a photocell on top of them. However this system is being phased out in most areas in favour of indiviually switched lamps as it gives them a longer time to repair a faulty one!
 
S

Spudnik

  • Thread Starter Thread Starter
  • #15
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

One way to find out if PME is available is to look at the posts.

Every third post should have a cable running down to the ground, and this is where the earth point is.

ANother way is to look at the OH lines coming to the house.

A single cable is normally concentric and sometimes proves an earth is available.

If there are two cables coming in then its a no.

However, all this needs to be approved by the DNO.
 
S

sjm

  • Thread Starter Thread Starter
  • #16
Re: Protection for cables suppling Consumer Unit, due to distance greater than 3 Metr

I've just had a similar situation. Asked both DNO and NICEIC about the need for additional protection of tails. The answers I got were definitely isolation (so fit a type S (time delayed) RCD) however, unless you're reducing the CCC of the tails, you can rely on the service head fuse for overcurrent protection. If I were you I would consider using standard tails inside of flexible conduit for the run in the cavity. Tails are th only cables allowed to run in the cavity but you should protect them from possible future cavity insulation. Also fixing the conduit could help to stop bridging of the cavity.
 
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