Mains cable size.

[Nickfromwales]

What about 35mm 4-core and have a second phase too? Any advantage to that ?

[Russell griffiths]

I probably won't need the 3 phase in the workshop, I don't intend buying any serious machinery, it always looks good in an auction but in reality the 3 phase machines are very large and will take up a lot of room, will stick to standard 240 diy/ semi pro stuff. 

Could the extra phase be run down to the new cu and then used for outside things like sewage treatment and potting shed. 

[Russell griffiths]

How is this earthed. Does an earth have to run back to the main switch by the meter? 

Oris it earthed to a ground spike by the new house by the cu. 

 

if it runs back to the main switch by the meter do I run a separate single earth or is the earth combined in the swa. Not the outer steel protection as I know this is not a recommended method any more. 

[Jeremy Harris]

14 minutes ago, Nickfromwales said:

What about 35mm 4-core and have a second phase too? Any advantage to that ?

 

4 core could give three phase at the house, as you may not want to export the PE down the cable, so could TT it at the house end (should be no problem getting a good earth given the lakes around).

 

35mm² would restrict the current to around 90A per phase, to stay within the 5% voltage drop limit, and that would be masses of power.  One phase could be used to run the house (90A is more than enough - I have our main supply fused at 80A), and another phase could be used to feed the garage/workshop, if need be.

 

If you think that the total single phase load will always be under about 90A, then you could just run a 35mm² single phase cable.  In practice this would be fine at the full 100A, as although the peak voltage drop would just exceed 5% (5.5% at 100A), in practice it's very unlikely that you would ever draw 100A.

[Jeremy Harris]

5 minutes ago, Russell griffiths said:

How is this earthed. Does an earth have to run back to the main switch by the meter? 

Oris it earthed to a ground spike by the new house by the cu. 

 

if it runs back to the main switch by the meter do I run a separate single earth or is the earth combined in the swa. Not the outer steel protection as I know this is not a recommended method any more. 

 

 

You can either export the earth down the 100m cable (might well mean increasing the cable size to get the loop impedance down) or you could just use an earth rod at the house end and have a TT installation.  I'd do the latter, as you'll get a very good earth with those lakes around.  It's cheaper, too, as for single phase you could run two core SWA, rather than 3 core.

[Russell griffiths]

Minimum cable depth. 

Soas an offshoot to this I'm going to have to put in a trench for this new cable, the problem I have is I don't know the location of the other services down the driveway, water, telephone. 

So if I go as shallow as I can on the elactric I may avoid bumping into my  50mm water main. ??????

thanks for all the input. 

 

[Nickfromwales]

50mm . No problem running 2 showers then lol. 

[Russell griffiths]

21 minutes ago, Nickfromwales said:

50mm . No problem running 2 showers then lol. 

Meter is by the main road, it runs for 180m and then T off to cabin in then travels another 200m to 2 other holiday lets cabins we have onsite. 

A burst pipe is the thing I have nightmares about. 

I check our water meter every week to check it isn't spinning around like a nutter.?????

[ProDave]

I would not rely on just the DNO's earth over that distance. I would probably still connect to it and add my own local rod(s) making it a true Protective MULTIPLE Earth

[Nickfromwales]

Aren't you better off with just a rod at that stage so you can't get two separate earths at two different potentials?

( Didnt Harold mention this in one of his rants? )

[ProDave]

That is debatable and some have very strong opinions.

 

If you just rod it and use a TT earth, then your earth impedance will never be as low as a PME earth. However I would never just export a PME earth that distance.  By adding a local rod, you are reinforcing the concept of the multiple earthing. A true PME is earthed at many places along it's route though no doubt the DNO stick a PME sticker on any TNCS supply even if there is no multiple earthing (PME is always TNCS, but TNCS is not always PME)

 

This is exactly what I have done for my house, even though my distance is about 20 metres. (my static caravan is on it's own TT earth as the regs prohibit connecting a caravan to a PME earth)

[Jeremy Harris]

12 minutes ago, Nickfromwales said:

Aren't you better off with just a rod at that stage so you can't get two separate earths at two different potentials?

( Didnt Harold mention this in one of his rants? )

 

Yes, there was indeed a rant at me over a typo related to exactly this.  I chose to keep my workshop PE separate from the incoming exported earth, which is a strict interpretation of the regs.  There's nothing in the regs I'm aware of that specifically says you can't add a supplemental earth, making, as Dave says, a true PME installation.  This is just an extension of the local LV distribution network topology, that has a number of intermediate earth connections along the length of the supply cables, in order to ensure that the earth impedance at the consumer end is always low.

 

If you want to stay strictly within a defined scheme, and not cause too much angst with whoever is signing it off, then it's probably better to just consider the consumer end of the long cable as not providing a PE, and using a standard TT earthing arrangement.  That will be understood well by whoever is signing things off if they get a bit pedantic.  There's no good reason why you can't just do exactly as Dave suggests though, as although unusual it is safe.

 

Edited July 29, 2017 by JSHarris

[Onoff]

8 minutes ago, Nickfromwales said:

Aren't you better off with just a rod at that stage so you can't get two separate earths at two different potentials?

( Didnt Harold mention this in one of his rants? )

 

In true PME the rods can be at different potentials anyway along the run. You can't guarantee ground conditions. At various points it could be very wet, bone dry, frozen and contracted away from the rod etc.

[Onoff]

This "underwater" thing and a 2-core SWA.....

 

IF the outer sheath got damaged then wouldn't that expose the armoured strands to potentially rapid corrosion as in if submersed? 

[Russell griffiths]

I think the underwater cables days are numbered, it will definitely get removed if and when we get consent for a new house. 

[Onoff]

I've just had a skim through BS5467 (it takes me away from the bathroom) ref armoured cables and it says specifically that the cable may be laid direct in "free draining soil" or in a duct itself in free draining soil. In a lake its hardly free draining! 

 

There was me thinking it's be the one time when laying it in blue MDPE might be the way to go!

[Russell griffiths]

So are we sort of saying you have an Earth that runs all the way from the meter aprox 100m but also have a spike local to the cu as belt and braces type of thing. 

 

If this earth runs  from the meter box is it a separate single to the swa or is it contained within the swa 

 

hope that makes sense. 

 

Any thoughts on depth this new cable should be at. Minimum depth required to avoid finding other services. 

 

So what is a TT earth and what is PME. 

Edited July 29, 2017 by Russell griffiths
Forgot a bit.

[Onoff]

22 minutes ago, Russell griffiths said:

So are we sort of saying you have an Earth that runs all the way from the meter aprox 100m but also have a spike local to the cu as belt and braces type of thing. 

 

If this earth runs  from the meter box is it a separate single to the swa or is it contained within the swa 

 

hope that makes sense. 

 

Any thoughts on depth this new cable should be at. Minimum depth required to avoid finding other services. 

 

So what is a TT earth and what is PME. 

 

Start here:

 

http://electrical.theiet.org/wiring-matters/16/earthing-questions.cfm?type=pdf

 

EDIT: Link old but good pics, same as in the current AMD3 On Site Guide I have here.....and I can't really scan that! 

Edited July 29, 2017 by Onoff

[Nickfromwales]

1 hour ago, Onoff said:

 

In true PME the rods can be at different potentials anyway along the run. You can't guarantee ground conditions. At various points it could be very wet, bone dry, frozen and contracted away from the rod etc.

But the consumer does not have access to these points. If you send a DNO generated earth AND provide a rod at the consumer location, and said rod earth becomes detached / high resistance, is that not where the problem lays?

Were not talking about the local area network, which isn't public domain, we're talking about someone's home here so best keep the T's crossed ?.

Ill get my coat and tin hat ready ?

[Jeremy Harris]

11 minutes ago, Nickfromwales said:

But the consumer does not have access to these points. If you send a DNO generated earth AND provide a rod at the consumer location, and said rod earth becomes detached / high resistance, is that not where the problem lays?

Were not talking about the local area network, which isn't public domain, we're talking about someone's home here so best keep the T's crossed ?.

Ill get my coat and tin hat ready ?

 

There's no logical reason to not have an additional local earth connection at the end of a long cable, it's just adding yet another multiple earth to a system that will already have several earth connections (you can see them on poles if you look around).  It's also allowable in BS7671 for the "customers earth" to be connected to the PEN coming in, as shown in this figure I've just scanned in from the 17th Edition (amendment 1, so not quite the current regs, but this is still the same):

 

[Onoff]

Easiest way to put this to bed about adding extra "rods".....

 

You've likely already got extra "rods" in the form of any metal services coming in...which you bond to. Another one won't hurt.

[Jeremy Harris]

16 minutes ago, Onoff said:

Easiest way to put this to bed about adding extra "rods".....

 

You've likely already got extra "rods" in the form of any metal services coming in...which you bond to. Another one won't hurt.

 

A point I made offline to the person that had a serious pop at me, but all it did was make him more abusive........................

 

I wouldn't have minded, but the first electrician I used was the one that had a problem with signing off the workshop with an additional earth rod connected to the exported PE, which is why I fitted an insulated box where the SWA came up through the floor, together with another box next to it to house the termination at the top of the long earth rod that goes down through the slab and deep enough into the underlying soil to have hit the water table, I think.  My original plan was just to connect the additional earth rod to the PE at the incoming side of the workshop consumer unit, but he wasn't happy with it.  No idea why, but it was his name on the chit, so I just accepted it and made the workshop TT.

Edited July 29, 2017 by JSHarris