Cable size

uksparks · Jan 5, 2015

Hi,

I am going to be connecting an out door power supply, away from the building, its all metal casing.

The supply is 10mm, I am using a 3 core SWA for part of the run, the inside part was going to be in twin and earth, but then I was just thinking, going on table 54.8 section 544.1, am I looking into this too much or does it need to have a 10mm earth as its extraneous metal which is fixed into the ground?

In which case I was just thinking of running 3x 10mm inside the building so the earth is 10mm too to them comply with min size of bonding conductor.

I am right to make it 10mm throughout the entire run aren't i?

davesparks · Jan 6, 2015

HandySparks said:
I think that this would be dependent on the method of connection to the rod. davesparks' option of brazing would count as maintenance-free, but it's hard to think of any other method that would be acceptable buried.

Exothermic welding is the preferred method for buried earthing connections, but is rarely going to be available to the likes of us mere mortals. Brazing is a reasonable and cheaper alternative.

Engineer54 · Jan 6, 2015

you don't gain much more reduction in Ra going deeper than 8'

I'd have to disagree with you there, the deeper you go, the better the will be the Ra value. Many of the rod installations on certain projects i've been involved in, have been extended (2 X 8') rods, but these have been cored/drilled Bentonite/Marconite filled holes. However, if we are talking ''driven rods'' you might have a problem going much deeper than 8', depending on the type of ground condition and method of driving the rod in the ground. If space between multiple rod locations is going to be a problem, i'd certainly give another 4' extension rod a try, especially if Ra values were coming up favourable after two rods...

Engineer54 · Jan 6, 2015

uksparks said:
So the tops of the rods, are you suggesting they are just left covered in mud? Seems wrong to me.

The tops of the rods need to be at least 20cm below FGL where exothermic or brazed welding connections are used without an earth pit.

davesparks said:
The best thing for linking rods together is bare copper conductors, it helps to lower the Ra as it then acts as horizontal electrodes.

But bare copper conductors are not cheap!

The correct bare copper conductor used in direct buried situations is called ''Hard Drawn'' (also used in LP installations) and is a lot more expensive than the soft copper used in normal cables. I wouldn't bother, just use normal G&Y insulated cable. Using bare copper cables can also interfere with Zones of Influence of the TT rod locations...

ImpededLoop · Jan 6, 2015

Is the building a steel structure or just steel cladding? If it's a steel structure it's possible it has a low enough Ra already without needing any rods.

uksparks · Jan 6, 2015

Engineer54 said:
The tops of the rods need to be at least 20cm below FGL where exothermic or brazed welding connections are used without an earth pit.

Ok thanks, I presumed it would have to be a bit into the ground. Whats the best method of brazing / soldering them? clamp round it and also solder it?

ImpeededLoop said:
Is the building a steel structure or just steel cladding? If it's a steel structure it's possible it has a low enough Ra already without needing any rods.

I would think its a steel structure looking at it, because when you look in the ceilings, all you can see is big fat steels everywhere, no bricks anywhere to see, so would imagine it's totally steel. I would not like to be 100% on this, but looking at it, theres no block work anywhere.

uksparks · Jan 6, 2015

If you look I have got to put the charger where the yellowish square is I have just marked on this photo.

The dots, I can then put a rod in at each, its just a 15m long bit of grass, so isnt too big a problem, cutting a slot and popping them in.

davesparks · Jan 6, 2015

Does that building have any lightning protection system installed?
Have you checked the EIC and any other paperwork to see what the deal is with all of this? If expect to see information in the electrical O&M documentation.

uksparks · Jan 6, 2015

Unfortunately they didn't have any of this information to hand, ill call them in a min and see if they can find it.

Engineer54 · Jan 6, 2015

uksparks said:
Ok thanks, I presumed it would have to be a bit into the ground. Whats the best method of brazing / soldering them? clamp round it and also solder it?

Exothermic welding is the prefered method, (soldering is NOT an option). Brazing you'll need a gas/oxygen set up and a person that knows how to use them. Best find someone that has the kit and get him to do the work. Anyone on this industrial/commercial estate, like a garage of some kind??

I would think its a steel structure looking at it, because when you look in the ceilings, all you can see is big fat steels everywhere, no bricks anywhere to see, so would imagine it's totally steel. I would not like to be 100% on this, but looking at it, theres no block work anywhere.

If you can test this structure with regard to it's ability to provide it's own earthing as an electrode then you may not need a separate TT system. But then if a PME supply and bonded to the buildings MET as it must be, it then can't be used to provide a separate TT system for a car charging point. In fact if this building is one big Ufer earth, ''Zones of Influence'' of any small TT system in the vicinity will probably be adversely affected...

If the structure is an all metal structure the DNO can i believe provide a sort of sudo TN-S supply via the PME network distribution cable... Members that work for DNO's here may be able to give you more information on this...

UNG · Jan 6, 2015

uksparks said:
I would drill diagonally, but its a metal double skin wall, brand spanking new building and I don't want to make a mess of it. Also that would then leave a slight issue of stuffing round the hole which could be resolved.

As for the joint, I was wondering this, can I do that with a wiska box? I have never tried glanding into the back of one.

A Dektite seal or small cable transit would close any cable entry route into the building

uksparks said:
There's were £4000 more min, I'm oonly talking a few earth rods here.

uksparks said:
The cost of running another earthing conductor in is not a problem, it's just the issue of the rods really, I appreciate your help on this, it's very interesting and had opened my eyes.

I think they were just rip off merchants to be honest.

The rods are cheap, it's more the hassle of putting them in.

It's always the apparently simple things that run away with the money in this type of job.

andyelectric · Jan 6, 2015

I've been following this thread with interest as I have no experience of earth rods. (We had a huge metal ship as earth).

uksparks, if I were you I'd be calling up one of your other sparky mates who has had experience of a similar installs (with respect to earthing) to share the job. That way you will see it completed properly. At present this thread seems to be a bit of all full on teach in.

uksparks · Jan 6, 2015

I have got hold of the original installation certificates, amoungst other things, it says:

Three phase 4 wire
TN-C-S Supply
PFC: 3.38kA
Ze: 0.13 ohms

16mm Earthing Conductor
10mm bonding to Water and Steel, no gas