Cable size

[uksparks]

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]

Maybe not at the moment it isn't, but I seem recall Tony of this parish had overheating Bonding conductors, and I don't fully trust BS951 clamps, or plumbers replacing them either properly or at all ;-) plus if you are going to all this bother in the first place why not ? it would not cost that much more in the scheme of things now that we are trying to achieve sub ohm TT rod nests, just my opinion of course

Not at the moment? its going to have to be a long way out of balance to get a neutral current large enough to bother 10mm.

I'm not disagreeing with you and do agree that your suggestion of 16mm is a better idea. Just wondering how much neutral current there will ever be in the installation, seeing as it is three phase and the phases should be well balanced as its a new install.

 

[davesparks]

Yes so the plan is to bang a stake in, get a Good reading and connect it to the end of the circuit.

A stake? what use is that going to be? wherever you decide to put it you need to put more than one stake in! otherwise its just a pointless exercise.

 

[ImpededLoop]

Just wondering how much neutral current there will ever be in the installation, seeing as it is three phase and the phases should be well balanced as its a new install.

It depends partly on where on the DNO network the neutral break occurs and whether other customers are also affected.

In theory there should be multiple earth rods along the CNE to limit the problem.

 

[spark 68]

Not at the moment? its going to have to be a long way out of balance to get a neutral current large enough to bother 10mm.

I'm not disagreeing with you and do agree that your suggestion of 16mm is a better idea. Just wondering how much neutral current there will ever be in the installation, seeing as it is three phase and the phases should be well balanced as its a new install.

Triplens ?

In all honesty I don't even know what the building is used for not disagreeing either just a good discussion, and interesting

 

[davesparks]

It depends partly on where on the DNO network the neutral break occurs and whether other customers are also affected.

In theory there should be multiple earth rods along the CNE to limit the problem.

The idea of having an earth nest connected to the MET is that you have a connection to earth under your control and so that you are protected against breaks in the CNE conductor between your installation and all of the DNO earth rods

 

[ImpededLoop]

The idea of having an earth nest connected to the MET is that you have a connection to earth under your control and so that you are protected against breaks in the CNE conductor between your installation and all of the DNO earth rods

Yes, totally agree with your thinking Dave.

Sorry, my post related to the potential magnitude of a CNE failure without any customer installed rods etc.

 

[davesparks]

Triplens ?

In all honesty I don't even know what the building is used for not disagreeing either just a good discussion, and interesting

Ah yes, the good old harmonic problem, that does put another spin on it.

If I was doing this I would have been looking at at least 2 earth electrodes each with a seperate 10mm back to the MET, so in theory under a lost neutral connection the neutral current would divide between the 2(or more) rods (assuming similar Ra for each rod)

 

[spark 68]

Ah yes, the good old harmonic problem, that does put another spin on it.

If I was doing this I would have been looking at at least 2 earth electrodes each with a seperate 10mm back to the MET, so in theory under a lost neutral connection the neutral current would divide between the 2(or more) rods (assuming similar Ra for each rod)

Ah! that is where my thinking slightly differed, I was thinking of linking all the rods together locally with a single cable back to the MET, I was also expecting maybe 4 rods or more to get a low enough Ra, soil type dependent of course

 

[uksparks]

A stake? what use is that going to be? wherever you decide to put it you need to put more than one stake in! otherwise its just a pointless exercise.

How do you know one rod will not be enough?

 

[ImpededLoop]

Ah! that is where my thinking slightly differed, I was thinking of linking all the rods together locally with a single cable back to the MET, I was also expecting maybe 4 rods or more to get a low enough Ra, soil type dependent of course

If you decide to connect the rods directly to the MET, bear in mind that they don't need to all be right next to the car charging point. It might give you easier cable runs.

 

[uksparks]

Ah! that is where my thinking slightly differed, I was thinking of linking all the rods together locally with a single cable back to the MET, I was also expecting maybe 4 rods or more to get a low enough Ra, soil type dependent of course

4 rods? I'm not suggesting for one min this is going to be acceptable, but sub 200 ohms is deemed as stable, so it would be easy to satisfy that requirement, but it's satisfying the other part of the equation.

 

[spark 68]

If you decide to connect the rods directly to the MET, bear in mind that they don't need to all be right next to the car charging point. It might give you easier cable runs.

The OP was planning on putting the rods close to the EVC to keep the cabling simpler, I was just rolling with that as he knows the site and I don't

 

[spark 68]

4 rods? I'm not suggesting for one min this is going to be acceptable, but sub 200 ohms is deemed as stable, so it would be easy to satisfy that requirement, but it's satisfying the other part of the equation.

Not for your purposes 200 ohms is anywhere near acceptable, we are aiming for about 1 ohm if not lower lol have you looked at the calc Richard Burns did on the other thread you posted earlier ? we are not talking about TT here

 

[davesparks]

How do you know one rod will not be enough?

Unless you are specially ordering 8' rods from someone like furse then you'll be using the same 4' long rods we all use no? A single 4' long rod will not provide a stable resistance to earth, you'll need at least 8' depth to get stability so will need at least 2 joined (i recommend solder/braze the joint to stop it rattling loose when you wallop it in)

Add to this I have never yet managed to get a single figure Ra with a single rod, and even if I did I wouldn't trust that to remain stable.

 

[uksparks]

No I missed that one.

4 rods equals 4 rod pits which equals more cash...