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]

It just does not. I am not dismissing you or belittle your post. I am all ears and would like to understand more. I would have though somewhere in BS7671 it would say something like to earthing systems cannot be in one location. Guess guess guess. Not fact!!!!

don't get many swimming pools on the railways although the unions are thinking about this 'right' in there current talks.

You aren't mixing earthing systems.

A PME supply has an earth rod connected at regular intervals from the substation right up to the end of the cable run. All you are doing is adding another electrode which is connected at the MET of the installation. The big danger with PME is when the neutral becomes broken between your installation and the DNOs earth rods, having your own rod connected to your installation mitigates the danger in your installation.

 

[Guest125]

If the Rods can achieve around sub ohm Ra they will be effective, also the way I read it they are as backup in case of a broken DNO Neutral to keep Ut low.

I hear you and it makes so much sense I am at a loss to why it is not done on all PME installations. There must be a reason no?

 

[davesparks]

Wont the fault then take the path with least resistance which is bound to be the PME earth which pretty much makes the earth rod pointless?

Its not there for the fault current, it's there to maintain the installation's earthing and bonding at true earth potential in the event of the loss of the supply CNE connection

 

[Guest125]

You aren't mixing earthing systems.

A PME supply has an earth rod connected at regular intervals from the substation right up to the end of the cable run. All you are doing is adding another electrode which is connected at the MET of the installation. The big danger with PME is when the neutral becomes broken between your installation and the DNOs earth rods, having your own rod connected to your installation mitigates the danger in your installation.

see post 122

 

[telectrix]

Wont the fault then take the path with least resistance which is bound to be the PME earth which pretty much makes the earth rod pointless?

not pointless. in the event of the supply N being lost, the rod will provide an alternative earth path for a fault on the installation.

 

[uksparks]

Ill have to go back and dig a hole to see whats there, I will not be able to just leave this and turn up on the day to do the job.

So are you saying connect the rod to the end of the circuit yes?

 

[Guest125]

I hear you all. I just cannot believe this is not a standard practice. Seems like sense in all PME supplies.

 

[Leesparkykent]

no connect the rod to the main earthing terminal not at the end of the circuit

 

[Guest125]

Ill have to go back and dig a hole to see whats there, I will not be able to just leave this and turn up on the day to do the job.

So are you saying connect the rod to the end of the circuit yes?

I have no reason to disbelieve what these guys are saying with regards to earth ridding to the MET.

 

[uksparks]

Not to the MET. The rod would earth the VCP only.

You say not to the MET...

Why doesn't it sound right? Adding an extra earth electrode to the PME supply is only going to improve it and make your installation safer in the event of an external neutral fault. This is exactly what is recommended in the regulations for swimming pools fed via a PME supply and is required practice for PME supplies in other parts of the world.

You say at the end of the circuit.

no connect the rod to the main earthing terminal not at the end of the circuit

You say to the MET

I have no reason to disbelieve what these guys are saying with regards to earth ridding to the MET.

You are now saying to the MET.

Which is it??

 

[davesparks]

So are you saying connect the rod to the end of the circuit yes?

No, although this is my opinion only at the moment as I can't reference regulations off the too of my head, my logic is as follows.
This earth nest (not just a rod, multiple rods interconnected) is going to have a low (sub 2 ohm hopefully) Ra so it will see its share of diverted neutral currents from the whole installation. I would prefer that this wasn't flowing through the circuit CPC so that of anyone working there in the future disconnects the circuit they aren't going to get a surprise when they disconnect the CPC.

 

[davesparks]

You say not to the MET...



You say at the end of the circuit.



You say to the MET



You are now saying to the MET.

Which is it??

I didn't say the end of the circuit, or if I did it was a mistake.

 

[Leesparkykent]

i havent got the BYB yet but ive had a flick through the BGB and from your situation i think this option of connecting an earth electrode to the MET is the best in this situation.

 

[uksparks]

Can I not connect it to the armouring to take it back to the MET or the CPC of the circuit? Do I have to run in another 10mm earth, I then have the hassle of protecting the earth cable when it leaves the wall and into the ground...

 

[uksparks]

I didn't say the end of the circuit, or if I did it was a mistake.

That was probably be misreading you.