Earth Cable To Sub Distribution Board.

[gerard]

Hi All,

I recently installed a new dist. board with 95mm sq 4 core swa cable. I used 35mm sq earthing cable even though I should have used roughly half the size of the live conductor. I used this because I had it, and was hoping it will be ok. I now fear that the cross section of the cpc. Is it possible to install an earthing point close to the new board to prevent me from installing a new earth cable?

Confused?!?

 

[telectrix]

can you calculate for the armour and add that to the 35mm, see if it satisfies the adiabatic?

 

[gerard]

I could only terminate the swa on one end with a compression swa gland, access to the other side made it impossible so I just used a brass bushing. So, although it's a good idea, I can't use the armour. Any thoughts on another earthing point?

 

[Darkwood]

The armour needs earthing correctly but if you run a seperate earth that meets the csa needs of the sub mains then you only need to earth the swa at the supply end, but it still must be earthed regardless of the additional earth cable.

 

[Knobhead]

Hi All,

I recently installed a new dist. board with 95mm sq 4 core swa cable. I used 35mm sq earthing cable even though I should have used roughly half the size of the live conductor. I used this because I had it, and was hoping it will be ok. I now fear that the cross section of the cpc. Is it possible to install an earthing point close to the new board to prevent me from installing a new earth cable?

Confused?!?

I could only terminate the swa on one end with a compression swa gland, access to the other side made it impossible so I just used a brass bushing. So, although it's a good idea, I can't use the armour. Any thoughts on another earthing point?

I’m sorry Gerard. But all the above would be totally unacceptable to me!

 

[Engineer54]

Hi All,

I recently installed a new dist. board with 95mm sq 4 core swa cable. I used 35mm sq earthing cable even though I should have used roughly half the size of the live conductor. I used this because I had it, and was hoping it will be ok. I now fear that the cross section of the cpc. Is it possible to install an earthing point close to the new board to prevent me from installing a new earth cable?

Confused?!?

Had you terminated this 4 core 95mm SWA correctly, you would not have needed any separate CPC. The armouring would have been more than sufficient for your needs....

 

[Bobster]

What's a compression SWA gland? Just a normal stuffing gland?

 

[Engineer54]

What's a compression SWA gland? Just a normal stuffing gland?

I'm hoping he means an external SWA gland, with the bushed stuffing section at the rear!! ...Fingers crossed like!! lol!!

 

[Bobster]

I do hope it's a cw gland.

By brass bushing I'm hoping its not just got the swa shoved through the hole in the panel.



edit:

I've seen it before so it wouldn't supprise me.

 

[Darkwood]

You can get liquid tight swa gland which is like a stuffing gland on the back of the swa gland to seal upto the sheath cw are not waterproof as many assume with their outdoor use.
Ive a feeling though this isnt the gland hes refering to.

 

[drew35]

The way it reads to me is that one end of the SWA is just sitting inside a brass bush.

 

[Bobster]

You can get liquid tight swa gland which is like a stuffing gland on the back of the swa gland to seal upto the sheath cw are not waterproof as many assume with their outdoor use.
Ive a feeling though this isnt the gland hes refering to.

Yes it's stated for outdoor use with cw glands, never seen a problem with properly fitted ones, even with a jet wash used to clean the plant down. The mistake most people make is over tightening the compression seal.

I have seen the ip65 swa glands, but I don't use them very often.

 

[Engineer54]

I prefer to use internally glued heat shrink than the standard shrouds in externally exposed locations, that are subject to adverse conditions...

 

[gerard]

The swa is passed into the panel at one through a 50mm brass bush. I considered this OK under the circumstances as:
1. I would never have been able to fit a cw gland
2. the cable has no chance of moving as it is under ground, well supported, will never move and completely protected from people and elements.
My only concern is the size of the CPC, it is 35mm sq and the rule of thumb is half the size of live conductor. I do think the cpc is sufficient but I would be happier if it was say 50mm sq. So, my original question was - can an earth rod be installed near a new db to compensate for the undersized cpc?

 

[johnboy6083]

In my opinion, there's not many places a gland can't be used. Even if you have to use the cone types with a jubilee clip like the ones found on some street lighting cut-outs. Not my favourite by a long shot, but would be better than not bothering at all.

As for the CPC, why can't the adiabatic be used? A 1 min calc, and you will be left in no doubt.

 

[Darkwood]

I sorry to side step your question here but the cable has to be glanded correctly regardless of how you found it, if you do any work on the sub-mains you take on the responsibility for the safety and condition of the existing supply to the submains, it may be buried but you cant predict issues like, subsidence, chemicals in the ground, inherent fault with the cable and any or other issues could create a fault down to the armour and if you havent correctly terminated it it could make the armour live without operating a protective device.

If you are in a situation as you say and truely can't gland it then i would suggest you cut a ring section of outer sheath away and fit a DNO style cable sheath earthing clamp (not a bonding clip!), then i would seal it with plenty wraps of amalgaming tape, this cable armouring must be earth at the supply regardless of the limited risks you perceive. Also because it has no rigid fixing to the dist' board you need to ensure the cable is fully supported and cannot be physically moved this way no movement will stop the termination from working loose.

In answer to your question the 35mm as you have realised is under sized and i would say no rodding the local ground, IMHO this isnt a solution and you will find it hard to prove compliance although others may suggest otherwise for which im open to discussion, rodding is really for TT where its your only option regarding earthing and a few other situations where you cannot export the earth but when your installation has a full earth provided system and you are not exporting it then you should be using the earthing provided.


I know you have probably looked over this time and time again but is it really impossible to fit a gland as this seems hard to believe or is it just really arkward and inconvenient.

I also drag up the correct glanding issue because if you can manage it then all your worries are solved regarding compliance.

 

[Knobhead]

Earth rod, where did that come from?

I'm off before I blow a gasket!

 

[gerard]

most definitely impossible to fit a gland, cable is 100% secure and in no fear of damage or movement. ok, so an additional earthing point is out of the question then!! in reality the current drawn by the board will never be large enough to render the cpc dangerous, but I would be happier with next size up cpc

 

[Bobster]

most definitely impossible to fit a gland, cable is 100% secure and in no fear of damage or movement. ok, so an additional earthing point is out of the question then!! in reality the current drawn by the board will never be large enough to render the cpc dangerous, but I would be happier with next size up cpc

I've fitted glands in seemingly impossible places before and with much larger cables. That's not my main concern.

If this is a new install, why did you not just work out the size of the cpc you need using the adiabatic equation? I'm sorry but something about this job seems like you've just thrown it in using a 'rule of thumb' and hoped it's ok.

 

[johnboy6083]

Do you know how to do the adiabatic?

 

[Carnegstar]

and all the cores of the swa are used?

 

[Darkwood]

The earthing is there as a safety path and/or functional earthing for electronic equipment you need to do the adiabatic equation to see if the 35mm will comply as the half size the phase conductor is a rule of thumb (cables 35mm or larger) and a quick way to know you comply but the adiabatic equation will eithe confirm compliance or show none compliance and if the latter additional earthing is required... dont forget the earth has to withstand fault current without failing so this is regardless of the max current demand of the board. I would also re-read my last post regarding earthing the armoured; its not a choice it has to be done regardless but as long as the supply end of the armoured is correctly earthed then you are not commited to doing the load end unless you are using the armouring as part or all your earth path, if you can get a DNO style earthing clamp fitted at the supply end then all you need to to correctly terminate the load side and you have effectively earth up the amouring and it can be used to back up the 35mm earth.

If you are not familiar with the adiabatic equation then you shouldn't be undertaking this work in the first place (i say this because i dont know your knowledge or qualifications) so dont take it as an insult, i have to make a judgment on you level of questioning.

 

[Bobster]

dont forget the earth has to withstand fault current without failing so this is regardless of the max current demand of the board. I would also re-read my last post regarding earthing the armoured; its not a choice it has to be done regardless but as long as the supply end of the armoured is correctly earthed then you are not commited to doing the load end unless you are using the armouring as part or all your earth path, if you can get a DNO style earthing clamp fitted at the supply end then all you need to to correctly terminate the load side and you have effectively earth up the armouring and it can be used to back up the 35mm earth.
.

Depends on the environment he is working in, M&Q is a must both ends are earthed.

 

[Darkwood]

You are correct Rob but was refering to the OP's situation, if i start detailing all the exceptions etc il be writing a novel on here im trying to give enough to work with without adding confusion.

 

[Bobster]

You are correct Rob but was refering to the OP's situation, if i start detailing all the exceptions etc il be writing a novel on here im trying to give enough to work with without adding confusion.

I do realise, just without knowing the environment the op works in, I wanted to point out that there are exceptions. 95% of the time your advice given above will be correct it's up to the op/ whoever reads this in the future to decide whether it's applicable to them.

 

[Darkwood]

I do realise, just without knowing the environment the op works in, I wanted to point out that there are exceptions. 95% of the time your advice given above will be correct it's up to the op/ whoever reads this in the future to decide whether it's applicable to them.

Yes agree looking from a 3rd person point of view i see where your coming from and probably was worth a mention as you have kindly done

 

[gerard]

I was originally going on the half the size of the live conductor theory, I used 35mm sq because I had it, but after I installed it I didn't feel comfortable. As I said, I could only gland one end - at the db. So, after all this, I will look further at glanding or clamping the cable at both ends - gonna be tough.

 

[trebor]

The swa is passed into the panel at one through a 50mm brass bush. I considered this OK under the circumstances as:
1. I would never have been able to fit a cw gland
2. the cable has no chance of moving as it is under ground, well supported, will never move and completely protected from people and elements.
My only concern is the size of the CPC, it is 35mm sq and the rule of thumb is half the size of live conductor. I do think the cpc is sufficient but I would be happier if it was say 50mm sq. So, my original question was - can an earth rod be installed near a new db to compensate for the undersized cpc?

Disregarding the installation arguments and sticking with the cpc sizing, I dont know of the rule of thumb for cps half the size of live conductors, surely the csa of seperate conductor which is a cpc for the armour should be calculated on 2 requirements
1. the Zs at far board complies with the protective devices needs,therefore the length and value of R1+R2 needs to be taken into account
2. It complies with adiabatic rules, which can only be determined after 1 has been calculated, it could well be that the 35mm is oversize for both


It is not good practice to add the csa of the armour and any supplemetary cpc together as you can not guarantee the split of fault current between the 2, good practice dictates the seperate conductor should be capable of fulfilling the requirements in its own rights

 

[Darkwood]

Your are correct in what you say Trebor but OP at the moment hasn't earthed the armouring correctly as he cant gland the supply end and was hoping that 35mm earth would alone suffice, my post was on the base that if he manages to gland the swa properly then it would comply on its own irrespective of the 35mm extra earth, we can assume hes not in a position to add an additional or run a larger earth.
Its a 95mm cable and would require a 50mm if not using adiabatic equation but we've already advised him to check if their is a chance the 35mm will suffice by using the adiabatic equation.

 

[Knobhead]

This all comes down to idleness!

I can’t make the gland off: there’s no excuse for this.
It’s what I had on the van: vans have engines, they move! Use it to go and get the correct cable. Cable, which you wouldn’t need if the SWA cable is made off correctly.

Both RoB and I have worked to M&Q, we would be looking for a new job if either of us pulled a stunt like this

 

[gerard]

I am going to get the earth fault loop impedance valve and work out exactly the pscc is, and if necessary, increase the size of the conductor. As for glanding the swa at the supply end, the best I can do is fabricate a clamp and connect to earth.

 

[SYKRAPS]

Should you not ask yourself these questions before you perform the install?