Tns main protective bonding conductor csa

[jonney88]

Hi just checking the main protective bonding conductor size on a tns system, there are two supplys the tails sizes are both (630x2 the supply earth is 240 and neutral is 630x1) persupply
designer aspecified main erthing conductorto met fothe main protective bonding as 2 x 240 i think this i overkill andthe lad on site have installedthis andare putting 50mm forall bonding i think it should be 25mm but looking at table 5.1 in guidence note 8 the table only goes upto 400mm tails
therfore wat size conductor to a: The met forthe bonding, b: The met to the pipework,steel etc
i thnk only nees 25mm

 

[Richard Burns]

SO long as you are sure it is TNS earthing arrangement then the main equipotential bonding does not normally need to be above 25mm².
However if it may be TCNS then 50mm² should be OK.
If you have a designers specification then it would be wise to check why they have specified 2x240mm² as you would be liable for the cost of changing it if there is a cogent reason.
this would seem to be an indication of a TCNS system with a dubious neutral connection!!

 

[jonney88]

its is 100% an tns system, there are two supplys to the lv panel,630mm per phase then a single 630mm for the neutral, and a seperat 240mm single as the earth,, both earths on the supply tag onto the integral earth bar inside the panel, then off the earthbar,, designer has said 2x240mm onto our earth bar for the bonding, then iv said thers no need it should have been only 25mm to feed the earthbar then 25mm to rest of things to be bonded am I not right??

 

[jonney88]

its is 100% an tns system, there are two supplys to the lv panel,2x630mm per phase then a single 630mm for the neutral, and a seperat 240mm single as the earth all of tis again for othr supply
,, both earths on the supply tag onto the integral earth bar inside the panel, then off the earthbar,, designer has said 2x240mm onto our earth bar for the bonding, then iv said thers no need it should have been only 25mm to feed the earthbar then 25mm to rest of things to be bonded am I not right??

 

[Pete999]

its is 100% an tns system, there are two supplys to the lv panel,2x630mm per phase then a single 630mm for the neutral, and a seperat 240mm single as the earth all of tis again for othr supply
,, both earths on the supply tag onto the integral earth bar inside the panel, then off the earthbar,, designer has said 2x240mm onto our earth bar for the bonding, then iv said thers no need it should have been only 25mm to feed the earthbar then 25mm to rest of things to be bonded am I not right??

Had a few beers this afternoon so forgive me if I sound a bit dense, 2x630? don't understand

 

[jonney88]

630mm singles two of them per phase with 1x 630m single for eutral, and two supplys with same size and numbe of cables

 

[Leesparkykent]

If it's TNS then 25mm is fine. Reg 544.1.1

 

[jonney88]

that's wat I thought should, didn't want to be wrong ther putting 50mm in for some reason

 

[Leesparkykent]

that's wat I thought should, didn't want to be wrong ther putting 50mm in for some reason

Belts and braces I suppose. Least it's not coming out of your pocket:smile5:

 

[jonney88]

well im there testing it so need to advise them otherwise but hardwork sometimes, people don't want to listen, can anything increase the size of the main protective bonding conductors exept the resistance of the cable in the point your trying to bond is a fair distance away

 

[Richard Burns]

If the main protective bonding conductors are also being used as circuit protective conductors for a remote power supply then the conductor must meet both csa requirements for earthing and bonding.

 

[uksparks]

630mm? Jeepers what is it that you are working on?

 

[shanky887614]

630mm? Jeepers what is it that you are working on?

it could be ali at that size or is it 3.3kv? sounds like a large factory to me but i could be wrong

 

[jonney88]

its an aeroplane hanger, it's only 2500amp

 

[uksparks]

What in an aeroplane hanger needs so much sodding power?

 

[uksparks]

its an aeroplane hanger, it's only 2500amp

Only 2500A, I like that, I only ever touch small fry

 

[jonney88]

Only 2500A, I like that, I only ever touch small fry

There's one machine on its own that requires a 1000amp and runs at a power factor of 0.4

 

[uksparks]

What does it do? Do you know?

 

[jonney88]

I know exactly wat it does, I can't say the company were working for moniters everything, takes half a day just to get on site

 

[davesparks]

Why are you questioning the designers? They have designed the installation and accept the liability for that design.
So what if you think it is over specced? There could be many reasons for this, especially in an environment with a lot of high tech equipment like that.
Where does the main bond for the lightning protection and structural steel terminate? These may well need to be a lot bigger than 25mm.