Urgent help please

[danny7299]

There is a 800amp supply been installed from a sub station to our building they have doubled up 2x 240mm so it's s 800mm cable ... What size earth should be in ?

 

[PC Electrics]

When I was at school, 2 x 240 = 480 ?

 

[danny7299]

yes thanks for that ...as you can see i am in a rush .... i need help on the earth size ... will explane later ....

 

[Andy™]

earthing type?

 

[danny7299]

tncs

 

[Andy™]

if same material, it must be half or greater of the size of live conductors (table 54.7)

or you could use adiabatic to find smaller size

 

[sparkytim]

BRB Pg 130

S> 35mm MIN CSA OF PROTECTIVE CONDUCTOR = S/2

S = LINE CONDUCTOR 480mm

S/2 = 240mm

or do adiabatic equation

 

[ian3012]

this job is way to big for me danny... but you still gotta go by the regs.

so there's 2 ways to go.... by reference or calculation.

table 54.7 (pg 130 regs) says that if your earthing conductor is the same material as your line conductor (eg if line is copper then earthing has to be copper)then S/2

applies.... for e.g. if you got 240mm of line then you need 120mm of earth etc

you will very probably be able to reduce this by using the adiabatic equation

reg 543.1.3... but to calculate that you'll need the pfc , characteristics of the protective device and the installation method of the earthing conductor.

 

[ian3012]

haha beat me to it tim.... just don't forget about conductors being the same material....

 

[ian3012]

actually why not phone the dno and ask them.... they might have a minimum size or if you speak to an engineer he'll just tell you anyway?

 

[danny7299]

Thanks chaps i thought it was half the size....

here is the reason for the rush... I have recently taken a new cutomer on ( taken the custom away from my old boss) and he had had the old customer for 15 years and had built the builing from scratch , they got rid of him because he was getting worse and worse at showing up...

so... i started working for them about a week ago when the had a huge problem and lost half of the power to the factory, a 400a cable had gone boom so they needed it replacing, when i was connecting it up the new cable i noticed that the bigest earth on the earth bar was the one i was putting in... and made me start asking questions.

i called the manager and asked him about it and he said that they would get hold of my old boss and ask him why he has put a 480mm cable in with only a 50mm Earth !!!

so got a phone call today , could i go stright to the factory as they where having problems... when i got there i relized they where having problems with my old boss, they had called him in and he was saying the what he had put in was correct and that he had been adviced by the electric board to put in what he has.

If i would have known why they where calling me in i could have turned up with my regs book and there for had hard evidence.... the argument was left with my old boss walking out...

so once again chaps thats why i wanted to know any many thanks for all your help!

 

[Andy™]

why do i have a feeling he has used table 54.8???

 

[danny7299]

what bonding... i think so too... but that doesnt help with who pays for it!! good job the factory still owe him some money !

 

[Andy™]

how long ago was this supply installed? any why did it fail?

tbh, if he was supposed to do it, but bodged it, then he should be given a chance to rectify. but he may not go for that. and the factory may not want him back to do it either. either way, id like to see his reasoning for the 50mm

 

[NozSpark]

Sounds like you are definately offf your old bosses Xmas card list now

 

[danny7299]

well i know the building was buit about 15 years ago... but i think the main 800a supply was upgraded and put in about 4 years ago.... the bit that went boom was a 400 amp sub feed, we dont know why it went boom , the readings where , 0Mohm between two phases, and there was about 60A running down the earth cable from the other phase.... the insurance company want the duct / cable ripping out , but the problem is that there is other cables in the duct and there has been a building built on top of the duct ! and no inspection hatch. i think there only option will be a camera or somthing!?

 

[Andy™]

can you not pull a new cable into the duct? and remove the faulty one?

 

[danny7299]

No as well as the faulty cables there are a load of other cables in there plus they have built a building on it ... I think the ducting has got crushed , we have cliped direct with a brand new supply , so there Is about 4k worth of copper left in the ground ... Think we should tie it to the fork lift and drag it out !!! And just mend anything that brakes !

 

[benji]

Am i right in thinking that you have 2x240mm4c connected in parallel and one cable is run underground and the other is fixed to the building above ground?

yours benji

 

[danny7299]

no...

the main income (800A) is 2x 240mm in parallel underground.... the sub main 400amp is a 240mm on cable tray ...