main bonding

[davelerave]

i'm a bit confused

'earthed' metal ductwork serving one room of a large building

does it require a main bond?

when i say 'earthed'-i mean it's bolted to the frame of the extraction unit just outside the building

thanks

 

[IQ Electrical]

i'm a bit confused

'earthed' metal ductwork serving one room of a large building

does it require a main bond?

when i say 'earthed'-i mean it's bolted to the frame of the extraction unit just outside the building

thanks

Is the extraction unit sitting on 'true earth'?

The only real way to tell is the 22kOhm test to the main earth terminal from the ductwork-if the reading is higher than 22kOhms then the part in question is not considered to be an extraneous-conductive part and should not be bonded.

 

[Knobhead]

Are there any fibre expansion joints in it? If there are they should have an earth bond across them. If possible bond it to earth it may not be in the regs but you can get considerable static build up in ducting.

1. It can give someone a hell of a belt if they touch it
2. It can cause explosions if there is dust in the air flow

It comes down to good practice

 

[IQ Electrical]

I think we're discussing 2 different things here, the op is asking about the requirements for main protective bonding to ductwork.

It's not good practice to bond non extraneous-conductive parts because of distributed fault voltages-this was the whole reason for the revised bonding requirements from 15th to 16th and eventually 17th editions of the regulations.

If the ductwork was an extraneous-conductive part, unless it was being used AS the main protective bonding conductor (not advised) then linking across would only serve to distribute the 'earthy' potential instead of limiting any bonding requirements to before the insulated joint only.

 

[davelerave]

yes-it's earthy and resistance to MET is much lower than 25kohm before cpc's are even connected

i'm just a bit confused on one issue- as it's bolted to 'exposed' metalwork outside i thought maybe it wouldn't be extraneous


would it be normal practise in large buildings to bond extraneous metalwork confined to a single room? the main supply is 400amp

but the room is served by a 63amp TPN sub-board?


also the ducting branches outside and enters at 3 different locations-would it be bonded at one location then?

 

[IQ Electrical]

If it's 'earthy' it must be bonded, if there's continuity on the branches then it can be bonded once, if the branches are electrically insulated from the 'earthy' initial run then they do not need to be bonded as they will not be introducing a potential and will not meet the definition of an extraneous-conductive part.

 

[davelerave]

yes there's continuity on all the branches and there's no insulating joints

so it doesn't matter whether it's connected to exposed metalwork outside then?-if it's 'earthy' it's extraneous whether it's earthed outside or not?i was confused on this


i think i'm right in saying if there's (main bonded) structural steel present a main bond can be taken from that-unfortunately that's not the case

 

[IQ Electrical]

yes there's continuity on all the branches and there's no insulating joints

so it doesn't matter whether it's connected to exposed metalwork outside then?-if it's 'earthy' it's extraneous whether it's earthed outside or not?i was confused on this


i think i'm right in saying if there's (main bonded) structural steel present a main bond can be taken from that-unfortunately that's not the case

Using the structural steel as the main protective bonding conductror can literally save thousands of pounds on industrial/large commercial installations yet it seems to be some sort of secret!

 

[Engineer54]

It's not a secret to those of us who tend to work on a lot of steel framed buildings, or where there are huge amounts of structural steel about. Industrial and large commercial installations is where you would encounter such conditions in the most part....

 

[Knobhead]

It's not a secret to those of us who tend to work on a lot of steel framed buildings, or where there are huge amounts of structural steel about. Industrial and large commercial installations is where you would encounter such conditions in the most part....

Hehe! You’ve just backed up my argument about running a CPC along with a SWA, what’s the point when you’ve got 100’s of earth paths!