bonding to structural steel in different rooms

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If you have for example a supply cable neutral of 185mm, then the bonding to the steel works etc, needs to be in 50mm earth.
However, if this supply feeds another board in an adjacent room which has a seperate steel structure, but is part of the same building, would the bonding to that need to be based on the main supply or on the db based in that room?
 
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the main building has a steel supported roof, large gurders etc, approx 60m x 20m, these gurders are built into stone walls except in two places were a few steel supports go to the floor.
In the adjacent building, there is two walls joined to the main building but none of the metal structure is joined. this building has a steel frame for one floor and roof.
I'm thinking that both structures would need to be bonded 50mm back to MET, but not sure. The supply in the smaller building is only 3phase 16mm, so would be alot cheaper to bond back to this db than running a 50mm back to MET
 
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what system type is this?

TNS, TNCS, TT?

we are talking equipotential bonding here i presume, what size bond goes to the water/gas? am just trying to figure out where the 50mm you qoute comes from?

holmslaw wrote

I would link them together using 120mm in two separate places

why 120mm?
 
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My guess - 185mm is bound to be supplied from a dedicated sub. If not, it is most likely still going to be TN-S.

If so, 25mm main eq bond is what is needed.



If it is PME (and OH MY GOD if it is), then the bond would need to be 95mm!
 
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our factory at work (tns) is similar steel beam/guirder set up. when i updated the bonding i ran 25mm (see 544.1.1 of the 17th) which is why i was wondering why you have chosen 50mm.

use the 25mm if you can and save the 50mm for when its required on another job.

remember you can use the steelwork as a bonding conductor should it be more convenient (subject to conditions). see the regs for this as i cant remember the number off hand.
 
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i'm pretty sure it is a pme system as far as I can see. the earth comes off the neutral at the supply.
I thought that if the supply neutral is over 150mm, then 50mm equipotential bonding is needed, not 95mm. Is this wrong?
 
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i'm pretty sure it is a pme system as far as I can see. the earth comes off the neutral at the supply.
I thought that if the supply neutral is over 150mm, then 50mm equipotential bonding is needed, not 95mm. Is this wrong?

That's correct as per 547-02-01 and table 54H.
 
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The size for a PME should be verified with the DNO as they can require larger sizes to be installed.
 
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The size for a PME should be verified with the DNO as they can require larger sizes to be installed.

Is that mentioned anywhere within BS7671?
How would we know to contact the DNO when BS7671 just points you to table 54H?
 
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