Bonding between two buildings

[topquark]

Agreed mate. With separate buildings if you come along after build (and 1st/2nd fix electrics) and do a test you'll find a degree of continuity (IE low(ish) resistance ... because of the bonding). If you do it during construction (once the concrete has gone off) you'll find the resistance so high it ain't worth considering.

Warehouses are a whole different ball game as, as you mention, they often share some common conductive structure (same with office blocks).

In reality, I guess, the risk is only (potentially) marginally worse than any 3 phase fault, where phases are used individually (IE fault on one phase appearing physically close to another phases line). The risk then is that the cpc (on one phase) can now be in physical proximity to another phase (not sure I'd wanna be there).

 

[Engineer54]

If you try getting continuity between adjacent steel structures you'll find a very high resistance (coz the steel is in concrete usually).

That shouldn't be the case at all!! As steel framed buildings should be effectively grounded at time of construction, that has been in force for as far back as i remember. Obviously the more up market the building is, the better the grounding scheme employed. Modern day steel frames that i've seen can have the vertical structural steelwork actually set in conductive mortar. But more usual is, ground mats, rods, or perimeter bare copper ring/loop...

Even modern concrete structures, will have re-bar break out points, so that the mass of connected reinforcement steel, can also effectivley provide lightning protection to the building.

 

[topquark]

Try reading the post just before the one you've posted (bonding)! I wouldn't disagree with what you've said at all.

I'd be interested to hear your reflections on the bond versus don't bond question though

 

[Engineer54]

Topquark,

I was basically only replying to your post highlighted in my post in this instance....

As to your question on the bonding issue, many of the the adjacent buildings and even some more remote buildings on projects i've worked on had bonding links between them. All of which were connected via removable links on the respective Main earth Bar arrangements.

On one particular project in Saudi, a medical city (as it was called) had a 185mm insulated conductor that connected every medical and service buildings Main Earth Bar on the project. It Also had a 240mm bare copper ring from a substantial ground field, that also connected to the Main Earth Bar of each building, again via removable links on the earth bar.... I've had very few conflicts during times of testing these systems, individually or as a whole....

 

[Engineer54]

Further to your question on my reflections on bonding between buildings. I have read your points on this matter and can agree on in principle on what you say. The thing is, when your dealing with many industrial facilities that have huge amounts of interconnecting metal plant and pipework, or say hospitals that again have several metallic systems, and various pipework systems. ...It becomes almost academic, as it's almost impossible to isolate any particular system from another, so parallel earth paths are going to always be a fact of life that we are going to have to deal with and accept i'm afraid!!! In small factory premises and the like it is far easier to manage, (if that's the right word) but your never going to get away from those parallel paths in reality. So better to effectively bond than not too....

 

[mechelec]

if the supply is TNS then it should not be a problem if you bond the the MET from each supply together. If it is TNCS then i would say no because if you lose the neutral to one incoming supply the current would find a path through all the bonded metalwork and could overload neutral earth conductors on the other supply

 

[Deleted member 9648]

if the supply is TNS then it should not be a problem if you bond the the MET from each supply together. If it is TNCS then i would say no because if you lose the neutral to one incoming supply the current would find a path through all the bonded metalwork and could overload neutral earth conductors on the other supply

Exactly why you should link the two MET's regardless of earthing system......if you dont the situation you describe would result in a dangerous PD between conductive parts in the same building.....The linking earthing conductor should be sized accordingly.
It's likely that in reality the two MET's will be linked via parallel paths through bonding and cpc's anyway.....better to provide a properly designed copper link in my view.....