Bonding concrete rebar do you need too?

SW:

We have a new building containing 3 machines that wash / clean food in the early stage of a process all equipment is stainless steel.

The building is steel frame, there is an incoming 3phase distribution my question is do you need to earth the concrete slab rebar, also there appears to be no lightning rods to bond the building is this mandatory?

You would need to connect the steel frame to the main earthing terminal for equipotential bonding purposes. You may not need separate earth electrodes for LPS as the steel frame may well be a very effective electrode for a self protecting structure in and of itself


And do you need to bond the concrete rebar to the main earth bar for this installation?

Not specifically, as you wouldn't expect an internal floor to be introducing a potential that is different to earth potential - it may well be that the rebar is already effectively bonded to the columns (and hence to the MET) due to the nature of the construction


Any guidance greatly appreciated 7671 is not clear on this with respect to the rebar?

If I were looking at it, I'd be thinking of connecting the slab edge to the columns to give me some further assurance that the columns were effective lightning protection electrodes if my risk assessment showed that I needed lightning protection. That would effectively connect the re bar to the columns and via bonding to the MET - but BS 7671 wouldn't specifically require that approach - it would be beneficial as a consequence of the LPS Risk Assessment and subsequent design. It may be that you then further decide that a buried perimeter electrode tape linking the columns might be an advantage in relation to external contact with metallic cladding to (in part) equalize the potential between any rise of voltage on the cladding (under electrical fault conditions internally) an someone standing on some wet ground outside the building.

Note sure that helps - as we don't know the specifics of the structure, but they are considerations the designer should make


Regards


OMS 

Many thanks for this, some very good points to consider.

Cheers,

SW

If it is already there, and you can do a Ze test on the building and separate the effects of any earthing via the mains supply, to not need electrodes for augmenting for LPS I'd want to see less than 10 ohms for a shed size building down to less than an ohm for aircraft hanger sized.

This is based on expecting about 10 ohms an electrode and an array electrodes a no more than 10m apart if you were looking at adding LPS to an insulated building. Not a hard and fast rule, more of an investigation threshold.

Some good consideration there thank you.

Hi All,


Doing a little more research, as long as the concrete rebar is not exposed it does not need bonding unless the floor/slab is discovered to be less the 50kOhms, or the exposed rebar falls in the previously stated.  Then it becomes an extraneous conductive part which would need bonding to the main earth bar.

BSEN7671 [418.1.5]


Kr,

Steve

Doing a little more research, as long as the concrete rebar is not exposed it does not need bonding unless the floor/slab is discovered to be less the 50kOhms, or the exposed rebar falls in the previously stated.  Then it becomes an extraneous conductive part which would need bonding to the main earth bar.

BSEN7671 [418.1.5]

I suspect a little confusion here. BS (not EN) 7671 regulation 418.1.5 only applies to installations where the means of protection from electric shock is a non-conducting location. That's an extremely unusual approach for modern installation - where it's much more usual for the chosen protective measure to be ADS (section 411) or Double/reinforced insulation (412) (or a combination of the two).

   - Andy.

Hi Andy,


What's your though with respect to "Bonding concrete rebar do you need too"?

Kr,

Steve

Hi Andy,


What's your though with respect to "Bonding concrete rebar do you need too"?

It's usually not necessary (at least from a BS 7671 point of view - lightning protection considerations might differ) - most modern habitable buildings in the UK will have a damp proof membrane of some kind between the structural slab and the internal floor surface (to comply with building regulations) - which effectively insulates the floor from the slab - thus the floor isn't an extraneous-conductive-part (as least as far as the internal electrical installation is concerned). There may be contact between the slab rebar and above-ground metalwork (either deliberate or fortuitous) but above ground bonding takes care of that. Also UK builders also know nothing about earthing or bonding, so by the time the electrician arrives on site there's usually several inches of solid concrete in the way of any possible connection point, unless the whole project is better managed than usual.


BS 7671 does demand re-bar bonding in a few special situations - concrete floored animal housing for example (section 705) - but these are very much regarded as special cases rather than the norm and deal with a much more demanding situation - from 'long wheel base' animals being far more vulnerable to shock from differing potentials to the presence of large amount of, ahem, 'organic fluids', that can make the concrete much more conductive.


   - Andy.

Cheers for this Andy some good points and logical.

Kr,

Steve

South Africa is a test bed for donner and blitzen , I used to sit in the garden on a summer's evening with a castle beer watching the free show.

I once saw a garden wall that was built using reinforced concrete slabs. There was a tree at the garden corner that was struck by lightning, the bark was stripped exposing the white nakedness of the trunk; also about 1 metre away the concrete slabs of the garden were exposed, probably as a result of induction. Had the reinforcing been earthed it may have discharged and avoided the damage..


Jaymack


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