Yeah I believe so

Hi guys thanks for all your help. My question is that if there are cable fed from a common source (MCC)( and the armour earthed by daisy chaining and connecting to the main earth)out to a local isolator should we then supplementary bond the SWAs at the local isolator together and to the uni strut and cable tray? And if so is it necessary to connect them to the earthing terminal of the isolator as well?

The metal work does not look like an extraneous conductive part, so no bonding required.


Could it be an exposed conductive part? If the supply and load cables are both SWA, it is difficult to see how any fault would affect the metal work. Now what's inside the GRP box and plastic switch? Could a loose conductor in there come into contact with a screw which is in contact with the metal work? If so, I think that it does satisfy the definition of exposed conductive part and therefore should be earthed.


It's a bit like metal back boxes - if a loose conductor came into contact with them, the fixing screws would amount to an exposed conductive part.


If the boxes are attached to a non-conducting board (which seems to be the case) then no green-and-yellow is required.

Thank you Chris. I have seen a lot on sites locally supplementary bonding on isolators or junction boxes and just wondered if it’s needed(usually daisy the armours together) Even if the cable try is bolted to the ground with small through bolts I couldn’t see it introducing a good earth potential.

If the daisy chaining is done at the isolator (connecting the swa together with 4mm)should we then connect that to the earthing terminal at the isolator?

Gary your comments is very interesting.  Recently, due to 7671 updates there are significant discussions taking place as to bond or not bond and the requirement to earth back to the main earth,   In other words, if a part is determined an extraneous conductive part ( not forming part of the electrical installation).  In various IET articles this is discussed and reference made to domestic service pipes such as water and gas.  However, as per this picture this is an industrial installation it needs further consideration.  First the extraneous conductive part  - needs to be conductive, not part of the electrical installation and very important likely to introduce earth potential.   Eg metal gantry for crane on the site, large submersible pump with metal walkway and hand rails.  If the resistance is low enough back to the source of the gantry, hand rail.  Then a person touching a metal pumping system under fault condition could be exposed to current flow through the body.   If the resistance is very high then there is no risk. Hence no requirement to connect back to the main earth.   The difficulty is at design stage this is not fully scoped which can led to running of earths post the installation.  Which can be costly. 


In relation to exposed conductive part as per previous posts here it is down to the risk and the what if scenarios. A plastic enclosure with the SWA glanded through to the main earth inside is to ensure all is tied to earth under fault condition.  The mode of failure to make the tray live or the uni strut live must be considered and risk assessed.  In this case it could be considered low from the picture.  


Hence, there is a very strong agreement that neither bonding or connection to main earth is necessary.  The major issue is cart and horse.  At design stage making the determination not to bond or earth can leave the project open to additional earthing when it comes to testing or handover.


A major gap in industrial type projects and the subsequent electrical installation certification is that the determination to earth/bond or not earth/bond is not documented sufficiently.  This can have impact at periodic inspection and testing intervals of the electrical installation and weather the person agreed with the level of earthing / bonding.     


Interested if other people working on industrial side as experiencing the same discussion given the recent articles and updates.

Welcome James. Have you been one of the silent watchers?


The difficulty with this kind of thing is that a lot of it comes down to local opinion, and of course, that varies widely. One of the few things which the 15th edition did was to try to define the bonding required, but then was hopeless because it became a case of "every bit of metal one can see" which was similarly wildly interpreted! I did have a particular dislike of commercial kitchens with all the supposedly moveable furniture (for cleaning) being firmly tied down by G and Y. The situation was in fact more dangerous because a kitchen became a virtual conductive location, and minor problems with portable appliances became major shock hazards, and extremely difficult to clean properly!


The inspection of such installations can be tricky, for all the reasons very reasonably listed above, but realistically few of them are significantly dangerous. Plastic boxes made of the proper material (glass-filled polymer often polypropylene) (Schnieder) are very rigid and do not suffer significant creep under load so banjos should be fine, but cheap ones without the rigid reinforcement are often unsatisfactory after a few years.

Local requirements added on top of BS7671, such as bonds to unconnected metalwork, are often unhelpful and put there to "make it safer" or some similar remark in a document produced by someone long gone, can become the bane of certain inspections. Inspectors need to remember that inspection is to BS7671 only, adding other bits should not be an EICR requirement (some have tried), as BS7671 is fairly reliable as far as safety goes. Is there a BS for Pirana nuts Mike, I suspect not, but they are quite useful? 

This reminds me actually so say you have a concrete tank (rebar) set into the ground then an electrical systems running over on a metal Platform with submersible pumps in the tank should the tank have been main bonded at the installation stage? Say the electrical system ie pumps have been earthed and all cable tray and armoured cables have been locally bonded together.