MPBC is it required

[pc1966]

My original reading of this was an automatic garage door, almost certainly earthed via CPC, so would it need separate bonding? Many of the above answers are right, but generally not establishing just what is actually risky here:

• Is a bond needed to avoid shock risk between simultaneously griped parts?
• Is any bond present large enough to avoid a fire risk under fault (open PEN) conditions?
• Is the presence of PME connected metalwork a significant hazard?

As @mainline pointed out a quick check for the first point could be done if something is currently isolated, as they accepted threshold for let-go shock is about 10mA, and the body about 1k, so anything below 22k presents a risk if it is large enough to be grabbed and then unable to let go. Other situations might not result in paralysis, but they might result in another accident from involuntary movement causing a fall, cuts, etc.

However, to have a fire risk you need any extraneous part to be in good enough contact that more than, say 10A or so, can flow for long periods to risk a 1mm (or 0.75mm flex) CPC from overheating. So a typical outdoor light is almost certainly an extraneous conductive part as it will leak to the true Earth, but it ain't seeing tens of amps flowing so does not need a separate main bond!

Realistically if you attempted an Ra measurement and it is not below 20 ohms or so then its not needing 10mm bonds, though the regs don't specify anything beyond the bond size. I guess the implicit assumption here is the bonds considered are big stuff: metallic service pipes, steel structures, foundation rebar, lightning conductors, etc, any of which could well be in the 1 ohm or less territory.

The open PEN risk for general metalwork is one of those aspects that is sort of addressed by special areas, and now but EV charging under their own rules, but it has caused some serious risks before in situations that are still not addressed.

UPDATE: Pupils and teacher suffer electric shocks at Burnley school

THIRTEEN children and a teacher needed treatment for electric shocks after an incident at a Burnley primary school.

www.lancashiretelegraph.co.uk

 

[Rockingit]

GN8 (6.3) Has this to say on the subject -

(701.415.2) Some insight into the subject of conductive parts that are not extraneous conductive parts can be gained from Regulation 701.415.2, relating to supplementary bonding in a location containing a bath or shower. Indent (ii) of the regulation states that metallic door architraves, window frames and similar parts are not considered to be extraneous conductive parts unless they are connected to metallic structural parts of the building.

 

[mainline]

PME is not prohibited on a building site.

Regulation 704.411.3.1 of BS 7671, which
states that a protective multiple earth (PME)
earthing facility shall not be used to provide
the means of earthing for a construction or
demolition site installation unless all extraneous-
conductive-parts are reliably connected to the
main earthing terminal.
Where consideration is being given to the use
of a TN-C-S system to serve a construction or
demolition site, reference should be made to
BS 7375 (see Notes in section 704 of BS 7671)
and to the relevant electrical distributor.
Although neither BS 7671 nor BS 7375 prohibit
the use of a TN-C-S supply, the Energy Networks
Association (ENA) state, in Section 6.2.2 of
Engineering Recommendation G12, Issue 4:
Requirements for the Application of Protective
Multiple Earthing to Low Voltage Networks that,
as it is usually impractical to comply with the
bonding requirements of BS 7671, a PME supply
should not be offered a construction site, except
for the supply to fixed buildings. As a result,
provision of a TN-C-S earthing system for a
construction site is highly unlikely and so an
alternative supply earthing arrangement must
be employed

 

[JK-Electrical]

From the ECA Technical Bulletin of March 2017:

How to determine if it is an extraneous-conductive-part, or not?

A simple measurement is undertaken. This is done by testing the resistance between the suspected extraneous-conductive-part and the MET or the nearest known connection to Earth, such as the circuit protective conductor (cpc) of a local circuit (so long as its connection to earth has been verified).

Published guidance by the IET suggests that if the ohmic value between the suspected extraneous part and Earth is greater than 22 kΩ, then the part in question is considered as not able to introduce a shock risk, and as such will not require bonding. To measure this sort of reading will require the use of the insulation resistance testing range (MΩ) where 22 kΩ will usually be displayed as 0.02 MΩ.

If the test reading indicates anything greater than 0.03 MΩ, then it may be concluded that the item in question is not likely to introduce a dangerous potential and therefore would not require bonding.​