Think it's best to say that opinions will always differ on this subject, personally I would always bond "Just in case", factor the price into whatever the job entails and explain reasons to the client. Sorted.
Unfortunately this "just bond it anyway" attitude can on certain occasions be more hazardous than not bonding.....
i.e.
a piece of metal with little or no continuity back to earth, will not complete a path for an electric shock.
Higher resistance = less current. Whereas lower resistance path = higher current flow, thus increasing the amount of the shock current.
It may help to consider TWO distinctly different fault scenarios:-
a) A faulty item of equipment where a line, (phase), conductor touches an external metal part, (e.g. say a dodgy washing machine).
B) A person using a damaged extension lead or doing a bit of DIY, failing to do any safe isolation and they touch an exposed line, (Phase), conductor.
In situation (a) we need a nice very low resistance path to increase the speed that the circuit breaker operates. Because during that time frame after the fault but before the circuit breaker operates all metal work connected to that same earth system with rise toward supply potential (230v).
In situation (B) we need a higher resistance to reduce the current flow! Thats one of the reasons why we use safety shoes insulated to 1000v and Non-metal stepladders and insulated screwdrivers etc.. etc..
This is a similar principal to the 230v shaver socket... Isolating transformer = NO return path for a fault current.
It is the bits of metal that are joined to the electrical installation, (Exposed conductive parts)
and the bits of metal making a reasonable contact with earth potential, (Extraneous conductive parts), that need the bonding.
Other bits of metal that just happen to be at a property but not introducing an earth potential will not be part of any fault circuit...
(unless you happen to choose to go and join them, bond, onto the installation anyway)
From what I recall the value of 22,000ohm (22kohm), between metal part and electrical earth, is level above which it is considered that the free-standing metal work doesn't need any bonding.
Less than 22k then generally considered best to bond.
As a certain member often says, it is the limited knowledge of just doing something cuz I always do it, without understanding why you do something that can be more hazardous.
IMHO earthing is one of those areas where many electricians still don't fully understand why they are doing something. Yet EVERY electrician has a responsibility to asses each installation they work on to asses that the earthing and bonding are appropriate, (reg 131.8), for the protective measures applied.
Remember it is those last few qualifying words.. "For the protective measures applied" that is pivotal in that reg.
e.g. there were difference of protective measure's between 16th & 17th domestic installations.
AND
Domestic & Commercial, also have their own set of differences for types of protection required, (a-la Ordinary persons, Skilled etc.. etc..).
The bottom Line:
There is no 'ONE-SIZE-FITS-ALL, STANDARD RULE' for bonding every installation in an automatic fail-safe manner.
The electrician needs to satisfy him/her self that they can sign the certificate for their work with a clear conscience.
coffeecoffeecoffee
