AMK said:

How should regulation 537.2.1.7 be interpreted and applied?

There is no such Regulation in BS 7671:2018+A2:2022 or BS 7671:2018, so the short answer is that it is neither interpreted nor applied - there has been no such Regulation for over 5 years!

However, 537.2.6 and 461.2 together effectively replace the requirement.

The parts of BS 7671 regarding isolation and switching were completely revised, and split into Chapter 46 and Chapter 53 (see page 5 of BS 7671:2018+A2:2022).

AMK said:

Regulation 531.2.2 allows the neutral to remain connected if it can be reliably assumed to be at earth potential.

And more fundamentally, Regulation 461.2 ? This also requires equipotential bonding to be in place. Some might argue that if there are no extraneous-conductive-parts, there is no equipotential bonding, so the neutral requires isolation regardless of the supply type and earthing arrangements.

Regardless of all that, in installations where the main switch at the origin is intended for operation by ordinary persons, if the supply is single-phase, the neutral must be disconnected (Regulation 462.1.201).

AMK said:

Some suggest that these methods are sufficient and practical, and that isolating the neutral is unnecessary and impractical.

Further, if you provide a separate neutral disconnecting link and fail to re-instate before re-energizing, that can be very dangerous in a three-phase system (see Regulations 431.3 and 442.3).

AMK said:

The Lead Engineer argues that both supplies, essential and non-essential, need to be isolated to work on the Essential Line side isolator, where the neutral is linked between the line and load. But is this really necessary?

I think that would be down to how one interprets Regulation 461.2 ...

I teach safe isolation and after I have done the fire alarm drill and where the WCs are I usually kick off with there is no such thing as safe isolation.

Looking at the EAWR 1989 Regulations 12, 13 and 14 and the guidance in the HSE HSR25 sets out the fundamental principles and the Statutory Requirement. The Management of Health and safety at Work Regulations 1999 make a Statutory requirement to carry out a risk assessment and to produce a Risk Assessment and Method Statement (RAMS). So no activity should take place until the RAMS is produced and the relevant person(s) have been instructed.

Additional Guidance for safe isolation may be found in IET Gudance Notes 2 and 3 and also the Electricians Guide to the Building Regulations. The IET guidance is to use a sensitive clamp meter around main protective bonding conductors as part of safe isolation procedures to detect the loss of PEN continuity. 

The original post asks about isolation of the neutral. I really do not like the statement, "Can this be verified prior to work commencing by a simple calculation of Un-E = neutral current x Zn (TN-S) or a voltage test between neutral and earth" You cannot verify safe isolation by calculation.

As for disconnection of the neutral Regulation 12(2) says disconnected and separation of the electrical equipment from every source of electrical energy. Is the neutral a source of electrical energy, discuss?

Isolation of the neutral or removal of a neutral link may in itself cause a shock hazard. If there are borrowed neutrals on the installation, when say a DB is isolated with a 4 pole switch the neutral is disconnected, that part of the installation may appear to be safe when tested but then may become live later when a load is switched on with a borrowed neutral. So you may have proved dead at a moment in time. 

When I have isolated big Form 4 panels that have incoming 4 pole ACBs  and even when the HV is isolated I will earth down the neutral. I remember working on a big site with 2 x a very large Form 4 Panel with a close coupled 11kV transformer supplied from a 33kV transformer and an on site engineer telling me as the RMU was isolated and earthed out I did not need to apply an earth on the neutral, I spent a bit of time explaining why I had earthed the neutral.

Removing neutral links should be carried out as live working with PPE and insulated tools and a volt stick to warn if live when disconnected.

Even with the supply isolated and maybe the supply fuses removed the installation may become live due to connections to other installations via bonding conductors.

I know of incidents from 2 particular industries I work with where metal street cabinets have become live through PEN conductor loss. The engineers now use a volt stick before they touch the street cabinet. 

So in summary it may not be a safe idea to disconnect the neutral depending on what the RAMS says.

I would advise to treat all installations as live at all times and keep a good quality volt stick clipped to the top of your shirt for ready and frequent use before you touch anything.

That is my brief synopsis of safe isolation not a comprehensive  lecture on how to do it!

JP

gkenyon said:

years!

However, 537.2.6 and 461.2 together effectively replace the requirement.

Do you understand 462.1? I don’t, neither for that matter do the technical bods at NICEIC. I would love to understand the latter part of that regulation!

John Peckham said:

Is the neutral a source of electrical energy, discuss?

Yes, it can be. When I first encountered it, I could not understand why cutting some T&E on an isolated (and locked off) circuit tripped the RCD. I eventually realised that the neutral was sufficiently above earth potential in the TT installation to allow circa 30 mA (or more) to flow.

In this particular domestic case, the amount of electrical energy was small, but not zero.

John Peckham said:

So you may have proved dead at a moment in time.

With all due respect, John, delete, "may".

I think I should leave the may in. This is the story of Colin who proved dead to replace a light fitting at the request of his wife. Having apparently proved dead he climbed the steps and when disconnecting the original light fitting he received a shock and fell from the ladder.

The sequence was switch on light. Go to the consumer unit and switch off the MCB and the light went out. Tested 2 pole tester before use.Tested with a 2 pole tester between line and neutral, line and earth and neutral and earth all indicated no voltage. 

So what happened to cause this? He went to hospital and was off sick for an extended period. He is a qualified electrician.

JP

One possibility is a borrowed neutral,  everything tested correctly as there wasn't a problem until the neutral conductors were disconnected at the light fitting. 

A JP brain-teaser!

I take the point, but for a start, he has not proved dead if he did not test the tester as the last stage.

I assume either

1) supply reverse polarity or perhaps the MCB in the neutral and an off-earth if nothing else was happening and shock to terra-frma

or

2) the neutral was also carrying current back to the board originating from another circuit breaker and when the loop-in loop out path was broken, a voltage between the two  open ends existed.

I'd hope both unlikely to go un-noticed  in an electrician's own home you would hope but maybe not if not inspected since moved in..

Mike

I agree with this possibility. Could the two-way switch have controlled the light at the top of the stairs (if that was the culprit)? Could the downstairs lighting circuit have supplied the common switch connection (permanent live), and the strappers have joined the switch on the landing? Could the switch live have risen to the landing light, but the neutral wired from the upstairs lighting circuit?

After watching a full season of “Shetland”, my money is on the wife!