If they have buttons I think they would be the single phase plus in types not the 3 phase ones. My big Tupperware box of lock off has a suitable device for every device I have come across. I would need to check my ancient copy of BS 3871 to see if the meet the 3mm requirement of isolation.

JP 

John Peckham said:

I would need to check my ancient copy of BS 3871 to see if the meet the 3mm requirement of isolation.

I can save you the time of doing that, John ... it's not there.

But than again, BS 5419, the old standard for LV isolators, also doesn't have a stated minimum gap , just "in development" rather than a table.

There are some interesting observations:

• BS 3871 Appendix D contains the statement "Complete isolation of the circuit in which a fault has been present usually occurs at the first or second instant of zero current following initiation of the fault". Meaning of course, that "back in the day" they thought that BS 3871 breakers provided isolation ...
• BS 5419 Appendix B has some advice on creepage and clearance distances, without providing tables of actual minimum distances.
• Both standards use a 2 kV "withstand" test across open contacts

So, it's clear that "back in the day" these products may well have been considered OK for providing isolation ... but they definitely don't meet the requirements for isolation of BS EN 61140 today (5 kV impulse withstand for CAT III, typically 3 mm gap).

I have had a quick spin through BS3871 and I cannot find any reference to contact clearance. That does not mean that the MCBs do not have a sufficient contact clearance but there is no requirement in the product standard. 

So this maybe the rabbit hole that GK may be luring you towards?

JP

John Peckham said:

So this maybe the rabbit hole that GK may be luring you towards?

Not a rabbit hole of my making ... I provided references in publications where this has already been stated by others.

See previous post (which I edited as you were typing) ... some of these older products can't be demonstrated to meet current requirements in BS EN 61140 (and hence BS 7671) for isolation ... simples !

 I assume however that the spark well versed in the current standards for isolation could always pull an upstream fuse or indeed use the main switch on the DB with the older breakers in, ...  thus only relying on the breaker for ADS, but not isolation for safe working downstream, much as I'm sure he or she presumably already does when changing light bulbs and so on what with the old method of changing the lamp into a live holder while held in the old flat cap, or just relying on the light switches having long since been deprecated.
Mike.

mapj1 said:

thus only relying on the breaker for ADS,

Arguably, 531.1.1 prevents that sadly. The device actually providing (intended to provide?) ADS needs to provide isolation (whether on purpose or by accident).

We can be as clever as we like in this Forum, but the only real test will be in court.

The device actually providing (intended to provide?) ADS needs to provide isolation (whether on purpose or by accident).

But presumably not necessarily provide isolation by the same means as disconnection for ADS. A rewireable fuse for instance isn't guaranteed to provide a 3mm (or whatever) gap when it blows - so isolation must be provided by pulling the carrier. So a push-button Wylex-standard MCB for instance, could similarly provide isolation by being pulled - so perhaps doesn't have to provide full isolation via its contacts? (OK not much use for conventional MCBs that are hard wired-in, but as a point of order).

    - Andy.

gkenyon said:

So, it's clear that "back in the day" these products may well have been considered OK for providing isolation ... but they definitely don't meet the requirements for isolation of BS EN 61140 today (5 kV impulse withstand for CAT III, typically 3 mm gap

So the myth buster in BPG 4 alluded to by AMK is wrong! The devices do not comply with 531.1.1 and, in any event, should not be used to isolate a circuit for electrical maintenance. 
To be honest, I always took the same view as BPG 4 and would not mention any issues in reporting documentation even where the devices proliferate in the older installations, but I think I will review that, given the comments by the guru of electrical compliance.

lyledunn said:

The devices do not comply with 531.1.1 and, in any event, should not be used to isolate a circuit for electrical maintenance. 

I would prefer to say "The devices might not comply with 531.1.1 ..."

On reflection, I ought to have said

So, it's clear that "back in the day" these products may well have been considered OK for providing isolation ... but they definitely don't might not meet the requirements for isolation of BS EN 61140 today (5 kV impulse withstand for CAT III, typically 3 mm gap)

There's every possibility that an older device may well have been designed such that it would pass tests and inspections of current standards ... but we do know it is highly unlikely to have actually been tested to the requirements of newer standards.

Interesting debate, just wondering if there is anything I can do to make it acceptable to use the existing BS3871 device.

Given in this case it's just protecting a sub main. Would it be acceptable to continue using the existing board and add a label stating that MCB's not suitable for isolation, isolation can only be achieved with main switch.

Second question, if the above is not acceptable. how would you feel about doing minor works like adding sockets to one of the other circuits on a BS3871 device or minor works to a circuit connected to another sub board fed by a BS3871 device.