In accordance with Best Practice Guide 4, the assertion that utilizing a BS 3871 circuit breaker constitutes a reportable non-compliance is classified as a myth

So long as you can show it meets the current  requirements of BS 7671 in terms of breaking times under fault and overload as applicable, and the wiring method is acceptable for the location (and SWA and pvc singles inside earthed containment probably will be) you can use any suitable breaker. This may well be perfectly  suitable. The fact that the standard was withdrawn in July 1994 does not in itself preclude using  devices specced to it.

Mike.

Yes, but it is time to be thinking about updating the 40 year old DB. Are there any BS 4293 RCBOs to accompany the ancient MCBs?

Regulation 531.1.1 might be a sticking point - if the BS 3871 breaker is to provide ADS for the new circuit, it needs to be suitable for isolation.

BPG2 Issue 3 (page 10) says that not all BS 3871 circuit-breakers are suitable for isolation.

mapj1 said:

can use any suitable breaker

Provided it meets all of the requirements - one in particular that could be an issue is Regulation 531.1.1

gkenyon said:

BPG2 Issue 3 (page 10) says that not all BS 3871 circuit-breakers are suitable for isolation.

The old push button plug-in Wylex breakers made to retrofit in their rewirable fuse boards are BS387, so unless unplugging them counts as isolation they would not be suitable for isolation, as the buttons cannot be secured.

Sparkingchip said:

unless unplugging them counts as isolation they would not be suitable for isolation,

That would be the same approach used for a fuse (of any type) as a ruptured fuse doesn't necessarily provide "isolation" but a removed or withdrawn one (to the standards recognised in BS 7671) does.

Similarly, products to BS 7288 (withdraw fuse or plug to isolate).

I would be very wary of using a fuse holder as a means of isolation, yes you may have pulled the fuse and have it in your pocket, but someone could simply move another fuse within the board and reenergise the circuit, as they can move plug-in BS 3871 MCBs in old Wylex fuse boards.

Sparkingchip said:

I would be very wary of using a fuse holder as a means of isolation,

Why, there are different approaches to "locking off" that prevent the fuses being re-inserted?

It just requires the correct lock-off kit used in the right way.

Yes I agree that just "pulling the fuse" is only part of the job ... but there is stuff out there if you want to prevent re-insertion.

they would not be suitable for isolation, as the buttons cannot be secured.

Aren't isolation and locking off two separate requirements? Locking off is only needed if the device is remote from the equipment needing to be isolated, and if needed, may be provided by means other than on the isolating device itself - e.g. in a lockable enclosure.

   - Andy.