Two 'Circuits' in one MCB

[Silly Sausage]

Going to tell us where this good practice is.

Arguably, it's subjective

 

[westward10]

Not actually guidance is clear in BS5839:6

 

[happysteve]

Not actually guidance is clear in BS5839:6

... which states that (for Grade D systems):

The mains supply to the smoke alarms and heat alarms should take the form of either:

1. an independent circuit at the premises’ main distribution board, in which case no other electrical equipment should be connected to this circuit (other than a dedicated monitoring device installed to indicate failure of the mains supply to the smoke alarms and any heat alarms); or
2. a separately electrically protected, regularly used local lighting circuit in which case there should be a means for isolation of the smoke alarm(s) from the lighting circuit (e.g. for maintenance)

For Grade E systems, (2) is recommended.

 

[markythesparky]

What does separately electrically protected mean?

 

[happysteve]

I'm not really sure. But there's a note underneath the above that says:

This does not imply that there need be separate electrical protection of the smoke alarm circuit; a single means of protection against overload or short circuit (e.g. a fuse or miniature circuit breaker) serving the entire circuit is acceptable.​

Perhaps they're trying to distinguish "proper" lighting circuits from those derived from (switched) fused spurs? Not really sure, though, sorry.

 

[markythesparky]

I would say that this means that it is acceptable to run smoke detectors on the same circuit as the lighting. But not two circuits, one for smokes and one for lighting, (2 cables) lumped together in one OCPD.

There should also be a means of isolating the smokes from the lighting, ie a switch or FCU.

In fact neither 1 nor 2 in the post before the last suggest that two cables in one OCPD is acceptable.

 

[happysteve]

There should also be a means of isolating the smokes from the lighting, ie a switch or FCU.

There is a further note (they just keep coming, don't they?!):

A number of manufacturers use a fixed base for the electrical connections, and the smoke or heat alarm can be readily removed by use of a tool operated release tab, etc.​