I will start off this reply by stating that my expertise is in fields where BS7671 is generally not applicable so the following is in relation to systems design in general rather than compliance with BS7671.

However I do have considerable experience with requirements for safety systems, emergency systems, etc. so I will expand on my earlier comment.

Safety systems as defined in BS7671 are aligned with my experience and where there is reference to a safety system it is generally looking at the functionality being provided. Where there is reference to a safety circuit it is referring to the wiring of a particular sensor/alarm sounder or similar which is part of that safety system. A safety system should be designed on fail safe principles but also be designed so that a fault in one circuit will only affect that circuit, and with the fail safe principle should if possible provide fault detection (e.g. an alarm should fail to the alarm condition).

If a single fault is not to affect more than one safety system (or safety circuit) it follows that there must be separation of the cabling. However if the cable is both fire resistant and armoured or otherwise mechanically protected then neither a fire nor mechanical damage should affect multiple circuits.

(It is at this point I run into problems with the wording of 560.7.7)

The wording implies that if you don't have fire resistant metallic screened cables the only solution is separation. I can't see that there is a difference between fire resistant metallic screened cables and normal cables run in a steel pipe, since the latter will be protected from fire and mechanical damage. However if separation is required then I don't see any option but to provided it (though how much separation is needed is another debating point).

The reason for protecting the safety system cables is that they are normally relied upon during abnormal situations such as fire, explosion, flooding, earthquake (though not so much in UK) or natural disaster. Medical IT cables are not expected to have to cope with such situations but rather medical emergencies during which the normal systems will be expected to be operational.

Of course I was not part of the committee writing BS7671 so I am quite happy to have this challenged.

Alasdair

I will start off this reply by stating that my expertise is in fields where BS7671 is generally not applicable so the following is in relation to systems design in general rather than compliance with BS7671.

However I do have considerable experience with requirements for safety systems, emergency systems, etc. so I will expand on my earlier comment.

Safety systems as defined in BS7671 are aligned with my experience and where there is reference to a safety system it is generally looking at the functionality being provided. Where there is reference to a safety circuit it is referring to the wiring of a particular sensor/alarm sounder or similar which is part of that safety system. A safety system should be designed on fail safe principles but also be designed so that a fault in one circuit will only affect that circuit, and with the fail safe principle should if possible provide fault detection (e.g. an alarm should fail to the alarm condition).

If a single fault is not to affect more than one safety system (or safety circuit) it follows that there must be separation of the cabling. However if the cable is both fire resistant and armoured or otherwise mechanically protected then neither a fire nor mechanical damage should affect multiple circuits.

(It is at this point I run into problems with the wording of 560.7.7)

The wording implies that if you don't have fire resistant metallic screened cables the only solution is separation. I can't see that there is a difference between fire resistant metallic screened cables and normal cables run in a steel pipe, since the latter will be protected from fire and mechanical damage. However if separation is required then I don't see any option but to provided it (though how much separation is needed is another debating point).

The reason for protecting the safety system cables is that they are normally relied upon during abnormal situations such as fire, explosion, flooding, earthquake (though not so much in UK) or natural disaster. Medical IT cables are not expected to have to cope with such situations but rather medical emergencies during which the normal systems will be expected to be operational.

Of course I was not part of the committee writing BS7671 so I am quite happy to have this challenged.

Alasdair