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Dr Zzeus - May 2021


Do fire alarm mains isolators need to be double pole isolators, and should it be on a Residual Current Device (RCD)?

One of the major changes in the 2017 BS5839-1 standard was to remove the requirement for the fire alarm system isolator to be a double pole isolator. So, a simple answer would be 'no'.

Many fire alarm technicians and engineers don't quite realise how much of BS7671 must be used when installing fire alarm systems, and some don't even realise that BS7671 even exists!

Clause 25.2 of the standard, the recommendation to provide double pole isolation, has been replaced with a recommendation to provide local, safe isolation, which need not be double pole.

Note 2 says that safe isolation is required under the Electricity at Work regulations 1989, meaning any isolating device must provide safe isolation. BS5839-1 references table 537-4 in BS7671 for the correct selection of an isolating device, which in turn specifies a BS EN 600669-2-4 isolating device. It also states that “numerous switching devices do not meet the requisite isolation performance criteria”.

Every isolator, switch and protective device that can disconnect the mains supply to the fire alarm and detection system must be situated in a position inaccessible to unauthorised persons or be protected against unauthorised operation by persons without a special tool.

Note 5: A special tool in this context is a tool not likely to be carried by a member of the public. Slot screws wouldn't be satisfactory, since numerous articles can be used as screwdrivers, “no flip out fuses”.

Every isolator and protective device that can isolate the supply to the fire alarm and detection system, other than the main isolator for the building, should be labelled, clear and in durable fade resistant material.

The main supply final supply circuit to all parts of the fire alarm and detection system must be dedicated exclusively to the fire alarm and detection system and should operate no other systems or equipment. The circuit(s) should be derived from a point in the building’s electrical distribution system close to the main isolating device for the building. The supply shouldn't be connected via a card or coin-operated meter or similar device.

The number of isolating devices between the incoming supply to the building and the fire detection and fire alarm system power supply unit should be kept to the minimum.

In certain systems within exceptionally large buildings, in which networked control panels or distributed power supply units are used, the recommendation might be impracticable.

If, with the agreement of the interested parties, a variation is adopted, whereby the mains power supply to this equipment is derived from one or more local distribution boards, the amount of isolating devices between the main incoming supply and the local distribution board should still be kept to the minimum.

In plain English, what do you need to do?

You need an isolator that can only be disconnected by a key or special tool that meets the EN BS600669 standard, and must have a 'FIRE ALARM. DO NOT SWITCH OFF' label (whether incorporating a protective device or not) that serves only the fire alarm circuit.

What about an RCD?

BS5839-1:2017 clause 25.2 (i) The circuit supplying the fire detection and fire alarm system shouldn't be protected by RCD unless this is necessary to conform to BS7671. Where a Residual Current Device is necessary for electrical safety, a fault on any other circuit or equipment in the building shouldn't be capable of resulting in isolation of the supply to the fire detection and fire alarm system (put on its own RCBO).

About the author

Tom Brookes

Tom has a PhD with his dissertation, Creation of pathways for the professional recognition of fire alarm engineers in the UK, gaining maximum credits; he has recently become the first Chartered Engineer in the UK based solely on fire alarm knowledge and skills and is keen to help other fire alarm engineers gain professional recognition.

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