Mains power supplies to fire systems – more than just another electrical circuit? | Carvell Group

Mains power supplies to fire systems – more than just another electrical circuit? | Carvell Group

Christopher Carvell MIET MIFSM PCQI, MD and Principal Consultant at Carvell Group, discusses the compliance issues around mains power supplies to fire systems and the compliance requirements an electrician needs to be aware of.

In essence, every non-domestic building will have some form of electrotechnical fire safety system installed within it which requires a mains power supply or multiple mains power supplies.

These systems could be fire detection and fire alarm systems, voice alarm systems, emergency voice communications systems (disabled refuge) and more recently introduced evacuation alert systems.

These systems obviously need to comply with BS 7671, however as the mains power supply forms part of the fire safety system one needs to ensure that compliance with the relevant standard for the fire safety system e.g., BS 5839-1 in the case of fire detection and fire alarm systems is adhered to.

It should be noted that in any instance where any conflict is identified between BS 7671 and the relevant fire safety system standard, then the fire safety standard takes precedence.

Let’s take a look at he headline compliance considerations that present themselves to the electrician installing these circuits:

Power supplies and means of isolation

The circuit to the system needs to be uninterrupted and dedicated solely to the fire safety system. The unnecessary use of RCDs isn’t permitted.

The number of protective and isolating devices between the origin of the supply and the means of local isolation to the fire system needs to be kept to a minimum. In practice this would comprise of a dedicated circuit emanating from the first electrical distribution board in the building’s electrical distribution system.

In very large buildings where there are several power supplies needed for the fire safety systems then this will be impracticable, and it’s more likely that supplies will be derived from more local distribution boards. In such instances this is a variation to the standard. Dealing with variations is covered further in this article.

Each power supply needs to have a local means to safely isolate the supply to the fire safety equipment. Selection of an isolator is important as many electricians select switches that don’t afford isolation as defined in BS 7671.

All power supplies need labeling at each local isolator with specific wording that is prescribed within the standard, so in the instance of a fire alarm “FIRE ALARM DO NOT SWITCH OFF” this label also needs to be applied to the protective device for the dedicated circuit.

This is commonly understood by electricians, however what is often an oversight is the need to label all protective devices going back to the origin with the wording “WARNING THIS SWITCH ALSO CONTROLS THE SUPPLY TO THE FIRE ALARM SYSTEM”.

The reasoning for this is that the isolation of a supply to a sub board which houses the dedicated circuit could inadvertently disconnect the mains power supply to the fire safety system.

Cables, wiring and connections

All mains power supply circuit cabling needs to match the colour of the ELV fire safety system cabling. This is normally red so that it is distinguishable from other electrical circuits.

The LV mains power supply cabling shall be segregated from the ELV fire system cabling so particular attention is needed when entering the item of fire safety system equipment in that it needs to be a separate cable entry to the ELV system cabling.

All cabling needs to be fire resistant and in buildings over 30m in height that aren’t protected by a sprinkler system, or buildings where there is a phased evacuation strategy, the fire resistance of the cable needs to be enhanced from the standard rating of 30 minutes to 120 minutes.

Methods of cable support need to be non-combustible and withstand similar temperature for a duration to that of the cable. The cable needs to be supported at intervals strictly in accordance with the cable manufacturer’s recommendations.

In the unlikely event that a joint is required then the method of jointing should withstand similar temperature for a duration to that of the cable. The box in which the joint is made shall be clearly labelled “FIRE ALARM”.

Cables which pass through walls, floors, and surface cables below 2m from the finished floor level need to be mechanically protected. This doesn’t apply to steel wire armoured cabling and mineral insulated compound cabling.

In instances where cables pass through external walls, they need to be contained in a smooth bore sleeve of metal material sealed into the wall with the sleeve sloping downwards towards the outside and plugged using a waterproof compound to protect entry of rain, dust or vermin.

Fire stopping and fire sealing

Cables which pass through floors, walls, partitions and ceilings need to be sealed using fire-stopping materials that ensure fire resistance of the construction isn’t materially reduced and that there are no spaces in which fire and/or smoke can pass through and spread. The same applies to cables installed within containment such as trunking.

Inspection and testing of wiring

All wiring needs to be inspected and tested in the usual way for insulation resistance, continuity, maximum circuit resistance and polarity with results recorded.


Any variations to the recommendations of the relevant standard e.g., BS 5839-1 in the instance of mains power supply circuits supplying of non-domestic fire detection and fire alarm systems need to be recorded.

It’s important for the electrician to understand that any variation needs to be agreed by all relevant parties prior to being introduced. Relevant parties include, but aren’t limited to, the fire system specialist, user, purchaser, enforcing authority or insurer.

It isn’t acceptable to introduce variations without this pre-agreement and retrospectively record them without agreement. Such situations are therefore not classed as an agreed variation and fall into the category of a defect.

Documentation and certification

The electrician needs to produce as fitted drawings which detail the following:

  • the positions of all power supply equipment
  • the type sizes and actual routes of all cables.

On completion of work the electrician is responsible for the production of the usual BS 7671 certificate for electrical safety which is understood.

What isn’t broadly understood is that to comply with the relevant fire safety system standard the electrician is to also complete an Installation Certificate in accordance with the recommendations of that standard with the extent of liability being the mains power supply circuit only.

The purpose of this is to certificate compliance of the circuit with the said standard e.g., BS 5839-1 for non-domestic fire detection and fire alarm system mains power supplies or BS 5839-9 for emergency voice communication systems (disabled refuge systems).


As electrical certification bodies don’t audit compliance of fire safety system standards as part of the electrical assessment, compliance with fire safety standards isn’t always being undertaken.

Unfortunately this results in electricians inadvertently delivering defective work and not producing the relevant documentation and certification for what is an intrinsic part of a fire life safety system.



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