Helpful video reference. We use eFIXX's electricians' Q&A "Do smoke alarms need a dedicated electrical circuit? - BS5839-6" as the video reference here. eFIXX is a UK electrical training channel produced by Electrical Training Solutions. Their Q&A series is particularly good at cutting through the myths that circulate on trade forums.
1. Understand what Grade D and Grade E mean
BS 5839-6 divides domestic fire detection into grades and categories. The grade describes the power source:
- Grade D - mains-powered with an integral battery backup. This is what you find in virtually every UK home built or refurbished in the last twenty years.
- Grade E - mains-powered with no battery backup. These are only acceptable in certain limited circumstances and should not be the sole protection in a dwelling.
- Grade F - battery-only. These do not require a mains connection at all.
The category describes how much of the dwelling the system covers, from LD1 (every room) down to LD3 (escape routes only). For a standard owner-occupied house, LD2 or LD3 is typical. This guide focuses on Grade D, which is by far the most common.
2. What BS 5839-6 actually says about the circuit
Clause 15.2 of BS 5839-6:2019 says that Grade D alarms may be supplied from a regularly used local lighting circuit. The key word is "regularly" - the idea is that if the supply fails, the occupant will notice quickly because the lights stop working too. A lighting circuit serves that purpose well.
The standard also says the circuit should not be one that is likely to be switched off by a time switch or by the occupant during the night. That rules out circuits fed through a timer or night-off switch, but a standard upstairs lighting circuit is fine.
3. The case for a dedicated circuit
The standard permits a lighting circuit, but it also says a dedicated final circuit is preferred. Here is why that preference makes sense in practice:
- A dedicated circuit cannot be accidentally turned off. Someone switching off the upstairs lighting MCB to change a bulb does not simultaneously kill the smoke alarm supply.
- A dedicated circuit with its own MCB makes the supply easy to identify on an EICR. Inspectors like it; it removes an observation.
- Voltage dips on a shared lighting circuit from LED drivers can occasionally cause nuisance trips on the alarm electronics. It is rare, but it happens.
If you are rewiring or doing significant electrical work, adding a dedicated 6A MCB for smoke alarms costs very little and is worth doing. If you are just replacing old alarms on an existing circuit, sharing the lighting circuit is fine.
4. RCD protection: what the standard really says
This is where the trade forums consistently get it wrong. BS 5839-6 says the circuit should preferably not be protected by an RCD, for one specific reason: an RCD trip could cut the supply to the alarms and leave the occupant unprotected without them knowing. A battery backup guard against that scenario, which is why the standard also says:
Where Grade D alarms have a compliant battery backup, RCD protection is acceptable. Modern 18th edition consumer units have RCD or RCBO protection on virtually every circuit, so in practice nearly all new smoke alarm circuits in UK homes are RCD-protected. This is fine, as long as the alarms are genuinely Grade D with a working battery.
What you should not do is connect Grade E (mains-only) alarms to an RCD-protected circuit, because an RCD trip would leave the property unprotected with no backup to alert the standard.
5. The interconnection requirement
BS 5839-6 recommends that all smoke and heat alarms in a dwelling are interconnected so that one triggering activates all of them. This can be done with a wired interconnect cable (a third core run between the alarms) or wirelessly if the alarm manufacturer supports it.
For a wired installation, all alarms share the same live and neutral but each has its own interconnect line that runs daisy-chain between units. When one detector is triggered, it pulls the interconnect line high, which wakes all the others. This is a low-voltage signal - typically 9V to 12V - but it still needs to be run with appropriate care.
6. How many alarms does a typical house need?
The minimum LD2 coverage for a standard two-storey house is one smoke alarm on each floor level in the circulation space (hallway and landing), plus one heat alarm in the kitchen. The alarms should be sited in the escape route so that someone waking to an alarm can reach the front door without entering the room of origin.
An LD1 system covers every room. This is worth considering for homes with an older occupant, a home worker who may sleep at unusual times, or a property with an EICR code requiring it.
7. Part P and notification
Replacing a smoke alarm on an existing circuit is not notifiable under Part P. Installing a new dedicated circuit for smoke alarms, or running new wiring to new alarm positions, is notifiable because it involves a new circuit from the consumer unit. A registered Part P electrician needs to either carry out the work or certify it after inspection.
In Scotland, Building Regulations came into force in February 2022 requiring all existing homes to have interlinked alarms to the LD2 standard. If you own rental property in Scotland, the requirements are more prescriptive still.
8. When to call us
Swapping an existing alarm on an existing circuit is a straightforward job. Adding a dedicated circuit from the consumer unit, or rewiring interconnect cable through a loft, is work for a registered electrician. Richard can certify the installation and issue the Part P certificate directly.
Need smoke alarms wired properly in Sandwich?
Richard can install mains interlinked smoke and heat alarms, wire a dedicated circuit and issue the Part P completion certificate.
Contact Richard