A mains powered alarm takes its supply from the property's electrical installation and keeps a standby cell for power cuts. The reason specifications call for them is not that they detect better, because they do not. It is that nobody can take the battery out of them for the television remote, which is the most common way a protected home becomes an unprotected one.
What mains power changes
The alarm is powered by the fixed installation and holds a backup cell that carries it through a power cut. Battery removal stops being possible on sealed units and stops being useful on the rest. In a rented or licensed property this is the whole argument: the protection no longer depends on a tenant replacing a battery in a device they did not choose.
How they are wired
Alarms are supplied from the fixed wiring, commonly from a dedicated circuit or from a lighting circuit depending on the design, with an interconnect conductor between units where the interlink is by cable rather than radio. It is electrical work, it should be done by a competent electrical installer, and it should produce an electrical certificate as well as leaving you with working alarms.
When the specification demands them
The grade in a dwelling specification is what decides this, and licence conditions for houses in multiple occupation frequently state it directly. Meeting the coverage with battery alarms where mains powered units were specified is a real failure even though every room has an alarm, and it is one of the most common reasons a well meaning landlord fails an inspection.
Service life still applies
A mains powered alarm is still a sensor with a finite life, usually stated on the back with a manufacture date. Mains power removes the battery failure mode; it does not stop the chamber ageing. Replacement on the stated schedule applies exactly as it does to battery units, and it is the part of the regime landlords most often miss because the alarms look fine.
A smoke alarm mains powered unit, and what the backup is for
Mains power is what allows a unit to be supervised and interlinked reliably, and the battery behind it exists for the power cut, not as the primary supply. Two practical consequences. The backup cell still has a life, so a mains unit is not maintenance free: it has a replace by date like any other and on many models a failing backup produces the chirp people mistake for a fault. And the alarm must be on a circuit nobody switches off, which in practice means a lighting circuit rather than a socket, because an alarm plugged into a spur that gets isolated at the board is a decoration.
Smoke alarms sale prices, and what a cheap alarm leaves out
The gap between the cheapest alarm on a shelf and a mid priced one is not the sensor, it is everything around it. Check for the standard first: a domestic smoke alarm should be marked to BS EN 14604, and a third party approval mark on top of that is what tells you somebody audited the factory rather than the paperwork. Then the battery: a sealed ten year lithium cell costs a few pounds more than a replaceable nine volt and removes the failure mode that actually kills people, which is a chirping alarm with the battery taken out. Then interlinking, because a bargain alarm is almost always standalone, and an alarm in the hall that cannot wake a sleeper upstairs is doing half the job. A silence button, a proper mounting plate and a visible expiry date are the rest of the difference, and the expiry matters: every alarm's sensor has a service life, usually ten years, after which it is replaced rather than tested.
Domestic smoke alarms, and which grade a house needs
The domestic standard describes systems by GRADE, which is about the power and the control equipment, and by CATEGORY, which is about coverage, and the two together are what a specifier means by a Grade D2 LD2 system. Grade D is the common domestic arrangement: mains powered detectors with an integral backup, interlinked, with no control panel. Grade F is battery only. Grades A to C involve a control panel and are used in flats, shared houses and larger properties. Category LD1 covers all circulation spaces and all rooms where a fire might start; LD2 covers circulation spaces plus the higher risk rooms; LD3 covers circulation spaces only, which is escape oriented and the minimum for many existing homes. New build in England commonly lands on Grade D LD2 and Scotland's requirement is effectively LD2 with a heat alarm in the kitchen. The reason to know the vocabulary is that it is what a letting agent, a licensing officer or a building control inspector will write in their requirement, and a quote that does not state a grade and a category has not stated what it is fitting.
Questions people ask about wired smoke detectors
Do mains powered smoke alarms need batteries?
They carry a backup cell so they keep working through a power cut. On sealed units it is not user replaceable and the whole alarm is replaced at end of life.
Can I fit mains powered alarms myself?
It is electrical work on the fixed installation, so it needs a competent electrical installer, and in a rented or licensed property you will want the certificate that comes with it.
Are mains powered alarms required by law?
Not by the statutory minimum for rented homes in England, which simply requires working alarms. They are required where the specified grade or a licence condition says so, which covers a great many rented and licensed properties.
How long does a wired smoke alarm last?
The sensor still ages, so the unit is replaced on the manufacturer's stated life regardless of being mains fed, typically about a decade. Mains power removes the flat battery, not the service life.
Do interlink smoke detectors have to be mains powered?
Not necessarily, and what matters more is whether they interlink at all. Radio linked battery units are accepted in existing homes and are what makes retrofitting practical between floors; mains powered units with a battery backup and either a hard wired or radio interlink are the norm in new work and in most rented standards. Two practical points decide the choice: whether a cable can be run without lifting floors, and who will change the batteries, since a sealed ten year cell removes that question and an alkaline one does not.
Are mains powered interlinked smoke alarms required, or recommended?
It depends where you are and what you are doing. In Scotland and Wales an interlinked system to the legal standard for homes is required, and mains powered units are the usual way of meeting it though sealed battery units with a radio interlink are accepted. In England it is required where alarms are installed as part of building work and is the standard most rented and licensing regimes ask for, while an owner occupier is under no duty at all. The common ground is the interlink rather than the wiring: a fire downstairs has to wake somebody upstairs.