A wired system carries both the signal and, in most cases, the power along cable chosen to keep working while a fire is burning. That cable specification is the part people skip past on a quotation and it is one of the few places where a genuinely cheaper installation is a genuinely worse one.
The cable is part of the system
Fire alarm wiring uses fire resisting cable so that a circuit passing through a compartment already on fire keeps working long enough for the rest of the building to be warned. Standard electrical cable does not do this. Where the standard requires enhanced performance rather than standard performance, that is a specification decision tied to the building, not an upsell, and it should be stated in the design.
Mains supply plus standby
The panel takes a dedicated mains supply, normally from its own labelled protective device that nobody will switch off during other work, and holds a standby battery sized to run the system through a mains failure and still sound the alarm at the end of it. Both halves get tested: a system that works on the mains and has a dead battery has no resilience at all.
Where a hardwired fire alarm is simply correct
New build and full refurbishment, where cable routes are open anyway. Buildings with many devices, where battery replacement across a radio system becomes a programme in its own right. Industrial environments where radio paths are unreliable. In all three the wired answer is cheaper to own even where it is dearer to install.
What to check on an existing wired system
Ask whether the installation certificate names the cable type and whether later alterations used the same. Extensions wired in ordinary cable are common and they quietly downgrade the whole circuit they sit on. It is a five minute question for a maintenance engineer and it is the sort of thing that only surfaces after an incident if nobody asks.
Questions people ask about wired fire alarm system
Can ordinary electrical cable be used for a fire alarm?
No. Fire alarm circuits use fire resisting cable so that they survive long enough to warn the building. An extension wired in standard cable downgrades the circuit it joins.
Does the panel need its own circuit?
It should have a dedicated, clearly labelled supply so that it is not isolated during unrelated electrical work. Finding the panel on the general lighting circuit is a common and avoidable defect.
How is the standby battery tested?
Under load during the periodic service, not by looking at it. A battery can hold a resting voltage and still fail the moment the mains drops, which is exactly when it matters.
Is a mains fire alarm always better than a battery system?
For anything beyond a single dwelling, yes, and the reason is supervision rather than power. A mains system with a panel monitors its own circuits and tells you when a detector is missing or a wire is broken, which a battery unit cannot, and it holds a standby supply so it survives a power cut. In a house, mains powered interlinked units with battery backup are the norm for new work and for most rented standards; sealed battery units with a radio interlink are accepted in existing homes and are what makes retrofitting possible without lifting floors.
Do mains fire alarms still work in a power cut?
They should, and that is what the standby battery is for. The panel takes a dedicated mains supply and holds a battery sized to run the system through a mains failure and still sound the alarm at the end of it. Both halves are tested, because a system that only works while the power is on is a system that fails in the one situation that often takes the power out.
Is a mains powered smoke detector the same as mains wired smoke alarms?
In a house, yes: the words describe the same thing, a detector fed from the mains with a back-up cell. On a panel based system the language is different, because the detector head is powered from the panel's own circuit rather than from the building's lighting or power, and it is the panel that takes a mains supply. If a quote mixes the two vocabularies, ask which of the two systems is being priced.