It's in thousands of premises. And many aren't the owner's.

Visonic is a long-standing wireless intruder detection house, today inside Johnson Controls. Its well-known family is PowerMaster: a compact panel that acts as both panel and communicator, with detectors, contacts, fobs, keypads and sirens over radio, and with models that include camera detectors able to send a sequence of what has just walked past.

And there is one detail that explains a lot: its radio is the same technology —PowerG— used by the other intruder detection brand in the same group. It is not a small-system radio: it is one of the best on the market. You'll find it in retail, hospitality, small offices, homes, clinics and workshops, but above all fitted by alarm companies on a monthly fee contract, by the thousand. And that changes the conversation completely.

A great many of these installations were fitted by an alarm company on a fee, and in that model the equipment is sometimes the client's and sometimes not, the alarm company holds the installer code and the transmission goes to their alarm receiving centre and only to theirs. It is not a con: it is a legitimate business model and for a lot of people it is the right one. But you have to know which situation you are in before asking for quotations.

There are three questions: who owns the installed equipment, who holds the installer code, and whether it can transmit to a different alarm receiving centre. If the equipment is the client's and access can be obtained, the sensible thing is often to keep the system, audit it, fix whatever is badly placed and change who maintains it — a good deal cheaper than starting from scratch. If the equipment is not theirs or is tied, you have to allow for replacing it. What we do not do is promise that everything can be reused before seeing the contract.

It is a wireless system from top to bottom, and that is an architecture decision, not a shortcoming: the panel is small and everything else arrives by radio. It is not a hybrid panel that grows on a bus with wired expanders spread around a building; it is designed for another size.

The radio does what has to be done and does it very well. It is two-way, so the panel knows whether each device is alive, with how much battery and with what signal, and there are no silent blind spots. It is encrypted, it hops frequency —which makes it much harder to knock out with a jammer— and it shares out the time between devices so they do not talk over each other. The range inside buildings is among the best in its class, and that shows in fewer repeaters.

The battery life comes from that: the devices last years on their batteries, and how many depends on how the supervision is configured, which is a setting with a price and one that can govern the grade. The building is split into partitions with their own arming, users and schedules; outwards there is transmission over the network and over the mobile network, and an app for the user. And the piece with most value in practice is the camera detectors, which turn an alert into something you can look at before moving anybody.

It has two things in its favour. The two-way radio: the alerts that are not intrusion —low battery, device lost, interference, cover open— arrive identified and in advance, so they become a work list instead of a fright in the night. On wireless systems, that family of alerts burns the user out as much as a false alarm does.

What it does not fix —and here we have seen it especially often, because these are installations that go in very fast— is the mounting. Detectors put where the bracket could be stuck rather than where the detection was needed. One looking at the shop window onto the street with the afternoon sun full on it. One in the blast from the air conditioning. An indoor one covering a terrace, or at the wrong height in a premises with a resident cat. With the log and the images that is diagnosed in an afternoon, and it is fixed by changing the place, the lens or the angle, not by turning the sensitivity down. And a zone under building works is isolated on paper and with a date for putting it back.

The European standards divide alarm systems into security grades according to the risk, and each grade demands things of the equipment, the power supply, the tamper detection, the supervision of the transmission and, on radio, how often each device is checked and how interference is detected. Which grade each model certifies is on its certificate, with a date, not on a website; which grade an installation meets depends on the whole build and is signed off by whoever hands it over. And at the top end of that scale there are installations where what is asked for pushes you towards cable: if that is the case, it gets said before selling. The receiving centre is a separate decision and here it is also the contract question: if it is tied to the one belonging to whoever fitted it, changing supplier can mean changing equipment. With the system free it transmits to whichever the client chooses, with the usual obligation: verify the alert before calling the police out, which is exactly what the image is for.

It fits the premises, the small office, the home, the clinic, the workshop: installations of a contained size, with no building works possible, that you want protected soon. It fits especially if you have to reach difficult points inside a building, because its radio is one of the best at that. And it fits, above all, when it is already fitted and the contract allows you to keep it.

It doesn't fit when the installation grows: this is not a hybrid panel that spreads through a large building with wired expanders and dozens of areas, and forcing it into that is asking it to be what it is not. Nor when the grade needed pushes you towards cable. And when a system like this has been in place for many years there comes a point where patching it costs more than renewing it; we say so when that point arrives, even if it means a higher quotation than the client expected.

The client handles the day to day: users and codes, who arms which partition, part-arming, schedules, arming and disarming remotely, inhibiting one zone temporarily, the history with its images and the state of the batteries and the signal. On installations that come from a fee contract this is usually new to the client, because they had access to almost nothing, and it is one of the things most appreciated at handover.

What it asks of the installer is the layer underneath: pairing devices, changing the supervision, the times or the confirmation logic, redoing partitions, touching the transmission paths and upgrading the version. And if it was handed over with a grade, every change means the documentation has to be reviewed. Batteries are decided on purpose: with few devices the client changes them if they are left the type in writing and warned that opening the cover sets the tamper off; with many it is a calendar item in the contract. And the warning that goes with convenient systems: how easy it is to switch something off from the phone is also how easy it is to leave it switched off, so the maintenance report starts with the list of what has been inhibited for a long time.

What is handed over and charged for: training by role and a folder with the plan showing every device, its battery type, its signal margin at handover, which detector verifies with an image, how the partitions and schedules were left, the ownership of the account and of the passwords, and who gets which alert and in what order. On an installation coming off a fee, that folder is the proof that the system is now theirs.