It is the distance at which the contact changes state, and it has to exceed the opening's real movement or the system reports faults nobody can reproduce.

Doors settle, frames warp and timber windows shrink and swell with the seasons.

A contact with a small tolerance fitted to a door that later sits a few millimetres proud will report open when it is closed.

That presents as intermittent faults, usually at particular times of year, and is frequently blamed on the panel.

Wide-gap contacts exist for exactly this and cost little more.

Measure the actual gap with the door in its worst position rather than at installation.

Only if they are rated for it - steel reduces the gap tolerance of a standard contact considerably, sometimes to nothing.

Ferrous material near the magnet and the reed switch distorts the magnetic field they depend on.

Manufacturers publish metal-mount or wide-gap versions with compensated magnets for steel doors, roller shutters and metal window frames.

Non-magnetic spacers help a standard contact but rarely restore full performance.

Roller shutters are the hardest case: large, moving, entirely metal, and usually outdoors. Dedicated shutter contacts exist for them.

Test the actual installation with the door in several positions rather than accepting the datasheet figure.

A standard reed contact can, which is why high-security versions detect an external magnetic field and report it as a tamper.

Holding a magnet against the switch keeps it in the closed state while the door is opened.

The attack requires access to the contact position, so recessed and concealed contacts are already harder to defeat.

Biased and multi-element switches respond to a specific field arrangement and detect the presence of a foreign magnet.

Insurers and alarm grading schemes often require this type on higher-graded installations.

Specify it wherever the door genuinely matters. The cost difference is small against the consequence.

Flush where the door and frame allow it - they are neater, better protected and far harder to interfere with.

Flush contacts are drilled into the frame and the leaf so only a small face is visible.

They cannot easily be reached, wedged or removed, and they survive knocks that damage a surface unit.

They require drilling, which is not always possible on glazed, metal or fire-rated doors without affecting the door's certification.

Surface contacts are quick to fit, suit retrofits and are the only option on some openings, but they are visible and exposed.

On fire doors, check that any modification is permitted before drilling - the door's rating depends on its construction.

They are reliable enough for most applications provided the panel supervises them and the batteries are managed.

Wireless removes the cabling, which is the main cost and disruption in a retrofit or a finished building.

The trade is a battery in every device, with a replacement programme and a low-battery warning that someone acts on.

Supervision is essential: the panel must poll each device and report one that has stopped responding, or a removed contact is indistinguishable from a closed door.

Radio range through a building is less than in free space, and metal doors and structures reduce it further, so a survey matters.

Higher alarm grades sometimes restrict where wireless devices may be used, so check the local grading scheme and the insurer's requirements.