There is a small white or grey block on the frame of your door, and a matching block on the door itself. On a metal door they are recessed into holes so only a disc is visible. On an overhead door there is a larger armoured version on the floor. Nobody looks at them. They are the cheapest devices in the entire security system, they are the first thing an installer treats as a commodity, and they are the reason your system is able to tell you anything at all about that door.
Everything downstream depends on them. The alarm that a door was forced. The alert that a door has been standing open for four minutes. The line on a report that says a loading bay door was open for ninety minutes on a Sunday. None of that comes from the reader, the lock, or the controller's cleverness. It comes from a switch screwed to a frame in a building that has been moving underneath it since the day it was installed.
That is the whole argument of this article: the humblest device in the system carries the most reporting weight, and it is the one most buildings have specified without thinking about at all.
A conventional magnetic contact is two pieces. One is a reed switch — a sealed glass capsule containing two thin ferrous blades, wired into the alarm circuit. The other is a magnet, with no wiring at all. The switch goes on the frame, the magnet goes on the door, and they sit close together when the door is shut.
When the magnet is near, the blades inside the capsule are pulled together and the circuit is closed. When the door swings away, the magnet leaves, the blades spring apart, and the circuit opens. The panel sees the change and reports it.
That is the entire mechanism. There is no electronics in it, nothing to configure, no firmware, and nothing to fail in any interesting way. It has one job: report whether the door is in the position it is supposed to be in.
Notice the precise wording, because the imprecision is where owners get misled. A contact reports position, not security. It does not know whether the door is latched. It does not know whether the strike engaged, whether the closer pulled the door fully home, or whether someone is holding it shut with a foot. It knows that the magnet is where it expects the magnet to be. A door resting against its frame unlatched, with the closer defeated and the latch bolt never having entered the keeper, reports exactly the same state as a door properly secured. The system will show a tidy green icon either way.
What "balanced" adds
A balanced magnetic switch — a BMS, sometimes sold as a high-security contact — does the same job, and is installed in broadly the same way, but the sensing arrangement inside is different.
Rather than a single reed responding to a single magnet, a balanced switch uses a balanced magnetic field arrangement together with a biasing magnet inside the switch housing itself. The switch is not simply asking "is there a magnetic field here?" It is holding a specific magnetic balance, and it reports an alarm when that balance is disturbed in either direction — too little field or too much.
The security property that follows is the whole point: a balanced magnetic switch is substantially harder to fool with an external magnet than a standard contact is. A conventional reed switch is, by design, a device that closes when it senses a field of roughly the right strength in roughly the right place; it has no way of distinguishing the field it is supposed to be reading from any other field of similar character. A balanced switch is designed so that introducing an additional external field disturbs the balance it is holding and produces an alarm rather than a silent hold.
We are not going to describe how a standard contact is defeated, what would be used to do it, or under what conditions it works — and you should be wary of any vendor who volunteers that in a sales meeting. The property is what matters to an owner making a specification decision. One device can be induced to report "closed" by a field that did not come from its own magnet. The other is built to notice that something changed. That difference is worth paying for on some doors and is genuinely not worth paying for on most.
Balanced switches also tend to be built for harder service — armoured cable, wide-gap variants for overhead and roll-up doors, tamper protection on the housing. Some of the real-world benefit at a loading bay is simply that the device is more robust than the commodity part, quite apart from the sensing arrangement.
The gap problem, which affects both
Here is the failure mode that costs BC buildings more nuisance than everything else in this article combined, and it applies to standard and balanced contacts alike.
Every contact has a working gap — a distance between switch and magnet within which it reliably reports "closed," and beyond which it reports "open." That figure is published by the manufacturer for the specific part, it differs between models, and it differs sharply depending on whether the contact is mounted on a steel door or a wooden one, because surrounding ferrous metal changes the behaviour. We are deliberately not printing a number here. The number that matters is the one on the data sheet for the device on your door, and any figure quoted from memory in a meeting is a guess.
The problem is that the gap is not a constant. Buildings move.
A door sags on its hinges. A closer is adjusted, or gives up. A frame shifts with the seasons and with the building settling. A door warps in a Lower Mainland winter — wet, cold, then heated from one side only — and does not sit flat again. A contractor rehangs a door and mounts the magnet a little off from where it was.
When that happens, the door stops closing squarely, and the switch-to-magnet distance at rest is no longer what it was on commissioning day. Two things go wrong, in opposite directions.
The door reports open when it is shut. The gap has grown past the working range. You get an intermittent, maddening fault: the door alarms at random, mostly in cold weather or when the wind is up, and always when nobody is standing there to see it. Somebody eventually asks for the point to be disabled so people can get some sleep, and the door is now unmonitored, permanently, with no record of why.
The door reports closed when it is not. This is the dangerous direction. The door is resting against the frame, unlatched, and the magnet is still just inside the working range. The panel is satisfied. The report is clean. Anyone can pull that door open by hand and nothing anywhere records it. Every dashboard in the building shows that this door has been secure all month.
Both of these are mechanical problems, not electronic ones, and neither of them will announce itself as a contact problem. They will present as an alarm nobody can explain, or as no alarm at all.
Your contact is a maintenance instrument
This is the part most owners have never been told: the door position contact is not only a security device. It is the sensor that makes your door hardware measurable.
The door held open and door forced open reports depend entirely on it. Held-open is the contact saying the door has been in the open position longer than the timer allows. Forced-open is the contact reporting an opening that had no corresponding grant. We have written separately about the four access control reports worth running, and those two alarms are the third of them. If the contact is out of adjustment, both reports are fiction — either full of noise or reassuringly, falsely, empty.
Read the other way, this makes the contact a diagnostic tool for the rest of the opening. A door that starts producing occasional forced-open alarms where it produced none for two years is usually telling you something physical: the closer is weak, the strike is drifting out of alignment, the door is no longer being pulled fully home. The contact is often the first instrument in the building to notice that a door is failing, months before anyone reports it as a complaint.
That is why the contact belongs in your inspection cycle rather than in the category of things installed once and never revisited. BC's Fire Code sets testing intervals for doors in a means of egress at Article 2.7.2.1, and our article on access control maintenance and what BC law requires covers those obligations properly. Confirm the edition governing your own building rather than taking any article's word for it, including ours. The practical point for this subject is narrower and applies to every controlled door, egress or not: when somebody is at the door doing a scheduled test, checking that the position point still reports correctly with the door closed and latched costs a few seconds and catches the silent failure above.
Where a BMS is justified, and where it is not
Balanced switches are not a building-wide upgrade. Fitting them to every office door is a way of spending money without changing your risk position, and the honest answer for most interior openings is that a good conventional contact, correctly mounted and kept in adjustment, is entirely adequate.
The doors where the upgrade earns its place have one or more of these characteristics.
The door protects something specifically valuable or specifically regulated. A cash room, a pharmacy or controlled-substance store, a secure records room, a cannabis storage area, an evidence or firearms room, a data centre cage, a high-value stockroom. The test is not how expensive the door is — it is what one undetected opening of that door would cost you.
The door is unobserved. A perimeter door on a quiet elevation, a rear loading bay, a roof hatch, a mechanical or telecom room off a service corridor. A door on a busy lobby is watched by dozens of people every hour. A door nobody passes for three days is defended entirely by its sensor.
A regulator, insurer, or contract has asked for a higher-grade contact. Some regimes and some policies specify a high-security or balanced device by name for particular rooms. If yours does, this is not a design decision — it is a requirement, and the specific grade named in the requirement is what has to go on the door.
The door is industrial and the commodity part keeps failing. Overhead and roll-up doors move far enough, and vibrate hard enough, that the wide-gap armoured versions are often the correct engineering answer independent of any security argument.
Everywhere else, spend the money on adjustment instead. A properly aligned conventional contact on a door that closes squarely outperforms a premium device on a door that sags.
What to ask for
Three questions, and an installer who works with this regularly will answer all three without difficulty.
Which doors have balanced switches, and why those? Ask for the list in writing, with the reason beside each door. If the answer is "all of them" or "none of them," neither is a design — both are a default.
What are the published working gaps for the devices on my doors, and what was measured at commissioning? The data sheet figure and the as-installed figure are different numbers, and the difference is your margin before seasonal movement starts producing faults.
Which door points are currently disabled or shunted, and who authorized each one? This is the most useful question on the list. Disabled points are how nuisance alarms get resolved, and they are almost never re-enabled once the annoyance stops. A building with six shunted door points has six doors it believes it is monitoring and is not.
The device on your door frame costs less than almost anything else in the system, and every report you will ever run about that door is only as true as that device is. It deserves five minutes of thought at design, and five seconds of attention every time somebody is standing at the door for any other reason.
