Electronic Access Control
Credentials, readers, controllers, electric locking hardware and the fail safe against fail secure decision that gets people trapped when it is made wrong.
How a System Fits Together
Every access control system, from a single keypad on a stockroom to a campus with ten thousand doors, is the same four parts. A credential, a reader, a controller and a locking device. Learn the four and any unfamiliar system becomes readable in a few minutes.
- Credential, reader, controller, locking device. The reader captures, the controller decides.
- Lock power must never run through the reader housing. Check for this on every survey.
- 125 kHz proximity cards broadcast a fixed number and clone in seconds.
- Ask what happens when the network drops. Offline decision making is not automatic.
The four parts
The credential is what the person carries or knows. A card, a fob, a phone, a fingerprint or a code. The reader is the device at the door that captures it. The controller is the brain that decides whether that credential is allowed through that door at that time. The locking device is what physically holds the door.
The critical point is that the reader never makes the decision. It captures and forwards. The controller decides. That is why a reader can be mounted on the unsecured side of a door without compromising the system, and why attacking a reader should not open a door on a properly installed system.
On a badly installed system it does, because somebody ran the lock power through the reader housing. Pulling the reader off the wall then exposes the lock wiring and the door opens. Check for this on every survey. It is common.
Credential technologies
Older proximity cards at 125 kHz broadcast a fixed number with no encryption and can be cloned by inexpensive hardware in seconds. A great many buildings still run on them. Say so when you survey, because most owners believe their card system is secure by virtue of being electronic.
Smart cards at 13.56 MHz can do mutual authentication with encrypted keys, which resists cloning properly, but only when the installer enabled it. A smart card reader configured to read the card serial number only is no better than a proximity card.
Mobile credentials move the credential into a phone over Bluetooth or near field communication. They are convenient, they are revoked instantly, and they depend on a charged phone. Keep a physical fallback for every mobile deployment.
Wiring and the controller
Legacy readers talk to controllers over Wiegand, a simple one way protocol with no encryption, which means the wire between the reader and the controller can be tapped and replayed. Modern installations use OSDP over an RS 485 pair with secure channel enabled, which authenticates and encrypts that link.
Controllers may be a panel in a cupboard serving many doors, or an edge device built into the reader at a single door. Panels centralise power and decision making. Edge devices simplify wiring and scale one door at a time.
Whichever it is, find out what happens when the network drops. A good controller keeps its decision list locally and continues working offline. A poor one stops admitting anybody, which turns an internet outage into a building wide lockout.
Electric Locking Hardware
The lock is where electronics meets the door and where most of the failures happen. There are three common families and each has a job it does well and a job it does badly. Fitting the wrong one produces a door that either does not hold or does not release.
- Electric strikes are the cheap retrofit and are unforgiving of door and frame misalignment.
- A door that buzzes but will not open is an alignment problem, not a power problem.
- Magnetic locks hold nothing without power and are heavily regulated for that reason.
- Electrified exit devices are the most code friendly option. Budget for the power transfer.
Electric strikes
An electric strike replaces the fixed strike plate in the frame with a hinged keeper that can swing out of the way. The latch stays extended. The frame lets it go.
Strikes are the cheapest retrofit because they need no change to the door, only to the frame, and they keep the mechanical lock fully functional so a key still works during a power failure. They are the default answer for a single interior office door.
Their weakness is alignment. A strike is unforgiving of a door that has dropped or a frame that has moved, and a misaligned strike produces the classic symptom of a door that buzzes but will not open. Fix the alignment rather than increasing the voltage.
Magnetic locks
A magnetic lock is an electromagnet on the frame and a steel armature plate on the door. Energised, it holds with anywhere from six hundred to twelve hundred pounds of force. De energised, it holds with nothing at all.
That property makes magnets inherently fail safe, which is required on some doors and prohibited on others. It also makes them heavily code regulated, because a magnet with no release path traps people. Codes generally require a request to exit device, a push to exit button and often an emergency release that cuts power directly, independent of any controller.
Magnets are also visible and are often installed badly, with the wiring run down the outside of the frame in surface trunking where anybody can cut it. Run the wire inside the frame or inside the door through a power transfer hinge.
Electrified locksets and exit devices
An electrified mortise lock or an electrified exit device puts the electronics inside the door hardware itself. The lever or the exit device is electrically controlled while the latch and the mechanical function stay intact.
This is the strongest and most code friendly option, because the door still latches mechanically, the exit device still releases in one motion whatever the electronics are doing, and the security does not depend on a strike or a magnet.
It is also the most expensive and requires power to reach the moving door leaf, through a power transfer hinge, an electric power transfer loop or a concealed hinge conductor. Budget the transfer, not just the lock. It is the part first time installers forget.
Fail Safe and Fail Secure
This is the decision that most affects whether people live or die in a fire, and it is routinely made by whoever was holding the screwdriver. It deserves to be made deliberately, in writing, with the building owner and the local authority having jurisdiction.
- Fail safe unlocks without power and protects people. Fail secure stays locked and protects property.
- Leaving must never depend on electronics working, in either mode.
- A maglock needs a hardware release that cuts the coil directly, not a controller request.
- Kill the power and walk every door. An untested mode is an unknown mode.
What the two terms mean
Fail safe means the door unlocks when power is lost. Safe refers to people. A magnetic lock is always fail safe because it cannot hold without power.
Fail secure means the door stays locked when power is lost. Secure refers to property. Most electric strikes are fail secure by default and many can be reconfigured either way with a jumper or a reversible plate.
Both are correct in the right place. A server room door should be fail secure so a power cut does not open it. A stairwell door on an exit route should be fail safe so a power cut does not trap anybody behind it.
Egress is never electronic
Whichever mode is chosen, leaving must never depend on the electronics working. That is the principle that resolves most arguments about this.
On a fail secure door the mechanical lever or exit device must still release the latch from the inside with no power and no credential. On a fail safe magnetic door there must be a hardware release path such as a break glass or a push to exit that cuts the magnet coil directly rather than asking a controller politely.
If the only way out of a room is for software to agree, the installation is wrong regardless of what mode it is in. That is the sentence to say out loud to a customer who wants a maglock with no mechanical release.
Documenting the decision
Write down, for every door, which mode it is in, why, what the release path is, and who approved it. Give the customer a copy and keep one.
That record is what protects everybody when a fire marshal inspects, when the building changes hands, or when somebody adds a door five years later and copies the wrong configuration.
And verify it physically. Kill the power at the panel and walk every door. Confirm that each one does what the document says it does. A mode that was set on a drawing and never tested on the door is a mode nobody actually knows.
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