You may already be familiar with the general theory of connecting together door access systems, but there are still some mysteries surrounding getting a system to work correctly and satisfying your customer. This article looks at the mechanics of the installation.
Readers & controllers
A reader will be fitted to the outer side of the door, sited at a reasonable height and ideally on the opening side of the door to allow people to flow past the reader and into the premises. If fitting the reader externally, make sure it is protected from rain entering, and that water can drain out easily. The reader will be connected to the controller by twisted pair data cable or multi-core alarm cable, depending on manufacture.
Door contacts
These are used to monitor the position of the door. When left open for an extended time, some controllers can be programmed to generate a door open alarm. They are also used by more sophisticated systems to detect that a door has been closed and to re-apply the power to the mag-locks or electric bolts.
The contact is usually a standard NC (normally closed) type and should not be fitted close to a mag-lock as the magnetic field can affect it.
Exit request (ER)
This is usually a normally open (NO) push switch, fitted near the exit door. Because of the high usage this button receives, it should be of a high quality. The cheaper buttons will fall apart after a short time. Connect directly to the ER input on the controller. This input has a timed function to release the door for a programmable time, usually five seconds. Do not wire it directly into the lock circuit, as the lock will only release for the duration that the button is pressed.
Break-glass (BG)
The break-glass is a use-once resettable switch that releases the lock in the event of a fire or if the doors need to be kept unlocked. A BG should always be fitted for safety, and is mandatory in most buildings to conform with fire safety regulations. The switch has a change-over contact, and can be wired either normally open or normally closed.
The standard colour for door systems is green. Two types are available: a standard glass pane, which must be replaced after every use, and a resettable plastic pane. The latter has the advantage of being easily reset, without having to find and fit replacement glass. The BG should be fitted at eye level, where it can be seen easily. If fitted lower and next to the exit request button, people often get confused and press the BG instead of the ER switch. In this instance, a resettable plastic switch is better.
The BG is wired in series with the lock at each door. It should not be wired back to the PSU because if the cable becomes damaged on the run back then the door might not release.
Never wire the BG into the ER circuit on the controller – if there is a fire you will have only five seconds to exit before the door locks again permanently.
Fire alarm relay interface
It is a requirement of many industrial sites that the door locks are automatically released in the event of a fire. It will be the responsibility of the fire alarm installer to provide a clean (volt-free) P36 C V P35 change-over relay at each door. The fire relay is wired in series with the door locks.
Fitting the fire relay at the common PSU box is not recommended as the lock power is then routed through inflammable cables instead of fire-proof cables. Neither is it advisable to wire the fire relay into the access controller or access PC. If the controller or PC locks up, or suffers power failure, then all control of lock release is lost, and the doors stay locked.
Electromechanical (EM) locks
EM locks are available in either fail secure or fail safe. Fail secure locks are normally locked and need a voltage pulse to release them (see Fig 1a), whereas fail safe locks are normally unlocked and require continuous power to hold them locked (see Fig 1b). Fail safe locks are often specified on industrial installations.
Depending on which type is used, there is a big difference in the power requirements. The fail safe requires a larger PSU to provide the continuous power to hold the lock active, without failing or overheating.
Many basic access kits include an AC lock, instead of the 12v DC variety. The AC lock has a much lower coil resistance of only 10R whereas 12v locks are normally about 70R. The higher current demand will kill the 12v PSU. Ensure the correct type is fitted, and never fit an AC lock to a 12v system. Locks are available in a couple of coil resistances. For higher reliability, lower stress on the PSU, and fewer cable volt drop problems, always fit a high resistance lock of about 70R.
A VDR (voltage dependent resistor) or, in the old days, a reverse fitted diode, should be fitted across each lock to short out any reverse voltage spikes. All lock coils store energy, and when the power is turned off suddenly the stored energy is discharged into the supply cable causing havoc with any nearby data cable. Therefore, always separate the lock power cable and the data cable, and never use the same cable for data and power.
EM locks are cheap and easy to fit, but ensure that the PSU has enough power for the lock fitted.
Magnetic locks
The disadvantage of EM locks is that they fall apart with frequent usage. Mag-locks are big, bulky, and expensive, but without any moving parts they are 100 per cent reliable and last forever.
They are ideally suited to high-usage office and industrial applications, but beware: the average current demand is 0.5A per mag-lock. You must specify a suitably large PSU to cope with the high demand, and heavy-duty cable must be used to carry the extra power, or twist several thin cores together. Mag-locks do not fail, so if the lock is not holding, check the lock alignment or check for voltage drop down the cable.
Electromagnetic bolts
This is a steel bolt surrounded by a magnetic coil, which shoots out when energised. The bolt mechanism is usually fitted in the door frame, and located into a hole in the door.
Electromagnetic bolts are unobtrusive and offer greater security, but they are tricky to install and require accurate alignment between bolt and retainer, and a precision closing door. Because they are electromechanical, they are not as reliable as mag-locks, and should be used where the extra security is required. They require a door contact to monitor when the door has closed, and extra electronics to energise the bolt only after the door has closed. Otherwise the bolt energises before the door has closed, and holds it open. Bolts are available as fail safe or fail secure.
Power supply unit (PSU)
As previously mentioned, power requirements vary from a short 100mA pulse to 0.5A per mag-lock continuous. PSU manufacturers are very optimistic as to their continuous power output. The true rating of power is RMS (root mean square), which equates to the continuous current that flows through a large resistor connected to the PSU. When calculating power requirements, apply the 100 per cent rule. If you need a 1A PSU, fit a 2A PSU. Alarm PSUs are generally better then 12v power packs, as they are set to 13.8v, which allows for a 2v line drop, and still capable of supplying 12v at the lock.
Multiple PSUs
While it is common practice to fit only one PSU for both locks and controller, it should be considered standard to fit separate PSUs for each (see Fig 2). In larger systems fitted with mag-locks, because of the high current demand, it is not possible to provide enough back-up battery power for both. In the event of a power failure, the batteries will take over and quickly discharge. Once the cell voltage has dropped to below 5 volts, the standard lead rechargeable cell is damaged for good, and will not recover. When the mains power is restored, the warped and shorted cells will be a heavy drain on the PSU and cause its early failure. Reliable operation of the controller will be affected by power surges, noise and low voltage, from over-stressed PSUs.
Improved reliability and fewer maintenance repairs will ensue if the controller is provided with a separate PSU and battery, and the locks supplied from another PSU, without batteries. When the power fails, the locks drop out but the controller continues.
Once power is restored, both locks and doors will work as normal. If batteries are used on high current demand systems, a PSU with battery cut-off should be used. This will disconnect the batteries when the voltage drops below 8v, thus saving and extending the battery life.
On electromechanical fail secure locks, battery back-up must be provided to allow the doors to open in an emergency.
Doors
Access systems can lock up as a result of lightning strikes, mains power surges, or simple failure. Therefore, on controlled buildings where the main entry door is controlled by an access system, a secondary method must be devised where staff can enter the building if the access system fails to operate. The door itself should not be overlooked. Half of all ongoing access faults are caused by door problems, such as ill-fitting doors, excessive flex in door and frame, doors not closing properly, doors not closing with sufficient force, and locks not fitted correctly. All access doors will need a door closure mechanism to ensure that the door is automatically closed. Time spent in ensuring that the doors close correctly will be paid back in fewer maintenance visits.
