Electromagnetic lock installed on a glass internal door.

Electromagnetic locks are ideal for use in many access control applications, particularly together with glass doors, gates and double swing doors, but there are considerations for the installer.

Electro magnets are the larger and heavier of the 2 components that comprise an electromagnetic locking device. They’re constructed of a laminated steel core around which hundreds of turns of fine copper wire are bound. Into the ends of this wire a low voltage current is connected giving a flow of either 12 or 24V DC and creating a magnetic field in a polarised steel core. The second component is a heavy steel plate called the armature.

Magnet and plate are brought together when the door is closed and as long as the current is flowing through the magnet, the armature completes the magnet’s path from north to south and this path draws the armature to the magnet and binds it there. Operationally, if the current is turned off, the door releases and if current is turned on, the door is secure. Electro magnetic locks will failsafe when power is lost – in other words when power fails the lock will release. When built into an overall access control system magnetic locks should be opened when fire alarms activate.

Different magnetic locks have different holding forces but in essence, lock power relates directly to both the number of amperes directed to flow through the copper turns and the overall number of copper turns that encase the steel core. Powerful magnetic locks are usually large. This is because low voltages are used, so huge numbers of wire turns are required to generate the magnetic attracting power required to form a secure seal.

Another reason for the larger size of magnetic locks is that electro mechanical and mechanical locks make significant use of the structural strengths of the door surrounds to form a secure seal. Conventional lock bolts are embedded in deep strikes in a door’s outer vertical frame and may even intrude into surrounding supports. Electric strikes are also internally fitted with both constant bolt and strike plate fixed deeply into the door surrounds. In both these cases, the actual strength of locked door is dictated by the strength of a heavy bolt and the structures that support it.

Electromagnetic locks, however, rely on magnetic force solely and have no physical bolt. Despite the fact such locks might appear to be vulnerable, a powerful electromagnetic lock is capable of offering enormous holding force. In most cases the force necessary to breach an electromagnetic lock would tear a door from its hinges and frames would be destroyed long before a 700kg seal was compromised by leverage.

Installation considerations

There are important points to bear in mind when installing magnetic locking devices. The first relates directly to the strength of the door and its surrounds. While some manufacturers indicate holding forces in tonnes, it’s important to install locks with a holding force commensurate with the strength of your door surrounds.

As a general guide, choose locks with a holding power of about 300kg when setting up internal access doors and upwards of 700kg when installing locks on external security doors. During the installation you need to be sure the door has no vertical flexibility. Because mag locks are usually installed at the top outside edge of the door, any force applied to the centre or lower edges will exert considerable leverage.

Also important is placement of the magnet and armature. You’re looking for exact mating with no overlapping.
Because of their failsafe characteristics, electromagnetic locks used on external doors demand special attention be paid to back-up power systems. The issue is that intruders may cut power to the lock in order to generate a failsafe release. It’s vital to ensure power supply is constant and the door state is monitored. Also take into account that reductions or fluctuations in power will result in a weakening of the lock’s holding power.

The disadvantages of electromagnetic locks include their large size and weight, a lack of aesthetic appeal and the fact they’re more expensive to purchase than mechanical locks and electric strikes. There’s also the fact the locks generate some heat during operation and demand inherent strength in doors and frames. Advantages include a lack of moving parts to fail, or be tampered with as well as the fact these locks can be surface mounted or flush mounted and will lock swinging or sliding doors, and glass doors. Magnetic locks also have very fast release times.