Tech Tips: Electromagnetic Locks, Wiring & Power Supplies

Even though most electromagnetic locking systems will include other components in the system, the power supply is the one absolutely necessary component for any electromagnetic lock installation,

Step 1 in design and approval is the wiring diagram. There are two types of diagrams.

Point to point shows all products and wiring connections needed between them. These diagrams are useful for future troubleshooting and are needed to develop a Riser diagram.

Riser Diagrams show only the number of wires and distance between products. They are useful in determining wire size and number of conductors needed for job.

Power Supplies come in many shapes and sizes. The unique requirement for a power supply intended for use with an electromagnetic lock is called the Fire Alarm interface; that is that the electromagnetic lock immediately unlocks when the premises fire alarm is actuated. There are a number of ways this can be accomplished, but IMO you are best advised to use a power supply designed with this feature, or at least use a power supply which is housed in a locked metal enclosure where splicing and interface connections can be performed and then protected from tampering or being otherwise disturbed. Code requires wiring and components to be enclosed and protected.

Most times the locksmith will not be providing the premises fire alarm, so when writing up the project for the customer, you need to include wording that states that a premises fire alarm is required, the electromagnetic lock must be interfaced to it, and the interconnection shall be “BY OTHERS.”.

Where a building has the FAS protection, NFPA 72-2010 requires that any device used to electronically lock a door in the direction of egress must connect to the FAS. The required unlocking function must occur prior to, or concurrent with, actuation of any public-mode notification appliances in the area(s) served by the normally locked egress doors. Further, all doors required to be unlocked by the FAS must remain unlocked until an authorized person resets the status of the FAS control unit. Examples of doors normally kept closed and locked include those doors opening into a stair enclosure or doors in the horizontal egress (exit) pathway leading from the building.

Additional and related requirements are in the building code and Life Safety Code (LSC). The LSC has the same unlocking requirements but it does not allow the actuation of a manual fire alarm to initiate the unlocking system.

Where the locked doors could isolate a person, such as in an elevator lobby, the LSC requires a two-way communication system for contact between the elevator lobby and a central control point, constantly staffed by personnel who have received training and who are authorized to provide emergency assistance.

Additionally, per NFPA 72-2010, where batteries serve as the secondary power supply, the system design may not use batteries to maintain the doors in a locked condition unless the fire alarm control unit has circuitry and sufficient secondary power to ensure the exits will unlock within 10 minutes of the loss of primary power. The exception to this rule: locks powered by independent power supplies dedicated to lock power and access control functions, which will unlock on loss of power, need not comply with the previous requirement.

I’ve seen the fire alarm contractor provide an interface relay which is operated by the fire alarm and I would bring a pair of wires to the location of this relay; and then some arrangement was made for the electrical connection and functional test of the access control’s unlocking in an alarm condition.

Sometimes the fire alarm contractor would supply the wiring between the access control and the Fire Alarm Control Panel (F.A.C.P.). It is still wise to be present for the functional test, to indemnify your company against malfunctions or problems later.

Most fire alarm contractors are familiar enough with power supplies with the fire alarm interface terminals, and only need to know if they are dealing with dry contacts or need to provide a voltage.

The circuits used are very basic, but they must be executed in a legitimate fashion, so that they work 100 percent and also so they can be inspected and approved by the Authority Having Jurisdiction (AHJ).

As we’ve told you in previous installments, the AHJ is the person who “approves”, and the agency or job title of the AHJ may vary from job to job and region to region. And of course there may be multiple AHJs perusing the system; the building inspector, the Fire Marshal, the electrical inspector…etc.

There are access control power supplies which use a plug-in low voltage transformer which connects to a power supply module. As mentioned, it is best to place modules like this in a protective metal enclosure. Enclosures should be clearly labelled, and the cover secured either with a cam lock or screws. Some jurisdictions may require the enclosure be painted red, since it has a connection to the fire alarm control panel.

It may sound like a great idea to use plug-in transformers, but in reality it is not advised. The concept is that by using a plug-in, you do not need an electrician to wire anything or supply a receptacle. You figure you will use and existing receptacle. However using an existing receptacle may be problematic. The closest receptacle may not be close to the system or the receptacle may be on a circuit powering other equipment and may already be at maximum capacity. The receptacle may be controlled by a wall switch and might be turned off at inopportune times.

I’ve also been on projects where I was working above the dropped ceiling, and found a convenient receptacle. Receptacles above dropped ceilings are not for the permanent connections to power supplies but for temporarily power for workmen and maintenance.

Other power supplies are furnished with line voltage connection points, and the step down transformer is in the metallic power supply enclosure.

You can have an electrician hard-wire a dedicated circuit from a breaker panel or convenient branch circuit to the power supply, or you can obtain a line cord and plug the power supply into a receptacle. All the same rules apply in these situations as do with plug-in transformers.

There are two distinct designs used for power supplies: linear and switching. The linear have been around forever and they use the large step down transformer.

The switching type are relatively new, frequently used for computer peripherals, and are smaller and do not use a step-down transformer. They are ‘efficient’ meaning that there is little heat generated, and their size is not proportionate to the power capacity.

Access control equipment will clearly state in the installation instructions if there are any restrictions as to what type power supply can be used with a device.

There may also be recommendations from the manufacturer of the access control product or the electric lock as to whether a single power supply can be used or if separate power supplies, one to the lock, and one for the controller are preferred.

This consideration may be based on more than the power capacity of the lock. It may have to do with the sensitivity of the access control device to ‘noise’ in the power supply used to operate it, or sensitivity of the lock to the absolute voltage level supplied to it or power quality issues such as surges, drop outs or noise.

Sometime noise is ‘injected’ by nearby equipment then carried along the line voltage supply and will ‘infect’ other systems, such as the access control. You may notice emission specifications on products indicating they do not emit noise, or they are not sensitive to noise sent in power or field wiring. Field wiring refers to cabling which connects the access controller to power, to the lock it is controlling and to card readers and station controls (example touch bars or REX switches)

Linear DC Power Supply: Linear DC power supplies were the mainstay of power conversion until the late 1970s with the advancement of switching power supply technology.

A linear power supply uses a big transformer to drop voltage from AC line to much lower AC voltage, and then uses a series of rectifier circuitry and filtering process to produce a very clean DC voltage.  

Switching DC Power Supply: Switching DC power supplies are the most popular form of DC power supplies in the market due to their exceptional power efficiency and performance.

Switching DC power supplies regulate the output voltage through a process called pulse width modulation (PWM). The PWM process generates some high frequency noise,

Switching power supplies are highly engineered, and since there are more components in a switching power supply as compared to a linear one, there are more things that can go wrong.

The electrical noise created by switching power supplies is sometimes referred to an oscillation. From my own experiences, when a switching power supply fails, it can have disastrous consequences. However switching power supplies have matured, and the technology and protective circuits have improved immensely.

You may wind up needing an electrician after all.