Powering the Systems That Are Protecting You

Dec. 3, 2018
Choosing the right access control power supplies for system reliability, scalability and optimum performance

Access control systems are relied on 24 hours a day, seven days a week, year after year to protect people and property in countless ways. Selecting and installing the right power supply provides a safe, energy efficient, scalable, access control system with enhanced and reliable performance.

If an incorrectly sized power supply is installed, the system may fail. If a power supply provides too much voltage to the system, the lifespan of the device may be decreased. With too low a voltage, critical backup batteries and systems will be undercharged and their performance compromised. Overload it, and it will blow fuses and breakers, bringing down the system and requiring expensive service visits. If it isn’t installed correctly and doesn’t meet AHJ and local code requirements, the certificate of occupancy may be withheld — a costly consequence of not specifying a proper power supply.

For these reasons, the power supply is one of the last pieces of the access control puzzle to consider. The totality of the final access control system — with an allowance for future additions and scalability — will dictate what type and how many power supplies are needed for any given application. Here's a general breakdown of considerations to keep in mind when specifying and considering power supplies for access control.

Success in the Numbers

For powering an access control system, look at the energy consumption of the entire system and all of its components. For example, a single lock may draw 125 milliamps at 12 volts. If there are eight locks, then we're looking at a full amp. A system with 100 locks will require multiple amps, with the factored-in addition of card readers, push buttons, request to exit devices, sensors, etc. Anything on the access control system and drawing power has to be included in the energy calculation.

Once the total power consumption of the system is calculated, say for our example 4.5 amps, we'll want to specify a power supply that provides at least 4.5 amps. In general, a best practice within the field is to add about 20 percent to the power requirements. The added capacity will allow the customer to easily expand the system without needing to install a new power supply for additional devices. It's very common throughout the life of a building that access control needs change, whether that means expanding or contracting. Headroom and flexibility are vital in avoiding headaches down the line. In this case, we would probably specify a 6 amp rated power supply.

With total device energy calculated, we now have to take into account the number of outputs needed. Power supplies most commonly come in 4-, 8-, and 16- output configurations. So, we can connect, theoretically, up to 16 doors to a single power supply. But if the 16 doors draw more amperage than the power supply can provide, multiple power supplies may be needed, even if it physically has enough outputs for the quantity of doors.

Access control systems are generally either 12- or 24-volt systems. Single voltage power supplies output either 12 or 24 volts, have to be specified accordingly from the factory, and cannot be reconfigured in the field. A dual voltage power supply can be switched in the field to be either 12 or 24 volts. Additionally, some power supplies provide for multiple voltages simultaneously. The advantage of a dual or multi-voltage supply is having fewer power supplies to stock and carry in the field, as well as flexibility for addressing a wider range of devices in your application.

Always Have a Backup Plan

Another important factor for power is the use and maintenance of backup batteries, a best practice for any security and safety-related system. Most integrators and installers use rechargeable sealed lead acid backup batteries in their power supplies, and most access control power supplies are designed to charge and maintain those batteries natively.

When a 24-volt power supply requires back up batteries, these batteries typically require between 27 and 27.6 volts to fully charge – more than the 24-volt rating of the power supply. What often happens is that some power supplies on the market generate that 27 volts to charge the batteries, and then they provide that same 27 volts down the line to the locking devices. Often those locking devices are rated for 24 volts (plus or minus 10 percent), which means the maximum voltage they want to see is about 26.4 volts. This conflict in power can shorten the lifespan of the locking device. Alternatively, other manufacturers reduce the power supply voltage to 26.4 volts, which is safer for locking devices but means the batteries may never charge to their full rated capacity.

For power supplies with backup batteries, consider models that have a dedicated battery charging circuit. This dedicated circuit generates the 27.5 volts needed for optimum battery performance, completely separate from the power supply’s standard 24-volt output used for powering the locking devices, protecting them from compatibility issues, overheating solenoids, and a reduced lifespan; while proving full charged backup batteries.

Further, some power supplies have available battery monitoring technology built in to detect the end of life for the batteries. Whether or not your power supply has this capability, it is a best practice to write the date of installation on your batteries, and to change them for new batteries about every three years. This will ensure that you have your full battery backup time in the event of an extended power outage.

Guaranteed Performance

Having a successful security strategy in place means making sure that all physical aspects of a security system — locks, remote and mobile monitoring applications, access control systems, cameras, networks, intercoms, emergency exits, perimeter sensors, lighting, and all other safety aids — are installed, appropriately powered and fully functional. Within the layers of a security plan, the total solution is only as strong as its weakest point.

Access control systems are critical for life safety and security. Power supplies are often the heart of these systems. Taking the necessary steps to identify current (and possibly future) access control needs, as well as customer and code requirements, will lead to a stronger, more secure access control system that will satisfy needs, and secure any opening, for years to come.

David Corbin is the Director of Product Management for Access Control Accessories at ASSA ABLOY Electronic Security Hardware. David has five years of experience in the access control industry and can be reached at [email protected]