Troubleshooting Power Supplies for Access Control Systems

Last week I was dispatched to service an access control installation. The system was comprised of about 12 readers distributed in three different buildings at a private middle school which had a campus and dorms. School was out for summer.

The system was network based. Each door had a reader, a door position sensor and an electric lock which connected to an interface at the door. Each interface was home run back to the head-end controller via RS-485.

The head-end controller was connected to the network. The head-end communication to the server was also on the network. System management was done via software installed on a PC, managed by an operator with a valid username and password.

I was told that the readers were ‘dead’ in one of the buildings. The building with the dead readers was the building in which the head-end was located.

There were two power supplies at the head-end: one for the electric locking devices and the other for the head-end and the readers and door interfaces. The power supply for the head-end and interfaces was down, and the head-end backup battery was depleted to where the head-end crashed. The failed power supply was toast.

I provided two new power supplies to replace the one which had failed. The power supply which had failed had already been recently replaced by one of our techs, and now this one, which was the same model as the original failed unit, had failed again.

The cause for the power supply failure was unknown, but the power supply’s output rating was theoretically more than adequate for the application. It was a regulated design and should not have caused a total failure.

Once the new power supplies were deployed, the system restarted. But then the customer immediately began complaining that all the doors were locked, and they were scheduled to be unlocked at this time of day.

The system administrator logged in, and sure enough, the internal clock in the head-end had stopped and needed to be reset. I was surprised how long it took for the system activity log to scroll through as the doors and head-end synced up with the server. The power had been out for quite a while and apparently no one noticed.

Avoiding Problems

Here are some ideas to help you to hopefully avoid situations such as this, or provide you with some effective measures to resolve problems once they manifest themselves.

Several factors need to be considered when selecting a power supply for a particular application.

Select a power supply which is properly rated for the project. Voltage drop must be calculated and compensated for, and the power supply current capacity should be 25 percent more than the calculated load requirement.

Power distribution and power management are best addressed during the planning stages of any project, not as you are driving to the installation, or when you are troubleshooting a system failure.

Access control and life safety systems are subject to codes, and NEC Article 725 and UL 294 are two of the most important. These relate to “power-limited” power supplies, cabling and devices, and you will usually be required to be in conformance.

Briefly, in a Class 2 circuit, the limit is 100 VA, with a maximum voltage of 30 and current of 8 amperes. (Note that Class 2 circuits are not considered a danger to personnel.)

The gauge and number of conductors, the type of insulation on the cable (Plenum or Non-Plenum) are both critical to the proper performance and safety of the installed system.

Performance of the system is dependent on the correct voltages reaching all the equipment which is a function of the appropriate gauge wire. The safety issues involve the heating and ignition of under-rated wiring, and the toxic fumes non-plenum insulated cable generates when exposed to flame or excessive heat and the possible endangering of lives if this non-plenum cable is inadvertently or deliberately installed in a plenum.