Douglas Krantz - Technical Writer - Describing How It Works
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Fire Alarm -- Description

Between the Fire Alarm Control Relay and a Door Holder Magnet should be a Buffer Relay to prevent the Magnet from destroying the Control Relay
Between the Control Relay and the Door Holder Magnet, a Buffer Relay prevents the magnet's turn-off voltage spike from harming the Control Relay.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

How is a Buffer Relay Wired Into a Door Holder Circuit?

By Douglas Krantz

Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut.

The door holder looks simple and innocuous enough, but it presents a special problem on installation.

The Door Holder is an Electromagnet

A door holder prevents the door from closing because it hangs onto the door using magnetism; making the magnetic field, electrical current passes through the coil of the door holder.

When the fire alarm system detects smoke, the current to the door holder turns off, allowing the door to close.

That much is obvious.

What's not obvious is electromagnets are hard on the relay contacts turning the magnet off.

Block diagram of an Auxiliary Relay being controlled by the Fire Alarm Control Relay
The door holders are switched on and off by an auxiliary relay - one that can handle the voltage spikes produced by the door holders when they are turned off. The auxiliary relay is turned on and off by the Fire Alarm Control Relay.

Transient Voltage Spike

An electromagnet, using the energy from the electrical current, builds a magnetic field, and saves this energy in the magnetism.

Once the electricity stops, the collapsing magnetic field generates a transient voltage spike. This voltage spike travels along the wires, back to the relay contacts.

Fire Alarm Control Relays are small, and each time this spike comes back, the contacts burn a little.

Burning Out the Contacts

Burning the contacts just a little every time the relay turns off the door holder is bad because the burning is cumulative. The damage from this repeated voltage spike adds up and eventually destroys the contacts of the relay.

Regular testing of the Fire Alarm System is especially hard on the contacts. When testing, each time the fire alarm system is put into alarm, the relay is exercised, and the door holders burn out the relay contacts even quicker.

To prevent this transient voltage spike from burning out the control relay contacts, an auxiliary relay with big contacts should be installed between the Fire Alarm Control Relay and the door holders. Then the auxiliary relay takes the voltage spike, and the Fire Alarm Control Relay only has to turn off the auxiliary relay.

Schematic diagram showing how to land the wires when using an Auxiliary Relay to control the doors
The Fire Alarm Control Relay is only used as a "pilot duty" switch to keep the Auxiliary Relay normally turned on. With an alarm, the Fire Alarm Control Relay switches off the Auxiliary Relay, which in turn switches off the door holders.


Compatibility, though, is an issue; even auxiliary relays can create problems for the Fire Alarm Control Relays.

From an internal capacitor, some off-the-shelf auxiliary relays, when they first turn on, have a huge surge current. This surge current will weld the contacts of the small Fire Alarm Control Relay. Once welded, the contacts of the control relay are stuck together, and the relay will never turn off.

Even though their published coil currents look like they're compatible with the Fire Alarm Control Relay, the fire alarm manufacturer does not accept these auxiliary relays as compatible. Call your fire alarm manufacturer's tech support to make sure the auxiliary relay is computable with the Fire Alarm Control Relay.

Auxiliary Relay Coil Circuit

The auxiliary relay coil is also an electromagnet. To protect the Fire Alarm Control Relay from the auxiliary relay's coil, type accepted auxiliary relays have an internal Flyback Diode already installed. (See the description of a Flyback Diode.)

Long Term Reliability

The most important thing to remember in all of this is that a door holder cannot be connected directly to a Fire Alarm Control Relay, an auxiliary relay is required.

Without this auxiliary relay, there is a good chance that sometime down the road the control relay will fail and the door will not stay open.

You can see how the fire alarm system works by ordering the book "Make It Work - Conventional Fire Alarms" by Douglas Krantz.

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