Fire integration with Door Holder unit

Buffer Relay Wired In Door Holder Circuit?

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. 

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.

A relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. The heart of a relay is an electromagnet (a coil of wire that becomes a temporary magnet when electricity flows through it).

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. 

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.

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

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.

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. 

Most modern fire alarm common controls manufactured today utilize common "on board" relay contacts that are rated at 24 volts DC. They are designed to interface with a fire alarm communicator for offsite monitoring of trouble, alarm, and supervisory conditions. In many instances, you must provide additional relay contacts to handle things like fan shutdown, fire dampers, etc.

110VAC door holders - Must utilize a 24VDC relay contactor that is triggered by a relay on the fire alarm's common control. Under no circumstances should you run the 110VAC for the door holder into the common control cabinet (although there are a number of older systems out there in which this was perfectly acceptable, electrical codes have changed). You must install the relay contactor into a separate box and ensure you maintain a physical separation between the high voltage and low voltage terminals. IMPORTANT! The door holders must utilize a 110VAC source that is separate from the fire alarm system.

24VAC door holders - Must utilize a 24VDC relay contactor that is triggered through the fire alarm's common alarm relay as well. You can install a relay block into the common control cabinet, but the door holders must be powered by a 24VAC source that is independent of the fire alarm system. Ensure that the cable entry for the 24VAC into the fire alarm common control enclosure complies with the manufacturer's instructions for non-power limited wires.

Signal Circuit Relays - For retrofit installations (where it may not be possible to run wires all the way back to the common control), Edwards (GE Security) manufactures the only UL/ULC listed signal (NAC) circuit relay module for use on any conventional supervised signal (NAC) circuit. The unit is listed as a "6554 Series Fire Alarm Relay". The relay contacts are rated at 120VAC or 30 VDC, 5 amps inductive, 3 amps resistive. The unit mounts inside a standard two gang (4x4) deep box and comes complete with a appropriately labelled stainless steel cover. You must exercise caution when employing this relay to trip anything other than a door holder. Most new signal appliances don't ring continuously, but are set to pulse temporarily, and some are programmed to silence manually. When either of these circumstances occur, the relay will "follow" the operation of the NAC circuit (in which case things like fan shutdown, elevator recall, fire dampers may not function in the manner required by code). There is a "workaround" for this however. You can employ a latching relay contactor on the circuit. This will require the operator to manually reset the device once the fire alarm system has been restored.

NOTE: If you're going to be employing a remotely mounted latching contactor, please ensure that appropriate instructions for resetting the circuit are posted on the fire alarm control (and/or remote annunciator) and that those instructions also include the location of the modules used to perform the required function.

The relay contacts are rated at 120VAC or 30 VDC, 5 amps inductive, 3 amps resistive.  The unit mounts inside a standard 4x4 deep box and comes complete with a appropriately labeled stainless steel cover. It's manufactured by GE Security/Edwards and it's listed as a "6554 Series Fire Alarm Relay".

NOTE: You must exercise caution when employing this relay to trip anything other than a door holder.  Most new signal appliances don't ring continuously, but are set to pulse temporarily, and some are programmed to silence manually.  When either of these circumstances occur, the relay will "follow" the operation of the NAC circuit (in which case things like fan shutdown, elevator recall, fire dampers may not function in the manner required by code).  There is a "work-around" for this however.  You can employ a latching relay contactor on the circuit.  This will require the operator to manually reset the device once the fire alarm system has been restored.  In this scenario, appropriate instructions must be posted on the fire alarm control (and/or remote annunciator).