Douglas Krantz - Technical Writer - Describing How It Works
Go to the Fire Alarm Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Go to the Science Article Map Page of Douglas Krantz - Technical Writer
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works

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

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.)
Was this
helpful?
Yes   No



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.







Douglas Krantz

Describing How It Works
writer@douglaskrantz.com
Text
612/986-4210

View Douglas Krantz's profile on LinkedIn



Ask
The
Technician

Readers Questions



Short Circuit
Free Subscription
I'll Send You the
Twice-Monthly
Fire Alarm
Newsletter

Get Short Circuit


Articles

How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More

How is a Buffer Relay Wired Into a 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... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More



Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.