Douglas Krantz - Technical Writer - Describing How It Works

What is "Power Limited?"

Power Limited starts out with the power supply. If the power supply isn't limited, and can provide high levels of voltage or current, a problem with the wire or devices can cause fires or injure people.

What is Power Limited?

Greetings Douglas,

I have a question regarding a Mircom FX-2003-6DS [Fire Alarm Panel] using the RM1008 relay adder card [8 Form-C relays, each with Comm, N.O., N.C. contacts]. The relay manual says 28VDC on the picture, which also shows:

All relay circuits are power limited and must use type FPL, FPLR, or FPLP power limited cable. (Is a low voltage AC from a transformer acceptable?)

All relay circuits must be connected to a listed power limited source of supply

I'm confused if you are allowed to use low voltage AC (under 30 VAC) to these relays. The building has added one door mag to the existing wiring coming from a step-down transformer. It then goes into a small PSW18 security board which converts the AC to DC voltage which then feeds into the on-board fire alarm relay card. The electrician fed the new wires from the new door mag and connected them into the existing wiring in parallel. Do the wires have to be in series or is this acceptable?

Thank you, JP

Power Supply

"All relay circuits must be connected to a listed power limited source of supply"

Whether it's a transformer, or it's a power supply of any type, it requires a stamp from a testing laboratory like UL, ULC, CE, FM, CCC, or other testing laboratory. The stamp means that the testing laboratory has tested it, found it to be safe, and put it on their "List" of things found to be safe (Listed). Included with that stamp has to be a classification of Class II or Class III electrical rating.

Class I means that the voltages and/or currents that the supply are capable of harming humans or starting fires. In essence, stay away from Class I power supplies.

Class II or Class III means the power supply isn't even capable of not being safe. The voltage and current supplied by that transformer or power supply is safe for humans and won't start a fire.

In other words, don't plug your power limited circuit directly into the wall or use a power supply that is capable of having too much voltage or current. It doesn't matter whether it's AC or DC, if the voltage or current is high enough to be a danger to humans or can cause a fire, don't use it. Stay with a Class II or Class III power supply.

As far as using a "Low Voltage" transformer, make sure of the Class II or Class III rating. I've seen transformers that produce 24 volts, but are capable of producing 10 amps of constant current. When shorted, that 10 amps inside the transformer can be more like 30 to 50 amps, at least until the transformer itself, or the building wiring, gets too hot and starts a fire. Even if the transformer produces only 24 volts, if it's not Class II or Class III, it's not fire safe.


"All relay circuits are power limited and must use type FPL, FPLR, or FPLP power limited cable."

On the wire, the lettered requirements show specifically what kind of insulation has to be used.
  • The first letter "F" stands for Fire Rated - the wire with this kind of insulation can be used for fire alarm systems
  • The second and third letters "PL" stand for Power Limited - the wire with this kind of insulation can be used in situations where the power has limits - the AC or DC voltages and currents are prevented from being high enough to be a danger to humans or cause a fire
  • The fourth letter "None", "R", or "P" provides further rating for the insulation - no fourth letter means that there aren't any extra ratings for the insulation, and it has limited use; "R" or Riser means the insulation is rated to, among other things, rise from one floor to the next; "P" or Plenum means that the insulation can be used in air handling spaces, like the air space above false ceilings (used for return-air in the HVAC system) - don't use it in the ductwork, though

Door Magnet (Door Holder) Circuit

Wiring everything is series is specifically done for "Wire Supervision". With wire supervision, if a wire breaks or comes loose, a trouble will show on the fire alarm panel. Someone sees and hears that there's trouble, and they call for service. But if something goes wrong with the door magnet wiring, the wiring isn't supervised by showing a trouble on the panel, the door magnets and their wiring are supervised by letting the door close.

The door magnet or door holder circuit is on a Class D fire alarm circuit or pathway. According to the NFPA Code, a Class D circuit (pathway) doesn't show a trouble on the fire alarm panel if something goes wrong with the circuit (pathway). Instead, whatever is being controlled by the circuit (pathway) will go into the fire-mode if the circuit fails.

With the door magnet, if anything goes wrong on a door magnet circuit, like a loose connection, broken wire, or loss of power, the door magnet will lose its magnetism and the door will close (into the fire mode). The door magnets or door holders, because they're on a Class D circuit or pathway, have what I call "Annoyance Supervision". If, for whatever reason, a door magnet doesn't work, people will get annoyed that the door won't stay open, and call for service.

Calling for service is the bottom-line reason for "Supervision". Because the door magnet circuit is a Class D circuit (pathway), the wiring by the electrician is acceptable.

Relay Contact Burn Out

I looked up the specifications on the Mircom RM1008A Relay Control Board. There are 8 relays on the board. The relays are in the 8 black boxes on the board. Compared to many relays, the physical size of the Mircom relays is small.

To find out how a relay works, look up:

Basically, a relay is just like a light switch on the wall, except rather than using a person's hand, it uses an electromagnet to "throw the switch".

There's a problem, though, with using the Mircom RM1008A Relay Control Board to turn on and off the door magnets. The Mircom relay is rated at 28 Volts DC, 1 Amp, Resistive Load. The words "Resistive Load" means that the relays are good for an incandescent lamp, but door magnets will burn the relay contacts and destroy the relay in short order.

As a fix to prevent problems later, look up:

These diagrams will work with AC or with DC.

I have had to fix quite a few door magnet systems because they were originally designed to only use the smaller fire alarm control relays.

Douglas Krantz
Life Safety
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