Fire Denyer – Fault Finding


This page is a work in progress and will be expanded as information becomes available.

To fault find and resolve issues, a good general understanding of the system is required.

Assuming a typical installation, the Fire Denyer system consists of a controller (switch) and battery pack that is usually powered from a nearby mains lighting circuit, a contactor (big relay) located in the switchboard and one or more series connected (daisy-chained) smoke detectors throughout the house.


  • Mains power is dangerous – don’t attempt this if you are not experienced with high voltages
  • Mains power must be applied while testing
  • The controller can be operated without a battery – but should have a battery connected
  • The fuse on the controller is for the load or contactor circuit
  • The controller supplies approximately 12V DC to the smoke detectors
  • The smoke detectors trigger the controller and are linked to sound alarms in all detectors
  • The smoke detector circuit uses a 4.7K end-of-line resistor

Getting Started

The controller needs a good mains power supply; phase and neutral. No earth is required. If the power supply is unstable or switched elsewhere, you have a problem other than the Fire Denyer installation.

Typical Faults

The Controller will not reset

  • A red flashing LED indicator shows that the controller is not reset.
  • Is the controller Reset – green LED indicator on or flashing occasionally and no red LED indicator ?
  • Or is the controller Reset and the contactor not closed ?

The controller must see the 4.7K end-of-line resistor:

  • Have you installed the loop wire at the last smoke alarm ?
    • check the loop wire at the controller while you are at it.
  • Are the smoke alarms wired correctly ?
  • Has the jumper been removed for the last smoke alarm in the circuit ?
  • Do you see approximately 12V DC on the Detector “+” and “-” terminals ?  No voltage when hushed – press Reset.
  • Do you see approximately 12V DC between “+12” and the Detector “-” terminals ?  Should be there always.
  • Using a multi-meter – check for approximately 2.49V across the Detector “I” and “-” terminals on the controller.  This represents a normal closed circuit.
  • The isolated “I” input if left open will not prevent the controller from resetting.  If this input is closed it will trigger the controller when opened.  Voltage is applied “I” to adjacent “-” to close this input.  Normally used for a manual call-point.

If the controller is reset and the contactor is not activated or closed:

  • Is the contactor wired correctly ?
  • Does the contactor work ?  Disconnect the contactor wire at the controller and apply 230V AC to the contactor coil to test it.  Note:  DO NOT BACK-FEED the controller from the contactor.
  • The controller switches the P (phase)
  • Is the controller supplying 230V AC to the contactor coil ?
  • Has the 2.5A fuse on the back of the controller blown ?

Note:  Incorrectly wiring the mains Supply and Load can destroy the controller.  Connecting the mains supply across the two “N” terminals will blow tracks off the circuit board as these terminals are looped.

Unusual Faults

Alarm triggering when lights operated

The reported fault was that switching a room light on/off would cause the Fire Denyer to activate. It is not known whether from one particular lighting circuit or more than one circuit.  It is suggested that the controller was swapped and the fault persisted – suggesting it’s not a faulty controller.  Assuming the mains power is good, as it was not mentioned that other appliances caused the fault, we will assume that there is not something like a neutral fault.

To begin with we assume that the Fire Denyer system is correctly installed wired and powered.  This leaves us thinking that the lighting circuit is creating noise that is upsetting the FD controller.  Noise that could potentially trigger the controller could get in via the mains wiring directly or be coupled into the smoke detector circuit.

  1. The FD controller could be run on battery with the mains power feed disconnected from the controller, therefore no path to conduct noise in.  Does the fault continue or is it solved ?
  2. If the problem persists, the noise may be coupled into the smoke detector circuit.  Is the smoke detector wiring run close to and in parallel with the mains lighting wiring at any point over it’s length – it should not be.  Don’t run the detector wiring in ducting or directly along-side mains wiring.
  3. If the problem persists, disconnect the trigger circuit from the detectors (“I” terminal) but leave the 12V power connected to the detectors.  A 4.7K EOL resistor will have to be connected from the “I” terminal to -ve so thet the FD controller can be reset.  Does the fault continue or is it solved ?
  4. If the problem persists, also disconnect the 12V power feed wires to the detectors.  Does the fault continue or is it solved ?

Once you have eliminated the mains power supply and the smoke detector wiring, the problem should be solved.

If you find the mains power wiring is causing the problem, there is some quite serious noise on the mains and you may have a fault somewhere else in the home – faulty wiring, switches or appliances.

If the fault is caused only when the smoke detector wiring is connected, the wiring is probably installed or runs too close to mains wiring somewhere. It may be possible to solve this by installing a ferrite core on the detector wiring.

The fire Denyer Controller does have internal mains filtering and input circuit filtering components as well as code based filtering on inputs.