Mixing Different Types of Smoke Detector on the Same Initiating Circuit

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firetech1
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Joined: Tue Feb 19, 2019 1:06 am

Mixing Different Types of Smoke Detector on the Same Initiating Circuit

Post by firetech1 » Sun May 05, 2019 4:44 pm

Conventional initiating circuits for a normal 24VDC fire alarm system operate using basic principles of electricity. The fire alarm panel sends 24 volts of DC electricity out to initiating circuit wires containing initiating devices that cause the fire alarm panel to go into an alarm state when a short occurs. The circuit is usually "supervised" by an end-of-line resistor at the end of the circuit so as to detect any open circuit occurrences. Common devices on an initiating circuit include manual pull stations, heat detectors, flow/pressure switches and smoke detectors.

Manual pull stations, heat detectors and flow/pressure switches cause the fire alarm panel to go into an alarm state when an internal switch is closed during an alarm state, thus causing a short in the circuit. However, two-wire smoke detectors operate on a slightly different principle related to electrical impedance rather than just simply having a switch close and short circuit created. A smoke detector going into alarm lowers the impedance of the circuit further which places the circuit into an alarm state. Yet it allows enough current to latch the detector into an alarm and causes the LED on the detector to activate (Shalna 2006).

Because smoke detectors rely on this principle of impedance rather than simply shorting the circuit (i.e. manual pull stations, heat detectors and flow/pressure switches), only specific brands and models of smoke detector can be used with specific brands and models of fire alarm panels. One may use a Mircom conventional pull station on an Edwards 6616 initiating circuit for example, or a System Sensor 5601A heat detector on a Simplex 4002 initiating circuit. Both of these would be acceptable devices to install as make and model of these devices are not relevant to their functioning. In contrast, smoke detectors place a different impedance on the initiating circuit depending on the make and model of detector. For this reason, smoke detectors must be "cross-listed" by the manufacturer of a fire alarm panel for use with a specific make and model of fire alarm panel. You cannot simply mix and match makes and models freely as you are able to with conventional manual pull stations and heat detectors.

Installing different types of smoke detectors on an initiating circuit causes the impedance values on the circuit to vary as different makes and models of detectors place different impedance values on the circuit. Fire alarm manufacturers therefore must decide on how many different detectors of a certain type that an initiating circuit can accommodate. Obviously mixing and matching different models gives an infinite number of impedance values for any given circuit in this case and the manufacturer cannot say with certainty what number of detectors per circuit is acceptable.

It is the best practice to use only one make and model of smoke detector per conventional initiating circuit in order to ensure the impedance values on the circuit remain within the manufacturer's acceptable range. An approved cross-listed smoke detector make and model must also be used (there are various resources to find which make and model of smoke detectors are cross-listed for which make and model of fire alarm panel, including manuals and online charts). A technician should always ensure they are using the correct smoke detector type when replacing detectors on a conventional fire alarm system. They should also be diligent about noticing different smoke detectors being mixed together on the same circuit when walking into a building for the first time.


REFERENCES:

Shalna, A J 2006, "Mixing different two-wire smoke detectors on the same initiating device circuit - not a good idea", IMSA Journal, May/June 2006 edition, p.28.

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