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Powering Safety Systems - Giving Yourself Significant Power 'Head-Room' Is Good Practice When Installing Safety Instrumentation
One of the most common sources of Fire and Gas detection system failure is power supply interruption and disturbance. Power supply specifications of an instrument refer to the voltage that must appear at the instrument power terminals, not the power supply rated output. There are many factors that impact voltage reaching the fire or gas detector, any one of which may cause an unwanted shutdown or damage to the instrument.
The most fundamental factor to consider is wire resistance between the power supply and the device but there many other factors.
The total power or voltage required to be supplied per instrument is the "Power required by the Instrument" plus the "Power loss in the wiring". Perhaps surprisingly, on installations with long wiring runs the "Power loss in the wiring" could exceed the "Power required by the Instrument".
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