I was looking at the wiring for several newer devices I have (namely some Siemens PE-11T smoke detectors and a SpectrAlert P2R) and started wondering about the logic behind the three terminal configuration, where two terminals are marked positive and one terminal is negative. When wired properly, the positive feeds in and out of the device connect to separate terminals, while the negative wiring shares a common terminal.
Since these are usually found on the bases of “plug-in” devices like the P2R, I initially thought it may be to provide supervision if the device itself was removed. However, the presence of a jumper that automatically completes the circuit again when the device is removed contradicts this theory. What is the benefit of this setup over two or four sets of terminals?
for conventional detectors, there’s a diode between the two positive terminals on the base. when the detector is plugged into the base, it shunts that diode.
so basically what happens is if you pull the smoke detector out of the base, the circuit still works. however, the diode will no longer be bypassed by the smoke detector (since it’s been removed), so the FACP can use reverse polarity to make sure all of the detectors are installed. whenever the FACP reverses polarity, if a detector is pulled, the diode will block the voltage triggering a trouble on the FACP. however, if all the detectors are installed, all of those diodes are shunted, and when the FACP uses reverse polarity to check the circuit it’ll all work fine.
you’ll notice addressable circuits (devices, bases, etc.) usually don’t require any of this.