Refer to this Wiki article about Dual Loop supervision.

I don't feel like playing 20 questions today... what are MAC, F&G?

Is this a pull station for a fire alarm system? Why have you not called the alarm guy?

MAC is a manual call point. One of those units to raise alarm by usually breaking glass but essentially the same as a pull station. F&G is Fire and Gas.

I understand the EOL resistor, I just can not get an answer that I am happy with for why the diode is included. I believe it is for a short circuit but I am not 100%

Thanks,

Can you expand on what you mean by "polarizes the device"? I am correct in saying the EOL resistor will allow for different loads (and hence currents) between normal opperation, alarm opperation and open circuit (no current in the latter). The diode will then allow the system to identify a short circuit?

A manual call point does not usually need to be polarized.

Many F&G circuits use a reversing polarity circuit when "alarm" occurs. In this case, the diode polarizes the device, and the current is pass through, essentially bypassing the resistor for a short circuit.

It would help greatly to know the manufacturer of the equipment.

The EOL will not allow for different loads (this is not an analog circuit) in this configuration.

It is difficult to help you without knowing what equipment you are using.

Wjmfire, answer number two sums-up all you should need to know. The resistor is in place to allow the alarm system to see that the wires connecting the pull station are continuous (not broken). The diode is in parallel to guaranty proper polarity.