Theoretically, what would happen if you put too many devices on an IDC or NAC and the current draw of the devices exceeded the output limit? Would they just be underpowered, or could it damage the panel, too?
Most older or low-end panels are fitted with fuses on the outputs to prevent what you described from occurring. A current draw exceeding the fuse rating would blow the fuse and kill the circuit.
For newer panels, I’m not quite as certain as to how they operate. I know that my Siemens SXL-EX will shut down the NACs if the board detects an over-current condition, eliminating the need for replacing fuses. My DMP XR-500 does the same for its bell circuit. Again, I’m not sure if this exact configuration applies to other panels as well, but the operation is likely similar.
Ah, I see. So the principle is similar to what happens in a house. If you run too many appliances, you overload the circuit and there may or may not be safety features to prevent damage. Thank you!
Electronic protection on NACs can take several routes. Most modern panels have supervisory circuits that can sense a short or low resistance on a NAC and prevent it from turning on. Overload protection can use a very low value resistor in series with the NAC, something like 0.01 ohm. A sensitive chip “reads” the voltage drop across that resistor and triggers the circuit to shut down when overloaded.
An IDC is a little different. The amount of current that can be drawn is limited by the circuit design. Contact devices like pull stations simply short the circuit together. It doesn’t matter if there is 1, 10, or 100, once a station is pulled the circuit is at whatever it’s design current is.
2-wire smoke detectors are another matter. Each one draws a small amount of current from the circuit. Something around 100 micro amps (0.0001 Amp). Each panel and listed smoke detectors are rated for the number of detectors safely allowed. There is a circuit that monitors the current in the IDC and there is a trip point where it switches from Normal condition to Alarm condition. When a detector triggers into alarm it adds a low value resistance across the circuit, something around 500 ohms. As additional detectors are added the circuit condition gets closer to that trip point. At some excessive number of detectors the IDC will go into alarm, not because a detector tripped into alarm but from the excessive standby current draw.