I'm explaining how fire alarm systems work for new technicians or other employees who deal with these systems. This is my 4th video in a string of many Khan Academy-styled lessons. This video reviews the principle of NAC circuits using diodes and reverse polarity to trigger their A/V devices. It also introduces relays, or more specifically, alarm contacts on a fire alarm panel.
![](https://s2.save4k.ru/pic/khzyHhQRqVk/mqdefault.jpg)