There are two kinds of optional equipment that can be included in a circuit:
- control devices
- protective devices
These devices are not required to make a circuit work.
A control device is something that allows us to determine where and when electricity flows. Most control devices either open or close the path of the circuit. Light switches, thermostats, and time clocks are examples of common control devices found in circuits.
A protective device is used to protect either the load or the path from excessive heat from overcurrent or overvoltage conditions. Most protective devices open the circuit path if excessive current is flowing in the circuit. Common examples of protective devices include fuses and circuit breakers.