Electrical protections have a very important function in buildings: when an electrical fault occurs, they disconnect the affected circuits immediately. If an electrical circuit is left connected with an active fault, the consequences can include major component damage, fire, or electric shock.
Most electrical protection devices use thermal or magnetic mechanisms to disconnect faults, and many protection devices use both types.
- Thermal protections use the heat released by the high electrical current that occurs during a fault. The heat expands a contact inside the protection device, opening the circuit and interrupting the fault current.
- Magnetic protections respond to the magnetic fields caused by fault currents, opening the circuit in response. Since there is no heating involved, magnetic protections act faster than thermal protections.
Magnetic protections may seem like a better option because they are faster, but this is not the case - each protection mechanism is suited for different applications. Devices like electric motors and lamps with ballasts have a brief inrush current when they activate, and the magnetic protection must not trip with this current. Instead, the magnetic protection must be calibrated for much higher currents, such as those found in short circuits and ground faults.
There are cases where a high current lasts longer than normal, indicating a fault. For example, an inrush current is normal when a motor starts, but an operating current above the motor’s nameplate value indicates an overload. In this case, a thermal protection device heats up and trips with the overload current.
There are many types of protection devices, each designed for different applications. The following are some of the most common types found in residential and commercial buildings:
- Miniature circuit breaker (MCB), also known as plug-in circuit breaker, which is normally used for loads below 100A.
- Molded case circuit breaker (MCCB), which can be used for larger loads up to 2500A.
- Ground-fault circuit interrupter (GFCI) and arc-fault circuit interrupter (AFCI), which are used to protect against ground faults and arc faults, respectively.
- Motor circuit protector (MCP), a magnetic protection designed for motor-driven loads, as implied by its name.
- Motor protection circuit breaker (MPCB), which is similar to an MCP, but including thermal protection as well.
- Fuses, which are thermal-only devices that disconnect a circuit by melting. Fuses can only be used once, unlike circuit breakers, since they are destroyed during a fault.
Some appliances and equipment need more than one type of protection to achieve full safety. For example, power outlets in bathrooms are protected by a circuit breaker at the distribution board, but they also use an integrated GFCI.
Circuit breakers are arranged in distribution boards, which have special contacts and terminal blocks to simplify connections. Each distribution board must be specified with enough spaces for all the circuits that will be connected. Note that circuits with two live conductors use 2-pole breakers, while three-phase circuits use 3-pole breakers. These circuit breakers with more than one pole use several spaces in the distribution board.
Circuit breakers and other electrical protections are only reliable if they are suited for the application at hand. Using high-quality products is important, but their capacity can only be specified after a detailed electrical load calculation.