Breakers as Overcurrent Protection: Thermal, GFCI and AFCI Explained

A circuit breaker marked 20 amps does not push 20 amps into a circuit. Electrician U explains that breakers are safety switches, called overcurrent protection devices in code, that sense too much current and open the circuit. The video, made with support from ABB and shot in its ReliaHome P-Series plug-on neutral panel, goes through how that works and which specialty breakers add protection.

Thermal and magnetic tripping

Inside each breaker is a small bimetallic strip. As it heats, it separates and sets a spring that pulls the circuit open. Current below the handle rating is fine; once it creeps over, the breaker shuts off. The second element is a magnetic trip for short circuits, such as touching a black and red wire together, where something like 100 amps could flow. The video puts the thermal trip at roughly 120 to 130% of the handle rating and the short-circuit trip at about 200%.

Without a breaker, plugging a vacuum, a toaster and a microwave into one circuit would keep raising the current. The wire insulation would start to melt, the wire could glow red hot, and it would probably start a fire in the home.

Poles are not added together

A 2-pole 20 has two poles connected by a handle, and each side monitors 20 amps because there is a 20 amp wire on each side. Current makes a complete loop from the transformer through the panel, the load and back. The breaker is therefore not a 40 amp device; the total is 20 amps. The same goes for a 3-pole 20, which is 20 amps in total across three conductors, not 60.

GFCI breakers

The red GFCI breakers use a small current transformer that monitors hot and neutral, which should be perfectly balanced. A slight imbalance means current is leaking somewhere, so the breaker trips. Near water, such as a washing machine or a sink, or outside with bare feet, that imbalance may be a person's body. GFCI breakers come in single-pole, 2-pole and 3-pole versions, and in ratings such as 30, 50 and 60 amps for 2-pole models. The video recommends a trip test on every one installed.

AFCI and dual-function breakers

A combination arc fault circuit interrupter (CAFCI) trips on both series arcs, where a wire connects to something in line with it, and parallel arcs between two wires. It uses an algorithm that recognizes the waveform of a series fault. The breaker has separate series and parallel test positions, and the video advises testing each, resetting between them.

A dual-function breaker does both GFCI (red) and AFCI (white) protection, with a GF test and an AF test. These breakers have indicators, and blink codes in the literature mean something. Two blinks, for example, point to the last thing that happened. The video notes that factory products occasionally arrive with defects such as a missing screw or a problem with one of the panel's teeth, so installers should test everything and not walk away until the install is complete.

Tandems and brand compatibility

A tandem 20 puts two 20 amp single-pole breakers in the space of one, so it does not give 240 volts between them. Filling a panel with them allows twice as many circuits. Another version has a 2-pole 40 in the middle, with 240 volts between those two, plus individual 20 amp breakers on the outside.

Breakers are molded case types that fit only certain panels. Forcing another brand's breaker into a panel can start a fire, and such breakers are not listed for use with it. The video says to check the manufacturer spec.

ABB bought GE's residential breakers and panels, so ABB is a direct replacement for GE. ABB also owns Steel City metal boxes and Carlon blue boxes. The video closes by noting that loads call for current based on their resistance, so the load, not the breaker, sets the current flow.