Tech Explained

How a Circuit Breaker Works (Thermal and Magnetic Trips)

Inside a standard breaker are two separate trip mechanisms, one for slow overloads and one for sudden short circuits. Here is how they work and how to read a trip curve.

By IES team, Innovative Electrical Services (IES), Auburn, CA · Reviewed November 2026 · 2 min read

Key takeaways
  • Breakers protect the wire from overheating; they are not sized to the appliance.
  • A heat-sensitive bimetal strip handles overloads: the more overloaded, the faster it trips.
  • A magnetic coil handles short circuits and trips almost instantly.
  • A trip curve shows both together as a band of time versus current.

A breaker is a switch that turns itself off when the current gets too high. Most residential and light commercial breakers are thermal-magnetic: they combine two different sensing methods in one small case.

The thermal trip: for overloads

Current passes through (or near) a strip made of two metals bonded together. The metals expand at different rates when heated, so as the current warms the strip it bends. If it bends far enough, it releases a latch and the contacts spring open.

Because it works by heat, the thermal trip is an inverse-time device: a small overload takes a long time to trip it, and a large overload trips it faster. This matches how wire heats up, and it lets motors start (a brief high current) without nuisance trips.

The magnetic trip: for short circuits

The current also passes through a coil. At normal current the coil’s magnetic pull is weak, but a short circuit can produce hundreds or thousands of amps. That pulls an armature in and trips the breaker almost instantly, in a cycle or less, without waiting for anything to heat up.

time to trip (log)current, as a multiple of the breaker rating (log)1×2×5×10×Thermal: overloadsbimetal heats and bendsMagnetic: shortstrips almost instantlyThe band between the lines is thenormal tolerance from the maker.
A thermal-magnetic breaker has two trip mechanisms. A small overload heats a bimetal strip, which bends and trips after seconds or minutes. A short circuit pulls a magnet and trips almost at once.

Reading a trip curve

Manufacturers publish time-current curves. The horizontal axis is current, often as a multiple of the breaker’s rating, and the vertical axis is time to trip. Both are logarithmic. The curve is drawn as a band because of normal manufacturing tolerance: any trip time inside the band is considered correct.

  • The sloped upper part is the thermal region (overloads).
  • The steep drop is where the magnetic trip takes over (short circuits).

Engineers use these curves to coordinate devices so the breaker closest to a fault trips first, leaving the rest of the building running.

What else is in a modern breaker

  • GFCI breakers add a sensor for current leaking to ground (see GFCI and AFCI).
  • AFCI breakers add electronics that recognize arcing.
  • Interrupting rating (such as 10,000 or 22,000 A) is the largest fault current the breaker can safely interrupt; it must be suitable for the fault current available where it is installed.

Why a breaker trips again and again

Repeated trips are the breaker doing its job. Swapping in a bigger breaker without bigger wire removes the protection for that wire. Our guide on a breaker that keeps tripping covers safe first steps.

Sources and further reading

  1. ABB, What you need to know about MCB trip curves (PDF)
  2. NFPA free access to codes and standards (read the NEC online)

Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.