Circuit breakers are divided into types based on their instantaneous tripping current. This is the minimum current at which the circuit breaker will discontinue the flow of electricity, or trip. It does this to protect devices plugged into the circuit from sudden rises in levels of current. A type D breaker trips when its current is 20 times its rated current.
Type D breakers are generally found in industrial settings. According to the The Electrical Guide, Type D breakers protect devices such as transformers or welding machines — items that can tolerate higher surges of electricity than home appliances.
Circuit breakers either use coils or strips. In the first case, current passes through the coil causing a magnetic field that attracts a metal plate and trips the breaker if the current gets too high. In the second case, the strip has two metals that expand at different rates when heated. If the current gets too high, the strip bends and trips the breaker. Type D breakers may use either of these methods.
Every circuit breaker should include a visible marking of its rating. The rating consists of the breaker's current rating, measured in amperes, followed by its type. Therefore type D breakers will include the letter D preceded by a number.
David Perez is a technology writer based in San Jose, Calif. He has worked as a Web designer and a copywriter, with articles appearing in the "Metro Santa Cruz" newspaper and online. Perez holds a Master of Fine Arts in creative writing from Goddard College.