HOW CIRCUIT BREAKERS WORK
MAGNETIC vs THERMAL - In general terms, magnetic breakers are more accurate over a wide temperature range and are available in precise ratings, so they are better suited for critical equipment protection applications. Thermal type breakers are suitable for wire and circuit protection, are available in higher ampere ratings, and are usually lower in cost.
Magnetic-hydraulic circuit breakers - Magnetic-hydraulic circuit breakers are ideal for accurate protection of sensitive equipment. These breakers operate on a magnetic field caused by electrical current passing through the coil of an electro-magnet. They are available as instant trip for special applications or with a built-in time delay accomplished by a hydraulic delay mechanism. Various delay characteristics are possible as well as custom trip ratings as low as 0.020 amperes. The point at which the breaker trips is independent of temperature, but the delay time is affected slightly by temperature resulting in faster tripping under high temperature conditions.
The following pages, excerpts from our manufacturer's catalogs, are provided to give you some insight into the workings of hydraulic-magnetic circuit breakers. Please keep in mind that specific references ( such as time delay curve numbers ) will vary between different manufacturers, but the concepts are the same.
Thermal-magnetic circuit breakers - Thermal and thermal-magnetic breakers are best suited for general purpose applications such as protecting wiring systems from overload and short-circuit damage. These breakers trip as the result of temperature caused by electrical current passing through the thermal element of the breaker. As a result, the trip point is affected by abient temperature to some degree and therefor must be de-rated to operate in high ambient temperatures. For protection of wire and general purpose applications this is not necessarily a disadvantage, but it can be a problem when trying to protect sensitive electronic equipment. The magnetic part of "thermal-magnetic" refers to the magnetic trip element that comes into play at short-circuit currents. It has no effect at normal tripping current.
Thermal circuit protectors - Thermal protectors are generally lost cost, low current (20 amperes max.) devices designed to provide overload protection but have very limited short-circuit interruption ability. They should not be used for short-circuit protection, but should be used downstream of properly rated circuit breakers (Plug-in equipment, for example). Ambient temperature is a factor in applying these protectors.