A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.
Typical hospital two-pole breakers are rated at 20 amps. Circuit breakers are designed to carry 100% of their rated current while the NEC dictates an 80% or less rating application. This is to offset any transients (sudden current spikes) or inrush currents applied to the circuits. For example, lets imagine you have a fume hood and its has failed its safety test (specifically the leakage current). The OEM tells you the solution is to purchase and install a low leakage transformer. The transformer arrives and you plug it into the wall and "Bam!", #12 circuit breaker trips. You reset the breaker and attempt a second, then a third try and all three times you still get the same results. You place an amp meter on the breaker panel wires to measure the amperage. You plug the transformer into the wall with the unit off then turn the switch on and the amp meter reads 16.83 amps. You read the transformers specifications and it says the transformers output is 12 amps.
There are two reasons:
First, there is an inrush current applied when the switch is turned on and delivers more current to the breaker causing it to trip. Second, although you have a 20 amp circuit breaker due to the inrush current being applied the breaker thinks its a short since it exceeds its 80% initial capacity. Lastly, the breaker can hold up to 20 amps without inrush currents being applied.