In the United States, the term “low voltage” with reference to power circuits usually refers to circuits of 600 volt or less potential.
This photograph shows a typical low-voltage (480 volt) circuit breakers in a “Motor Control Center” (MCC) panel, for 480 volt 3-phase industrial power circuits:
Note how each circuit breaker has its own on/off handle, for manual operation. These circuit breakers, like most low-voltage breakers, are capable of turning off (“tripping”) on their own when high current is detected, but must be turned on (“closed”) manually. That is to say, they lack “close” and “trip” solenoid coils present in larger circuit breaker units which would permit remote operation.
Some low-voltage circuit breakers utilize thermal elements to detect overcurrent conditions, much like the circuit breakers traditionally found in residential applications. When this thermal element inside the circuit breaker becomes too warm with current, it mechanically forces the mechanism to trip and open the contacts. Other low-voltage circuit breakers are magnetically operated, tripping when the magnetic field caused by conductor current becomes excessive. In either case, the trip mechanism for a low-voltage circuit breaker is typically contained within the circuit breaker itself.
A close-up photograph shows one of these breaker panels, containing two separate three-phase circuit breakers inside:
Note how the “Sump Pump” circuit breaker has been placed in the “off ” position, its handle locked there by a padlock, a danger tag attached to notify any personnel of the reason for the breaker’s lock-out.
This next photograph shows a different brand of MCC (manufactured by Gould) where each unit contains not only a circuit breaker, but also an entire motor starter assembly (contactor, overload heaters, and associated switch contacts) for controlling a three-phase electric motor. One of these motor control “buckets” has been removed, revealing the line and load bus connections in the rear:
Industrial circuit breakers such as this are typically designed to be unplugged for ease of maintenance and replacement. If the “bucket” units are heavy, a lifting hoist is provided on the MCC to facilitate their removal and replacement.