490.21 Circuit-Interrupting Devices
(a) Circuit breakers installed indoors shall be mounted either in metal-enclosed units or fire-resistant cell-mounted units, or they shall be permitted to be open-mounted in locations accessible to qualified persons only.
- An accessible mechanical or other identified means for manual tripping, independent of control power
- Be release free (trip free)
- If capable of being opened or closed manually while energized, main contacts that operate independently of the speed of the manual operation
- A mechanical position indicator at the circuit breaker to show the open or closed position of the main contacts
- A means of indicating the open and closed position of the breaker at the point(s) from which they may be operated
- The continuous current rating of a circuit breaker shall not be less than the maximum continuous current through the circuit breaker.
- The interrupting rating of a circuit breaker shall not be less than the available fault current the circuit breaker will be required to interrupt, including contributions from all connected sources of energy.
- The closing rating of a circuit breaker shall not be less than the maximum asymmetrical fault current into which the circuit breaker can be closed.
- The momentary rating of a circuit breaker shall not be less than the maximum asymmetrical fault current at the point of installation.
- The rated maximum voltage of a circuit breaker shall not be less than the maximum circuit voltage.
DANGER — DISCONNECT CIRCUIT BEFORE REPLACING FUSES.
Exception: More than one switch shall be permitted as the disconnecting means for one set of fuses where the switches are installed to provide connection to more than one set of supply conductors. The switches shall be mechanically or electrically interlocked to permit access to the fuses only when all switches are open. A conspicuous sign shall be placed at the fuses identifying the presence of more than one. source.
Where more than one switch is installed with interconnected load terminals to provide for alternate connection to different supply conductors, each switch shall be provided with a warning sign identifying the presence of more than one source. Each warning sign or label shall comply with 110.21.