Switchboard Ratings

Interrupting Rating

Interrupting rating is the level of current that a protective device (fuse or circuit breaker) can safely interrupt without damage under specified conditions. The interrupting rating of the switchboard depends on the interrupting rating of the circuit protection devices and the rating method used.

According to the NEC® Article 110.9, equipment designed to interrupt current at fault levels must have an interrupting rating sufficient for the nominal circuit voltage and the current which is available at the line terminals of the equipment.

There are two ways to meet this requirement, the full rating method and the series rating method.

Full Rating

The full rating method requires selecting circuit protection devices with individual ratings equal to or greater than the available fault current. For example, if 65,000 amperes of fault current are available at the service entrance, every circuit protection device must have an interrupting rating of 65,000 amperes.

Series Rating

Because the full rating method adds expense to a switchboad design, UL listed series-rated switchboards are available for many applications at a lower cost. The series rating method also requires the interrupting rating of the main circuit protection device to be equal to or greater than the available fault current, but subsequent downstream circuit protection devices connected in series can be rated at lower values.

For example, a building with 42,000 amperes of available fault current might use a breaker at the service entrance with an interrupting rating of 42,000 amperes and additional downstream breakers with a lower interrupting rating, such as 18,000 amperes.

Series-rated breaker combinations must be tested in series in order to be UL listed.

Short Circuit Withstand Rating

Short circuit withstand rating refers to the level of fault Rating current a piece of equipment can withstand for a specified time without sustaining damage. The standards for short circuit withstandability are set by Underwriters Laboratories (UL Standard 891).

Bus structures and bracing are designed to withstand a specified current for a specified time. The short circuit withstand rating of a switchboard is determined by the combined withstand, interrupting, and current limiting capabilities of the bus and overcurrent protective devices in the switchboard and any overcurrent protective devices ahead of the switchboard that may supply and protect it.

Ampere Rating

The ampere rating refers to the current a switchboard or protective device can carry continuously without deterioration and without exceeding temperature rise limits.

Voltage Rating

The voltage rating of a switchboard must be at least equal to the system voltage. The voltage rating of a switchboard can be higher than the system voltage, but never less.

References