Switchboard Ratings

Switchboards (and Switchgear) is a type of apparatus that helps control the flow of electricity in a power system. Electrical ratings are limits that determine how much electricity the switchboard can handle without getting damaged or causing problems.

These ratings are important because they help ensure that switchboard/switchgear equipment is safe to use and won’t cause any accidents. These ratings tells us how much power tolerance the equipment can handle at its maximum capacity without reducing the expected service life.

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 switchboard 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.