Technical information

INDUSTRIAL MACHINERY

STEP 2

Modify the available short circuit current within a portion of a circuit in a panel

due to the presence of current limiting components as specified in SB4.3, when

applicable;

This second step takes into account the effect of any current limiting devices in

the feeder circuit. Current limiting refers to the ability of a protective device to

clear a short circuit fault in less than one half cycle and typically within one

quarter cycle. Current limiting devices will reduce the levels of Ipeak magnetic

and I2t heat energies.

SB4.3 establishes the guidelines for three methods of using feeder components

that limit the available short circuit current.These options include power

transformers, Listed circuit breakers and fuses as described in these sections:

• SB 4.3.1: Branch circuits supplied by a power transformer with an isolated

secondary.

• SB 4.3.2: Listed circuit breaker marked “current limiting” in the feeder circuit.

• SB 4.3.3: Branch circuits supplied by a Class CC, G, J, L, RK1, RK5 or T fuse in the

feeder circuit.

These sections identify the feeder components that limit the short circuit current

available and guidelines in applying them. For many years, fuses were the only

protective devices that were considered to provide current limiting performance.

Today, circuit breakers exist that can open high fault currents in just a few

milliseconds, very comparable to fuses.These sections also identify the Ipeak, I2t

and let-through values that must be used when applying a current limiting

component.

Using high fault rated components can eliminate the need to apply feeder circuit

current limiting provisions.This makes its simple to achieve a higher panel rating.

See page 99 for more information on component high fault short circuit current

ratings.

Figure SB4.1 Let-through values for

current limiting circuit breakers from

UL 508A Supplement SB

Table SB4.2 Let-through table for

UL Listed fuses from UL 508A

Supplement SB