COUNT ON THE HIGH INTERRUPTING RATING OF OUR MOST WIDELY-USED CLASS L FUSE When it comes to protecting service entrance equipment, feeder circuits, and circuit breakers, A4BY fuses have been the industry's favorite. The A4BY is a 100% rated device and may be applied at continuous currents up to its ampere rating. A 4-second minimum time delay at 500% rating allows the A4BY to pass normal current surges and to coordinate with ground fault relays.