Arc-Flash Mitigation by Adjusting a Main Breaker Trip Unit
A 480 V, 2,000 A switchboard is fed by a 2,000 kVA transformer with 5.75% impedance. The calculated bolted-fault current at the switchboard is 41.8 kA, and the available arcing current is 19.6 kA. A recent incident-energy study, performed using the IEEE 1584 equations and a 0.5 s clearing time, reports 12.4 cal/cm² at an 18 in working distance. The main breaker has an adjustable electronic trip unit with a 2,000 A sensor, long-time pickup at 1.0, short-time pickup at 4.0, and short-time delay at 0.3 s; instantaneous is disabled. The plant wants to reduce the label to 8 cal/cm² so electricians can wear a lighter 8 cal/cm² suit instead of a 12 cal/cm² suit. Their proposal is to lower the short-time delay from 0.3 s to 0.1 s and to change the long-time pickup from 1.0 to 0.8. The downstream 400 A feeder breakers have short-time pickup at 5.0 and no adjustable delay.
1. Will the proposed settings achieve an 8 cal/cm² label, and what technical issues must be checked?
Read the worked answer
For a fixed arcing current and distance, incident energy is approximately proportional to clearing time in this time range. Reducing the short-time delay from 0.3 s to 0.1 s could reduce 12.4 cal/cm² to roughly 4.1 cal/cm², which is below 8 cal/cm². The 19.6 kA arcing current is well above the main short-time pickup of 4.0 times 2,000 A, or 8,000 A, so the short-time element should trip. However, the calculation must be rerun with the actual trip curve, because the 0.1 s delay is a delay band, not the total clearing time; add the breaker opening time and any interface timing. The long-time pickup change from 2,000 A to 1,600 A does not reduce arc-flash energy for a 19.6 kA fault, because long-time pickup is not the tripping element. It can create nuisance tripping and may compromise selective coordination with downstream 400 A breakers and upstream protective devices. A coordination study, not just an arc-flash calculation, is required before lowering the setting.
2. What other mitigation options should be evaluated, and why might a maintenance switch be preferable to permanently lowering the short-time delay?
Read the worked answer
Other options include current-limiting fuses, zone-selective interlocking, bus differential protection, remote racking and operation, increased working distance, and a temporary arc-flash maintenance switch. A maintenance switch temporarily places the breaker in a faster trip mode only while qualified workers are near energized equipment. That approach can achieve low incident energy during maintenance without permanently sacrificing coordination during normal operation. Permanently reducing the short-time delay may cause the main to trip for downstream faults that should be cleared by feeder breakers, especially if the downstream devices have significant clearing times. Current-limiting fuses may reduce incident energy, but the available fault current and the fuse let-through characteristics must be checked, and the switchboard rating must remain adequate. Any option must be evaluated with an updated incident-energy study using the actual equipment configuration, electrode gap, enclosure size, and working distance.
3. What documentation and acceptance steps should be completed before the new label is issued?
Read the worked answer
A licensed professional engineer should issue a revised arc-flash study and coordination study that show the new settings, trip curves, arcing currents, clearing times, and calculated incident energy at each working distance. The study must use the actual breaker trip unit settings and manufacturer time-current curves, not generic assumptions. The arc-flash label must be updated to match the study and must state the nominal system voltage, arc-flash boundary, incident energy or PPE category, working distance, and required PPE. Before energizing, verify the installed settings by reading the trip unit and, where practicable, by primary injection testing. Keep a setting sheet and a record of the approved study in the electrical safety program. Finally, update the switching procedure and train affected workers on the new label and on the conditions under which a maintenance switch may be used.