Racking a Starter Bucket Under a Stale Arc-Flash Label
You are the EPT technician at a 480Y/277 V MCC lineup fed from a 2000 kVA transformer. The arc-flash study labels the main breaker line side at 18 cal/cm² at an 18-inch working distance, with a 6 ft arc-flash boundary. The study assumed a 0.15-second clearing time. While pulling settings for an unrelated coordination question, you find the upstream 600 A feeder relay actually has a 2.0-second time delay, because a downstream device was replaced two years ago and the study was never revised. A 100 hp combination starter bucket has a blown fuse, and production wants the bucket racked out during the run. On site: 8 cal/cm² daily wear, a 25 cal/cm² arc-rated suit, a 40 cal/cm² suit, and a remote racking tool in the shop, 20 minutes away.
1. Production asks whether the 25 cal/cm² suit is enough. What is your answer and the reasoning behind it?
Read the worked answer
No - the suit is above the posted number, but the posted number does not describe this job.
- The label states 18 cal/cm² at an 18 in working distance. A 25 cal/cm² suit has an arc rating above that, so at face value the suit covers the label. That is where the easy part ends.
- An incident-energy label is a calculated result for a specific fault current and a specific clearing time. This study used 0.15 s. The field-verified clearing time at this location is 2.0 s. The label is invalid and cannot be the basis for PPE selection.
- The 8 cal/cm² daily wear is not adequate under any reading of the data, and the blown fuse is direct evidence that arcing energy has already been released inside this bucket.
- Correct sequence: treat the equipment as unlabeled, re-run the calculation with the verified settings, and hold energized work until a valid label exists. Do not negotiate upward from the posted number.
2. Using the verified 2.0-second clearing time, estimate the incident energy and boundary, and state what controls are actually available.
Read the worked answer
Incident energy scales approximately linearly with arcing time, and boundary scales with the square root of energy.
- E = 18 cal/cm² × (2.0 s / 0.15 s) = 240 cal/cm²
- Boundary factor = √(240/18) = √13.3 = 3.65, so a 6 ft boundary becomes roughly 22 ft
No PPE on site, and no commercially practical suit, covers 240 cal/cm². There is no PPE solution - the only compliant control is to de-energize:
- Open and lock out the upstream 600 A feeder, verify absence of voltage, and rack the bucket with the source off.
- If energized racking were somehow required, remote racking only helps if the operator stands outside the re-derived boundary (about 22 ft here). Confirm the tool lead and cabling length before counting on it - a 10 ft remote tool is not protection at a 22 ft boundary.
- Treat the 2.0 s setting as a defect to be corrected, not a condition to be worked around.
3. What must be documented and communicated before this MCC is worked energized again?
Read the worked answer
Capture the finding, fix the root cause, then relabel - in that order.
- Record the field-verified time-current settings of the 600 A relay (pickup, curve, time dial, instantaneous) and the date/technician, and attach them to the study file as the basis-of-design change.
- Direct the study owner to re-run the arc-flash and coordination calculation with actual device settings and the replacement device's data. The root cause here is a coordination study that was never updated after a device swap; fixing only this one label leaves the rest of the lineup wrong.
- Re-label every affected enclosure and post the revised boundary before any energized work is permitted.
- Document the interim control used today (de-energized racking with lockout), the blown-fuse investigation, and the fact that the 25 cal/cm² suit was rejected despite exceeding the posted 18 cal/cm². That decision is the defensible record if the question comes back later.