Series Rating, Selective Coordination and a Stale Arc Flash Label on a Hospital Essential System
A 320-bed hospital is adding a 500 kW chiller to its essential electrical system (EES). The chiller is fed from a new 400 A ATS connected to a spare 400 A breaker in the 480 V main switchboard, which has a 2,000 A main and a series rating of 65 kA marked for the listed main/branch combinations; available fault current at the board is 41 kA. The ATS is marked 30 kA short-circuit withstand. The coordination study shows that for a 6 kA fault on the load side of the ATS, the upstream 400 A breaker in the main board trips at 0.28 s while the ATS input breaker trips at 0.31 s. The existing arc flash label at the switchboard, dated 2012, reads '8 cal/cm², PPE Category 2'; the new study calculates 13.5 cal/cm² at the same working distance with the chiller on generator backup. The contractor wants to keep the existing label because it is still legible and to energize before the Joint Commission survey next month.
1. Assess the plan for interrupting rating and selective coordination. Which items must change before energization, and why are they separate issues?
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Three distinct problems - do not let them be traded off against one another.
- Withstand rating. The ATS is marked 30 kA and the available fault current at its line terminals is 41 kA. Unless the 400 A feeder breaker and the ATS form a listed series-rated combination documented by the manufacturer and acceptable to the authority having jurisdiction, the ATS is applied beyond its rating. The switchboard's 65 kA series rating covers the specific listed main/branch pairs inside that board; it does not automatically extend to new equipment added downstream.
- Selective coordination. At 6 kA the upstream main-board breaker (0.28 s) wins over the ATS input breaker (0.31 s). That is miscoordination in the worst direction: a fault downstream of the ATS trips the upstream device and removes more of the essential system than necessary. The EES must be selectively coordinated - the hospital article invokes the emergency-system coordination requirement, and EES coordination is required in its own right. Fix it by adjusting the upstream device (higher short-time pickup or a deliberate delay with verified withstand) or by selecting an ATS breaker that coordinates, then re-run the study and submit the time–current curves.
- Documentation. If series rating is the chosen route, the specific combination must be listed, marked, and shown on the study and the equipment schedule.
These are separate because a device can be adequately rated and still miscoordinated, or perfectly coordinated and still under-rated. Both must be satisfied independently.
2. Can the 2012 arc flash label stay? Explain what the label must show and the practical consequence for the electricians.
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No - the label is now wrong and must be replaced before the equipment is worked on.
The 2012 study did not model the new chiller or the generator-backup configuration. The new calculation gives 13.5 cal/cm² at the same working distance, which changes both the required PPE and the arc flash boundary. NFPA 70E requires the label to be updated when a significant change alters the arc flash hazard, and the study itself is to be reviewed at intervals not exceeding five years. A legible but superseded label is more dangerous than no label, because workers will trust it.
The replacement label must show the incident energy (or the PPE category with its corresponding arc rating) at the stated working distance, the arc flash boundary, the nominal system voltage, the date, and a reference to the study behind it.
Practical consequence: 8 cal/cm² corresponds to PPE Category 2. At 13.5 cal/cm² the electricians need arc-rated clothing and equipment rated at least 13.5 cal/cm² - in practice the next category up - plus the larger boundary, which may affect barricading, switching locations and who is permitted to operate the board. Update the switching procedures, the PPE inventory, and the pre-job briefing before the chiller is energized; the electricians must not be asked to work on the old numbers in the interim.
3. List what you would require from the contractor and the equipment vendor before you authorize energization.
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Documents to receive and review (a hard hold point, in writing):
- Revised coordination study with time–current curves for every device in the new path, demonstrating the downstream device clears first at all fault levels, and confirmation that the upstream delay is within its withstand.
- Short-circuit study update including generator contribution and motor contribution from the new chiller, with the 41 kA figure re-verified.
- SCCR and series-rating documentation for the ATS and the new feeder breaker: the manufacturer's written statement of the listed combination, or a replacement device rated for the available current. The 'vendor says it's fine' email is not documentation.
- Arc flash study and printed labels reflecting the as-installed configuration, with the methodology stated.
- Settings sheets plus field proof - secondary or primary injection results showing the breakers are set as the study requires.
- Commissioning test records: ATS transfer under load and failure-mode testing, generator load test, breaker trip tests, and confirmation that any ground-fault protection on the EES coordinates (the hospital rules limit the levels of ground-fault protection and require downstream coordination).
- A commissioning plan with defined hold points and a signed statement that energization will not proceed until the hold points above are closed.
If any item is open at the survey deadline, the correct answer is that the equipment stays de-energized: a regulatory survey date is not a technical justification for energizing equipment with an inadequate withstand rating.