Mechanical plant electrical.
The cooling plant is where electrical work hides inside mechanical work orders. A chiller job is a refrigerant job, a rotating-machinery job — and a 480 V, capacitor-buffered, drive-fed electrical job that answers to three different articles before the first coupling bolt moves.
A drive is a rectifier and an inverter — Article 330's list, by name
Motor drives appear explicitly in Article 330's scope, alongside UPS systems, as equipment containing rectifiers and inverters. The consequences mirror the UPS room: the dc bus inside a VFD holds charge after the disconnect opens, the discharge interval on the drive's label is a real number that has to actually elapse, and the meter that verifies zero energy must be rated for what it is measuring. 'Drive is off' and 'drive is safe to open' are separated by minutes of capacitor dwell and one deliberate measurement — the two things most often missing from a mechanic's version of the lockout.
Bypass contactors add the second trap: a drive in bypass has line voltage on its output terminals from the far side. Isolation means the drive input, the bypass path, and control power — the transfer-switch lesson, shrunk into a wall-mount enclosure.
The MCC rows, both of them
600 V-class MCCs carry two table-method rows: Category 2 with a 5 ft arc flash boundary under the 65 kA / 0.03 s parameter set, and Category 4 at 14 ft under 42 kA / 0.33 s. Which row governs is a question about your overcurrent protection's clearing time — not a choice. An MCC behind an older breaker with a long short-time band belongs to the second row, and the difference between suiting for Category 2 and Category 4 is decided by a protection study, not by preference.
Inserting or removing MCC buckets is a Yes on the likelihood table — the plant's equivalent of racking. De-energize the structure for bucket work, or carry the full analysis for doing it live, with the justification question answered first and honestly: a bucket swap is rarely 'not possible de-energized' in a plant with redundant trains.
Where 70E hands off
Electrical energy is one of several on a chiller: refrigerant under pressure, spring and rotational energy, hot water, pneumatics. NFPA 70E governs the electrical member of that set; the full lockout belongs to the broader hazardous-energy program under OSHA 1910.147, one program covering all sources with the electrical steps embedded. The field failure mode is two permits — a mechanical LOTO and an electrical one — executed by two crews who each believe the other verified the part both of them touch. One equipment, one energy-control procedure.
A seized cooling-tower fan gearbox, and a mechanic's lockout hanging on the MCC bucket handle — with the bucket never verified dead because 'the handle was off.' Handles operate mechanisms; only a measurement proves absence of voltage. The mechanical crew's training never included that sentence, and the electrical crew was never on the work order.
Drive or MCC work inside a mechanical order? Run the walk on the electrical portion as its own task — it usually gets a different answer than the wrench work.
This is the material of the 8- and 16-hour NFPA 70E classes — taught on the 2027 edition, against data center topology, about 55 times a year. Private onsite days run against your one-lines.