What to specify,
and what to ask for

Three different jobs on the same single-line diagram, not three sizes of the same part. What each is allowed to switch, why the AC-3 rating rather than the AC-1 figure on the same frame governs motor duty, a worked example where the duty cycle rather than the current picks the frame, and what an enquiry has to state for each device.

Three occasions get called switchgear testing, and each produces a different document proving a different thing. Design against routine verification, what site acceptance closes that the works cannot, the six tests worth knowing by name, and the four things that separate a report that evidences a claim from one that only looks like it does.

The service route decides the mounting, not the cooling design. Why ground level triggers enclosure-integrity, dead-front and clearance rules that pole height removes, loop-feed against radial-feed at the bushings, and a worked footprint check that eliminates a pad on a 3.5 m easement.

Six leads, and the link pattern that makes them star or delta. What U, V, W and the 1/2 suffixes encode, the dual-voltage case, a worked 45 kW line-current calculation, and why star-delta gives a third of the current and a third of the torque by two different routes.

Fire load and siting settle which one is admissible; insulation class, cooling, overload margin, part-load losses and maintenance settle what it costs to own. With a worked twenty-year cost-of-losses comparison showing why the cheaper transformer at the loading dock is not always the cheaper one.

Prospective, bolted, symmetrical, asymmetrical — what a spec sheet assumes you already know. The infinite-bus shortcut and where it overstates, a worked point-to-point calculation out to a downstream panel, and the equipment ratings the number decides.

Where each type sits, what it is rated against, and why the three are a cascade rather than three options to choose between. Includes the parameters a specification must state and a worked example of how a 1.2 m connecting lead nearly doubles the voltage reaching the protected equipment.

The industrial slice only — air, oil, SF6 and vacuum, by voltage class and interrupting medium. What a quotation has to state for two offers to be comparable, why withstand and making current are not free choices, and a worked breaking-capacity calculation from a transformer's rating and impedance.

One is wired in series and must never be opened under load; the other is wired in parallel and must never be shorted. Burden and accuracy class expressed differently for each, accuracy limit factor against voltage factor, and a worked check of whether a C200 CT stays within class through a fault.

"PDU" covers four distinct pieces of equipment — floor PDU, remote power panel, rack PDU and busway tap-off. Tier, monitoring grade, phase configuration and breaker coordination, with a worked phase-imbalance calculation and the enquiry checklist a comparable quotation depends on.

Eight variables set the price — ratio, accuracy class, rated burden, accuracy limit factor, insulation level, construction type, enclosure rating and quantity. Includes a worked burden calculation showing why a long secondary run favours a 1 A secondary, and what an enquiry must state to get comparable quotations.

The chain from utility entry to the rack, grouped by what each stage is for: conversion, continuity and subdivision. Defines PDU, RPP, busway and rack PDU, explains N+1 against 2N against distributed redundancy with a worked comparison at 300 kW, and shows how to read a single-line diagram as a fault-analysis tool.

Conductor grade and temper, the basis behind a current rating, thermal and electrodynamic short-circuit evidence, creepage at the supports, joint torque, laboratory accreditation and export packing — with a worked force calculation and a ten-question supplier evaluation table.