Torque and clearance leave no room for guesswork: HR for switchgear manufacturers

Six months after an LT panel is commissioned at a hospital, the electrical contractor runs a thermal camera over the busbar chamber. One joint on the incomer glows far hotter than its neighbours. The cause is simple. The fitter tightened it with a ring spanner by feel, because the bay's only torque wrench was out with another team that afternoon.

Switchgear rewards care that nobody can inspect once the covers go on. Two numbers decide much of a panel's life: the torque on every bolted joint and the gap between live parts. Both rest on one fitter at one moment, which makes them a people question as much as an engineering one.

Torque sets the contact pressure in a bolted busbar joint. Too little and the joint heats, oxidises and loosens further. Too much and the bolt stretches or the disc spring washer flattens, with much the same end. Clearance is the air gap phase to phase and phase to earth, and creepage is the path along insulator surfaces. A busbar support fitted a few millimetres off, or a lug turned the wrong way, can eat into either. Once shutters, barriers and covers are fitted, a visitor sees none of this.

• One torque wrench per bay, so whoever cannot wait for it uses a spanner and judgement.
• Torque values held in the senior's head, not printed on the drawing or a joint schedule.
• Contact surfaces left uncleaned, with joint compound skipped on a rushed day.
• No paint stripe on the nut, so nobody can tell later which joints were torqued and which were missed.
• Clearance judged by eye, because the gauge block lives in the test bay drawer.
• Month-end pace, where three panels must leave and the last joints of the day get the least attention.

Owners often assume the HV withstand test catches bad workmanship. It catches a great deal, especially a clearance that has collapsed. It says little about a joint tightened by feel, because a slack joint conducts perfectly well at test current on the bench. The fault appears under load, over months, as heat. A millivolt drop or micro-ohm reading across main joints helps, but many shops only sample it. The test is a backstop. The fitter is the control.

1. 1

   Print a joint schedule for each panel

   Every bolted joint on the busbar drawing numbered, with bolt size, washer stack and torque value beside it. The schedule travels with the panel.

2. 2

   Issue numbered torque wrenches against signature

   Enough wrenches that nobody waits. Each carries a tag with its last check date, and stores issues it like a measuring instrument.

3. 3

   Stripe every torqued nut

   A paint pen line across nut, washer and bar, applied only after the wrench clicks. An unstriped nut is, by definition, not done.

4. 4

   Initial the schedule joint by joint

   The fitter initials each joint number as it is finished. A checker countersigns all main joints and a sample of the rest.

5. 5

   Keep clearance gauges at every assembly bay

   Go and no-go blocks for the panel's voltage class, hung where the fitter works, not locked in a drawer.

6. 6

   Hold the routine at month end

   The production head agrees in writing that no panel leaves with blank lines on its joint schedule, whatever the dispatch date.

PANEL: ______  DRAWING AND REVISION: ______  BAY: ______  FITTER: ______.
COLUMNS: Joint no. | Location (incomer, main bus, dropper, outgoing) | Bolt size and grade | Washer stack | Torque value | Wrench no. | Surface cleaned and compound applied (tick) | Stripe applied (tick) | Fitter initials | Checker initials.
CLEARANCE CHECKS: phase to phase and phase to earth at each chamber, gauge used, pass or fail, initials.
FOOT: Total joints ______  Initialled by fitter ______  Countersigned by checker ______.
RULE: Any blank line returns the panel to the bay before the test bay will accept it.

A routine only works if fitters value it. That depends on how the shop treats their skill. Grade fitters by joint and clearance work, with a sample panel checked at each grade, and pay the grades fairly. Site contractors offer experienced fitters more money for commissioning work, and the switchgear retention page covers reasons to stay beyond pay. Fitter grades set against site rates are on the switchgear pay benchmarking page. The checker who countersigns needs written authority to send a panel back, which the switchgear role design page sets out.

• Count torque wrenches per assembly bay and ask how often a fitter waits for one.
• Open a finished panel and look for paint stripes on every main busbar nut.
• Ask your senior fitter which joints he re-checks on a rushed panel, and why.
• Find where the clearance gauges are kept and when they were last used.
• Check whether the test report records anything about joint torque.

Safe routines for energised testing and heavy panel handling are on the switchgear health and safety page. The switchgear manufacturers page lists the other strains, and the overview of people issues in electrical equipment firms compares them across the trade.