LEW case studies

LEW · Put your knowledge to work in context.

LEW stands for Licensed Electrician (or Electrical) Worker - a state or jurisdiction-issued credential that authorises a person to perform electrical installation work within that jurisdiction's scope rules. Unlike a purely national trade certificate, licensing is administered locally, so requirements, recognised experience and permitted task scope vary by issuing authority. The LEW case studies are 5 cases and about 4,494 words.

TT-supplied workshop: loop impedance, RCD fault protection, and a proposed second earth rod

A metal-fabrication workshop on an industrial estate takes 400/230 V from the estate's own 11 kV/400 V transformer. The supply arrangement is TT: the workshop has a single earth electrode at the main switchboard, measured at 42 ohms by fall-of-potential.

960 words, about 5 min 2

Office fit-out: six circuits bunched in one trunking at 40 degrees C

A contractor is installing final circuits for a refurbished office suite. Six 230 V single-phase circuits will share one 50 mm metal trunking along a ceiling route for 19 m.

810 words, about 4 min 3

Neutral overheating on a harmonic-rich 400 kVA transformer with power-factor correction

A 400 kVA, 11 kV/400 V Dyn11 transformer feeds a mixed office and small data-centre building. The load is dominated by switch-mode power supplies, LED drivers, electronic ballasts and inverter-driven chillers; the building also has a 100 kvar automatic capacitor bank for power-factor correction.

914 words, about 5 min 4

37 kW pump tripping on overload: voltage unbalance, ambient derating, and a proposal to raise the setting

A 37 kW, 400 V, three-phase, four-pole IE3 pump motor drives a cooling-water pump. The nameplate gives a full-load current of 68 A, a service factor of 1.15, class F insulation and a 40 degrees C ambient rating.

882 words, about 4 min 5

RCBO tripping on an LED lighting circuit: cumulative leakage versus insulation fault

A retail unit is protected by a 30 mA type AC RCBO on each final circuit and a 300 mA S-type RCD at the origin. The RCBO feeding the shop-floor lighting trips about twice a week, always between 17:00 and 19:00, shortly after the lighting is switched on.

928 words, about 5 min