Industrial Wiring and Power Distribution
Distribution arrangement
Industrial LT supply is distributed from a transformer through an ACB or MCCB to a main busbar, then through MCCBs to sub-distribution boards and machine feeders. Cables are sized for current-carrying capacity, voltage drop (generally not more than 5%), short-circuit withstand and derating for ambient temperature, grouping and enclosure.
Power factor and metering
- kW is real power; kVA is apparent power; kVAR is reactive power.
- Capacitor kVAr = P (tan φ1 − tan φ2).
- CTs and PTs step down for metering; CT secondary is 5 A or 1 A.
- Earthing system: separate earth pit resistance should be low, commonly ≤ 1 Ω for substations and ≤ 5 Ω for general installations, checked with an earth tester.
Worked example
A 50 kW load at 0.80 lagging power factor is corrected to 0.95. tan φ1 = 0.750, tan φ2 = 0.329. kVAr = 50 × (0.750 − 0.329) = 21.1 kVAr. Use a 20–25 kVAr capacitor bank with suitable switching and discharging resistors.
Traps
Sizing cable by kW alone, ignoring derating factors, and connecting a capacitor bank without discharge protection. A CT secondary must be shorted before removing the meter, otherwise dangerous voltage appears.