Power System Protection and Safety
This domain covers how faults are detected, isolated and made safe: relay types, breaker selection, earthing, and the electrical safety rules a technician is expected to apply on site.
Protection fundamentals
A protection scheme must be selective, fast, sensitive, reliable and economical. Current and voltage transformers step down quantities to relay level; a 200/5 CT gives a 40:1 ratio. Common schemes: overcurrent and earth fault for feeders and motors, differential for transformers and generators, distance (mho/impedance) for transmission lines, and Buchholz relay for oil-immersed transformer gas accumulation.
Breaking and clearing
- Fuse: cheap, one-shot, suits LT.
- MCB/MCCB: resettable, thermal-magnetic or electronic.
- ACB, VCB, SF6 breaker: progressively higher ratings and faster arc quenching.
Earthing and safety
Earthing keeps touch voltage low. Substation earthing uses plate or pipe electrodes and mesh conductors; earth resistance targets are typically 1 Ω or less for large substations and up to 5–8 Ω for small installations. A 30 mA RCCB protects people against lethal leakage; ELCB/RCCB is not a substitute for correct earthing. For electrical fires use CO2 or dry powder, never water.
Worked example: relay setting
A feeder CT is 200/5 A and the overcurrent relay is set at 0.5 A plug setting with a 1.0 multiplier. Secondary pickup = 0.5 A, so primary pickup = 0.5 × (200/5) = 20 A. A fault drawing 200 A gives a plug multiplier of 200 ÷ 20 = 10 times, which tells you how long the relay will take from its characteristic curve.
Traps
Confusing CT ratio with relay setting; stating that an RCCB detects earth leakage because it measures earth resistance (it measures the imbalance between phase and neutral currents); using water on an electrical fire; assuming higher earth resistance is safer; forgetting that protection must isolate only the faulty section, not the whole board.