Protection Coordination, Fault Analysis, and Electrical Safety
Protection coordination and fault analysis test whether you can make the correct breaker trip first and keep touch voltages safe. The exam covers relay settings, discrimination, DC feeder protection, and earthing safety.
Core rules
- Selectivity means the downstream device clears the fault before the upstream device; time grading needs a margin for breaker and relay error.
- AC feeders use overcurrent, distance, and differential protection; transformers use differential and restricted earth fault. DC feeders use high-speed breakers with current and di/dt settings.
- Fault current from a source is I = V/(√3·Z) for a three-phase AC source, with Z derived from short-circuit MVA.
- Earthing must limit touch and step voltage; frame leakage protection detects current in the earthed enclosure.
Worked example
A 25 kV source has 250 MVA short-circuit power. Z = V²/S = 25²/250 = 2.5 Ω. Three-phase fault current = 25,000/(√3 × 2.5) ≈ 5774 A. A feeder relay set at 600 A sees 9.6 times setting, so it trips quickly; an upstream relay set at 800 A must have a time delay of at least 0.3–0.4 s to preserve discrimination.
Traps
- Using phase voltage instead of line voltage in the √3 formula.
- Setting upstream and downstream relays with no time margin.
- Assuming a DC breaker clears on current magnitude alone; di/dt matters.
- Ignoring touch voltage after the fault is cleared.