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Egypt Engineering Consultant Electrical study materials
Egypt Engineering Consultant Electrical study resources

Egypt Engineering Consultant Electrical: Protection, Switchgear, and Coordination

Egypt Engineering Consultant Electrical · Work through your exam, one topic at a time.

All guide sections

Protection, Switchgear, and Coordination

What this domain tests

Protection, Switchgear, and Coordination tests your ability to select breakers and relays, set overcurrent and earth-fault protection, prove discrimination, and verify switchgear ratings. Egyptian MV practice commonly uses 11 kV and 22 kV switchgear with CT-operated relays, while LV boards use MCCBs and ACBs. The exam rewards correct grading arithmetic and clear zone boundaries.

Core rules and concepts

  • A breaker has several ratings: rated current, Icu (ultimate breaking capacity), Ics (service breaking capacity), Icw (short-time withstand), and making capacity. Do not treat them as interchangeable.
  • CT ratio and relay plug setting define primary pickup. A 200/5 CT with a 1.0 plug setting picks up at 200 A primary, not 5 A.
  • IEC 60255 inverse-time curves use t = TMS × 0.14/[(I/Is)^0.02 − 1] for the standard inverse curve. Set the downstream relay first, then grade upstream.
  • A coordination time interval of about 0.2–0.4 s is normally targeted between successive devices, allowing for breaker opening and relay reset.
  • Earth-fault protection, differential protection, and distance protection protect different zones; do not use one to replace another.

Worked example

A downstream relay has a 100/5 CT, plug setting 1.0 (100 A), and TMS 0.1. For a 600 A fault, M = 600/100 = 6. Then t = 0.1 × 0.14/(6^0.02 − 1) = 0.1 × 0.14/0.0365 = 0.384 s. An upstream relay with a 200/5 CT, plug 1.0, and TMS 0.2 sees M = 3; t = 0.2 × 0.14/(3^0.02 − 1) = 0.2 × 0.14/0.0221 = 1.27 s. The margin is 0.88 s, so discrimination is adequate at that fault level. Check other fault levels because curves can cross.

Traps that cost marks

Confusing CT secondary current with primary pickup; using the fault current instead of the multiple of setting; ignoring the instantaneous element; grading only at one fault level; and selecting a breaker by nominal current while ignoring Icu at the actual prospective fault level. Also, cascading is not the same as discrimination.

Exam-focused summary

Draw the single-line diagram, mark CT ratios and settings, and show the time-current margin at the grading point.