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HK Grade B Electrical Worker Exam study materials
HK Grade B Electrical Worker Exam study resources

HK Grade B Electrical Worker Exam: Protective Devices

HK Grade B Electrical Worker Exam · Work through your exam, one topic at a time.

All guide sections

Protective Devices

Protective Devices is where the paper tests device selection rather than device names. You must be able to show that the chosen device protects the cable against overload and short circuit, operates within the disconnection time, discriminates from the device upstream, and has an adequate breaking capacity.

The selection rules

  • Overload: Ib ≤ In ≤ It and I2 ≤ 1.45 It, where I2 is the current that reliably operates the device.
  • Short circuit: the rated breaking capacity must be at least the prospective short-circuit current at the point of installation, or back-up protection must be demonstrated.
  • Fault protection: Zs must satisfy the disconnection time; for a Type B MCB, Ia = 5 In, Type C 10 In, Type D 20 In.
  • Additional protection: 30 mA RCD for socket outlets, bathroom and outdoor circuits, and construction sites.
  • Discrimination: time and current grading between upstream and downstream devices; a 100 mA time-delayed RCD upstream of a 30 mA device, for example.

Device families

MCBs are available as Type B, C and D with different magnetic thresholds; MCCBs give adjustable thermal and magnetic settings; fuses (BS 88, BS 1361, BS 1362) offer excellent current limitation and I²t performance; RCDs are Type AC, A or B depending on the residual current waveform, with Type B needed for smooth DC from VFDs and PV; RCBOs combine both functions.

Worked example

A circuit has Ib = 45 A in 10 mm² cable with tabulated capacity 63 A and an ambient factor of 0.87, giving It = 54.8 A. Choose a 50 A MCB: 45 ≤ 50 ≤ 54.8, and I2 = 1.45 × 50 = 72.5 A, which is within 1.45 × 54.8 = 79.5 A. The board PSCC is 4.5 kA, so a 6 kA MCB is acceptable. If Type C is used, Zs must not exceed 220 / (10 × 50) = 0.44 Ω, so the measured 0.35 Ω is fine.

Traps that cost marks

Sizing on Ib alone and ignoring It; ignoring breaking capacity or back-up protection; assuming a 32 A MCCB discriminates from a 32 A MCB downstream; choosing a Type AC RCD for a VFD or PV circuit; and using a 30 mA RCD to excuse a failing Zs without saying what the RCD is providing.