Electrical Safety, Earthing, and Lightning Protection
What this domain tests
Electrical Safety, Earthing, and Lightning Protection tests protective bonding, touch and step voltage, earth-electrode design, RCD application, and lightning risk assessment. The exam expects you to distinguish protective earthing from functional earthing and to prove that a fault is cleared within a safe time.
Core rules and concepts
- The fundamental safety criterion is that touch voltage stays below the applicable limit, commonly 50 V AC in normal dry conditions, for the duration of the fault.
- In a TT system, the fault loop includes the local earth electrode and the source earth. A 30 mA RCD is commonly required because the loop impedance alone may not clear the fault fast enough.
- Earth-electrode resistance for a vertical rod can be estimated by R = ρ/(2πL) × [ln(4L/d) − 1], where ρ is soil resistivity, L is rod length, and d is diameter.
- Bond all incoming metallic services and structural steel to the main earthing terminal. Bonding is not the same as using the pipe as a protective conductor.
- Lightning protection follows a risk assessment: air termination, down conductors, earth termination, and surge protective devices. IEC 62305 provides the framework; the Egyptian Code and utility practice add local requirements.
Worked example
A TT installation has a 3 m rod of 16 mm diameter in soil of resistivity 100 Ω·m. R = 100/(2π × 3) × [ln(4 × 3/0.016) − 1] = 5.31 × [ln 750 − 1] = 5.31 × 5.62 = 29.8 Ω. For a 230 V phase-to-earth fault, the prospective current is 230/(29.8 + source earth resistance). Even with a low source resistance, the current may be only a few amps, so a 30 mA RCD is needed for fault clearance. Verify that touch voltage at the electrode remains below 50 V.
Traps that cost marks
Assuming an RCD replaces earthing; using a separate “clean earth” without bonding; ignoring step voltage near substations; designing lightning protection without a risk assessment; and treating a measured electrode resistance as valid without considering seasonal soil moisture. Also, bonding a gas or water pipe does not permit it to carry fault current as a PEN conductor.
Exam-focused summary
State the earthing system, calculate the loop or electrode resistance, and prove touch voltage and disconnection time.