Grounding, Bonding, and Electrical Safety
What the candidate must know
Candidates must separate earthing (a path for fault current), bonding (equalising potential between conductive parts), and functional earthing (a reference for signalling). Know the terms: protective earth, main earthing terminal, equipotential bonding, extraneous conductive part, exposed conductive part, and touch voltage. Understand why bonding conductors must not be switched or fused, and why a single poor joint can raise touch voltage across the whole installation. Safety topics include isolation and lock-off, proving dead, safe isolation procedure, and the role of RCDs as supplementary protection.
Traps that appear in questions
- Treating bonding and earthing as synonyms; bonding does not carry fault current in normal operation.
- Assuming a low earth resistance alone proves safety; loop impedance and disconnection time matter.
- Forgetting that extraneous parts (pipes, structural steel, tanks) must be bonded where they could introduce a potential.
- Skipping the prove-test-prove sequence when checking for dead.
- Believing an RCD removes the need for earthing.
Worked example / rule of thumb
A fault raises the hull to 100 V and the fault current is 2 A through a 50 Ω earth path. Touch voltage across a person standing on a bonded deck approaches that 100 V, which is dangerous. Improving the earth path to 2 Ω raises fault current and speeds disconnection. Rule of thumb: bond everything that could become live, then verify disconnection time, not just resistance.