The Job Card with Four Wrong Notes in the Margin
You are a third-year electrical installation student on work placement with a contractor fitting out a metalwork shop in Nicosia. Your foreman hands you a handwritten job card for a new radial socket circuit: three 16 A double socket outlets, 2.5 mm² twin-and-earth cable, protected by a 16 A Type B MCB, with 30 mA RCD protection at the origin. The supply is TN-S and the main earthing terminal is at the intake position. A helper has added four notes in the margin:
- 'The voltage in this cable is too small, so use a thicker one.'
- 'Earthing here is the same as the neutral, so we can skip the earth to the metal back boxes.'
- 'The RCD protects the cable from too much current.'
- 'The breaker is what stops you getting a shock.'
The foreman wants the notes corrected before the card goes into the job file, and each corrected note kept under 25 words so it fits the margin.
1. Rewrite all four margin notes using the correct technical terms. For each one, name the wrong term used and say what the correct concept is.
Read the worked answer
Wrong term: 'voltage in this cable'. Voltage is a property of the supply, not of the cable; a cable has a current-carrying capacity and a voltage drop. Correct note: 'Cable current-carrying capacity may be too low for the load; a larger conductor also reduces voltage drop.'
Wrong idea: 'earthing is the same as the neutral'. On a TN-S system the protective conductor and the neutral are separate conductors from the source onwards. Even where they are combined upstream (TN-C-S), the protective conductor must never be omitted. Correct note: 'TN-S: separate PE and N. Metal back boxes are exposed-conductive-parts and must be connected to the circuit protective conductor.'
Wrong device function. An RCD is not an overcurrent device; the MCB or fuse provides overload and short-circuit protection. Correct note: 'The MCB protects the cable against overload and short circuit; the 30 mA RCD gives additional protection against earth-fault shock.'
Wrong device function again. A 16 A Type B MCB will not operate at the tens of milliamps that flow through a person; the RCD is the device that limits the duration of a shock current. Correct note: 'The MCB will not clear a shock current; the RCD is the additional protection against electric shock.'
2. The helper argues: 'It's a metal shop, everything is bonded anyway, so a missing earth to a back box does no harm.' Give the technical answer you would put in the job file.
Read the worked answer
Bonding and earthing are not interchangeable, and neither is optional here.
- Earthing connects exposed-conductive-parts (the metal back box, the metal enclosure, the metal tool frame) back to the source so that a fault current has a low-impedance path and the protective device operates quickly.
- Bonding connects extraneous-conductive-parts (structural steel, pipework) together and to the protective conductor, so that no dangerous potential difference can appear between them.
If the back box is not connected to the circuit protective conductor, a live conductor touching it makes the whole box live, and the MCB sees no fault current at all - nothing operates until a person touches the box and something earthed. Bonding elsewhere in the shop does not fix that, because the fault loop still has to pass through the person.
Job-file wording: 'Metal back boxes on this circuit are exposed-conductive-parts and are connected to the circuit protective conductor; bonding of structural steel is a separate requirement and does not replace it.' Follow the current edition of the wiring regulation applied on the project (BS 7671 / HD 60364) and the job specification.