Industrial distribution and earthing — Industrial Electrical Practice

Sample questions

1. 01A replacement contactor coil marked 230 V has been wired between L1 and L2 of a 230/400 V three-phase supply. What voltage will the coil see?

   • A400 V, which is 230 V multiplied by √3
   • B230 V, because each line is 230 V to earth
   • C690 V, three times the line-to-neutral value
   • D460 V, twice the 230 V line-to-neutral value

   Why: Between any two lines of a UK 230/400 V supply the voltage is 400 V, which is the 230 V line-to-neutral voltage multiplied by √3 (about 1.73). A 230 V coil across two lines will not last. Doubling 230 V is tempting, but it ignores the fact that the line voltages peak one after another rather than together. 230 V is what you read from one line to neutral, not between two lines.

2. 02You are choosing enclosures, cables and glands for a food factory where the plant is hosed down every day. Which regulation of the Electricity at Work Regulations deals directly with equipment exposed to wet, dirt, dust and corrosion?

   • ARegulation 6
   • BRegulation 16
   • CRegulation 5
   • DRegulation 14

   Why: EAWR regulation 6 requires equipment that may foreseeably be exposed to things such as mechanical damage, weather, wet, dirt, dust or corrosion to be built or protected to prevent danger from that exposure, so far as is reasonably practicable. Regulation 5 is tempting because it is also about equipment, but it deals with equipment being pushed beyond what it is built to handle. Regulation 14 covers live work and regulation 16 covers competence.

3. 03A maintenance manager says the 24 V DC control circuits on a packing line are outside the Electricity at Work Regulations because the voltage is so low. Is that right?

   • AYes, EAWR only applies to circuits within BS 7671
   • BNo, but only as the line is fed from a 400 V board
   • CNo, EAWR applies at work whatever the voltage
   • DYes, EAWR only applies above 50 V AC or 120 V DC

   Why: The Electricity at Work Regulations apply to all electrical systems at work and have no voltage limit; they apply wherever danger may arise. BS 7671 has its own scope, but EAWR is not tied to it. The 400 V board answer reaches the right verdict for the wrong reason: the control circuits are covered in their own right, whatever feeds them.