Inspection, testing and maintenance — EV Charging Installation Practice

Sample questions

1. 01At handover, a B32 EV circuit on a TN supply measures Zs = 1.05 Ω with the cable cold. Using the Appendix 3 0.8 factor against Table 41.3, what do you conclude?

   • AFails: it is over 0.87 Ω, which is 0.8 × 1.09 Ω
   • BPasses: it is under 1.09 Ω, which is 0.8 × 1.37 Ω
   • CFails: a cold B32 reading must be under 1.00 Ω
   • DPasses only if it is retested later at full load

   Why: Table 41.3 gives 1.37 Ω for a 32 A Type B device. Appendix 3 has the measured value meet Zs(m) ≤ 0.8 × the tabulated value to allow for conductor temperature rise: 0.8 × 1.37 = 1.096, taken down to 1.09 Ω. 1.05 Ω passes. 1.09 Ω is the tabulated B40 value, so 0.87 Ω is the B40 limit, the wrong row.

2. 02A three-spike test on a rod gives 48 Ω, 50 Ω and 52 Ω with the potential spike at its three positions. What does GN3 say?

   • ARepeat, as all three readings must be equal
   • BRecord 52 Ω, as the highest is the safe value
   • CRepeat, as the spread of 4 Ω is too wide
   • DAccept: the largest deviation is 4% of 50 Ω

   Why: The average is (48 + 50 + 52) ÷ 3 = 50 Ω. The largest deviation is 2 Ω, which is 4% of the average. GN3 says it is inadvisable to accept readings that differ by more than 5%; then you repeat with a larger separation to the current spike. 4% is acceptable.

3. 03During a three-spike electrode test the soil is very dry and the spikes read high. Where may brine be used to improve contact?

   • AOn the spikes and on the electrode
   • BOn the temporary test spikes only
   • CNowhere, as brine is not permitted
   • DOn the electrode under test only

   Why: GN3 says spike resistance can be reduced by driving the spikes deeper or watering with brine, but in no circumstances should that be used to reduce the resistance of the electrode under test, because it would flatter the reading. Reconnect the earthing conductor before re-energising.