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Measuring Zdb at the boardWhy test again with the earth reconnected?

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With the earthing conductor reconnected, repeat the tests at each distribution board other than the main one — a garage, workshop or summer-house board: earth lead on that board’s earth bar, line and neutral as before. Nothing is disconnected, so the parallel paths are in.

  • Zdb and the board’s Ipf go at the top of the schedule of test results for that board.
  • Zdb is the Zs of the distribution circuit feeding that board, so it takes in that cable as well as the supply. Record what you see.
  • For the main board itself, the Zdb box takes your Ze value.
One board or several? Ze exists only at the origin. Every board gets its own schedule of test results and its own Zdb box — at a sub-board it’s the loop impedance measured at that board. In a house with one board there’s no separate Zdb to measure: the box takes your Ze.
✋Probe on metal. A thin probe into a large terminal screw can sit in the hollow without touching anything. Go in at an angle, onto the metal.

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