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Ze, Zdb and Zs: where each one is doneWhere is Zdb measured?
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Three values, three places, and the middle one gets mixed up most.
| Value | Where | Parallel paths |
|---|---|---|
| Ze | At the origin — your main board, across the incoming supply | Removed — earthing conductor disconnected |
| Zdb | At any other board — garage, workshop, summer house | Left connected |
| Zs | At every circuit, at the point of utilisation | Left connected |
✋Zdb is measured at each board other than the main one. It is the Zs of a distribution circuit, measured at the board that circuit feeds. If a property has three or four boards you will have three or four Zdb values — and three or four test sheets, each carrying its own. For the main board itself, the Zdb box on the schedule takes your Ze.
Zs can be calculated instead of measured. Ze + (R1 + R2) saves you climbing into a loft to reach a lighting point. Both are accepted.
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- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing