11.5 Ω Earth Resistance and Three Quick Fixes
A 100 kVA transformer in a sandy-soil area has its neutral, the LT panel body and a nearby streetlight pole all bonded to a single 40 mm diameter, 3 m long GI pipe electrode. In May, a three-pole fall-of-potential test taken with the transformer disconnected gives 11.5 Ω. The utility's accepted limit for a transformer neutral earth is 5 Ω or less. The line staff propose three quick fixes: (i) pour two buckets of water and a kilogram of common salt around the electrode every morning through the summer; (ii) add a second identical pipe electrode 3 m away and interconnect the two with 25×3 mm GI strip; (iii) drive the existing pipe 1 m deeper. The test itself was taken with the potential spike only 8 m from the electrode and the current spike 25 m away.
1. Can the 11.5 Ω reading be trusted? Assess the test method and state what you would change.
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Partly - re-test before acting on the number.
- Fall-of-potential needs the current spike far enough away that the electrode's resistance area does not overlap it; the usual guide is at least five times the electrode depth, so 15 m or more for a 3 m pipe. At 25 m the current spike is acceptable.
- The potential spike must sit in the flat part of the potential curve, normally at about 62% of the current-spike distance, i.e. about 15.5 m here. It was driven at 8 m - roughly 32%, close to the electrode and in the steeply rising part of the curve. That reading is not valid and is usually higher than the true value.
- Correct method: disconnect the electrode from the system, take readings at about 52%, 62% and 72% of the current-spike distance, and accept the result only if the three agree within a few percent (a flat curve).
- May is the right season - the dry-season value is the worst case and the one that must meet the limit.
So re-test properly first. If it still reads above 5 Ω, the electrode system is genuinely inadequate.
2. Evaluate the three proposed fixes on engineering grounds.
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- (i) Water and salt daily. Works for a few days, then leaches away. Salt accelerates corrosion of the pipe and the joint, contaminates the soil, and depends on somebody remembering every morning. An emergency measure at best; not a fix.
- (ii) A second identical pipe 3 m away, bonded with GI strip. Right direction, wrong arithmetic. Two identical electrodes spaced about one length apart do not halve the resistance - expect roughly 60% of the single-electrode value, say 11.5 → about 7 Ω. That is still above 5 Ω. Wider spacing (at least twice the electrode length) gives a better factor, and sandy soil usually needs three or more rods.
- (iii) Driving the existing pipe 1 m deeper. Adds some surface area, but in dry sandy soil the improvement is small and unpredictable, and the pipe may refuse to go deeper. It also leaves the transformer neutral, the panel and the streetlight pole dependent on one electrode - a single point of failure, and a hazard if that electrode is damaged.
None of the three, taken alone, reliably reaches 5 Ω.
3. What electrode system would you actually specify, and how would you prove it works?
Read the worked answer
Specify a system, not a patch.
- Keep the transformer neutral, LT panel body and streetlight pole on a common bonded earth - they must be bonded anyway - but give the system enough electrode surface: at least three 3 m rods, or a correctly rated maintenance-free chemical electrode, spaced at least twice the electrode length apart and interconnected with 25×3 mm GI strip, joints brazed or made with proper clamps, not twisted wire.
- Use a backfill compound (bentonite or proprietary earthing compound) instead of salt and charcoal - it holds moisture, does not corrode the electrode and does not need recharging every summer.
- Protect the joints: the strip-to-pipe connection must be above ground or fully encapsulated. Most in-service earth failures are failed joints, not failed electrodes.
- Prove it: re-run fall-of-potential on the completed system in the dry season at 52/62/72% spacing and record the value - it must be 5 Ω or less. Check bonding continuity from transformer tank to panel to pole with a low-resistance ohmmeter. Enter both results in the earth register.
- Re-test annually before summer; a value drifting upward is the warning that a joint or the backfill has failed.