Substation Earthing Failure and Touch Voltage Before Energization
A 33/11 kV substation has two 5 MVA transformers and an earth grid of 12 pipe electrodes. In May, an earth tester shows an overall grid resistance of 3.8 Ω. Four electrode pits that were disconnected and tested individually read 18 Ω, 22 Ω, 25 Ω and 30 Ω; the remaining pits were not tested. Soil resistivity is 180 Ω·m at 1.5 m depth. A lineman reports a tingling shock when touching the metal enclosure of an 11 kV switchgear panel, and a multimeter shows 48 V between the enclosure and a nearby earth pit. The enclosure bonding strip reads 0.4 Ω end-to-end, but the bolted joint at the panel is corroded. The 11 kV neutral is earthed at the transformer through a separate strip. The design target for this substation is 1 Ω or less. A new 11 kV outgoing feeder is to be energized the next morning.
1. Interpret the earth resistance and the 48 V measurement. Is the installation safe to energize? Show how the corroded joint matters under fault conditions.
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For a 33/11 kV substation of this size the design target is usually 1 Ω or less; 3.8 Ω is about four times the target and the individual electrodes at 18–30 Ω confirm poor dry-soil contact. The 48 V between the enclosure and the earth pit is a touch-voltage warning. If a fault current of, say, 1,000 A flowed through a 0.4 Ω joint, the joint would drop 400 V; even 100 A would drop 40 V. The corroded bolted joint is therefore not a cosmetic issue-it can put a dangerous potential on the enclosure while the grid itself is at 3.8 Ω. The installation should not be energized as it stands. Repair the bonding joint, re-test, and verify touch and step voltages before the new feeder is charged.
2. Give a sequence of corrective actions and the acceptance criteria you would apply before energization.
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- Isolate and lock off the switchgear; clean or replace the corroded joint and the bonding strip; use a proper bimetallic joint and torque it. 2. Test the bonding path end-to-end; a few milliohms to tens of milliohms is expected, not 0.4 Ω. 3. Add electrodes: 4–6 more pipes or rods at 3 m depth, interconnected with GI strip, with bentonite or native soil backfill; do not just pour water. 4. Measure overall earth resistance in dry conditions; aim for ≤1 Ω for this grid, and in any case a value that keeps touch voltage below the permissible limit for the fault clearing time. 5. Check the 11 kV neutral earthing and the lightning arrester earths; bond separate earths where the standard requires it. 6. Measure touch and step voltage at the panel, or calculate them from grid resistance and fault current; 48 V must be reduced. 7. Obtain a signed test sheet and check that the 11 kV earth-fault relay trip time is coordinated. Energize only after the bond repair and a repeat measurement; if the grid cannot reach 1 Ω before the deadline, the feeder can be charged only under a documented risk assessment with restricted access and temporary earthing, not as a normal operation.