41 V AC at Test Station TS-14
A two-person crew is running a close-interval survey on a 6 in. coated gas line that shares a corridor with a 138 kV transmission line for 3 km at a 12 m offset. At test station TS-14 the technician's handheld meter shows 41 V AC between the pipe and a temporary copper ground rod. It has been raining for an hour and the soil is saturated. The interrupter (1.2 s on / 0.8 s off) is powered by a portable generator sitting in the truck bed 8 m away. A trainee wearing nitrile work gloves reaches toward the TS-14 terminal to clip on a test lead. One spotter is flagging traffic on the adjacent two-lane road. The crew's procedure sets a 15 V AC touch-potential action level.
1. Identify the hazards present and state what must happen before anyone touches the TS-14 terminal.
Read the worked answer
Stop the task first, then reason.
- 41 V AC is above the 15 V action level in the crew's own procedure. The pipe is almost certainly energized by induction from the parallel 138 kV circuit. Wet soil and wet skin have removed most of the resistance that normally limits current through a person.
- Nitrile gloves are chemical PPE, not electrical PPE. The trainee is about to become a parallel path from pipe to earth through hand, body and boots.
- Secondary hazards: one spotter for live traffic, a generator in the truck bed (fuel, CO, ungrounded frame, cords in standing water).
Controls before resuming: keep everyone clear of the terminal; confirm the meter is working by checking it on a known source, using a CAT III/IV instrument and inspected leads; if work proceeds, use rubber insulating gloves with leather protectors and a voltage rating above the measured value with margin, work one-handed, stand on an insulating mat, and bond the reel/frame so the operator is never between two different potentials.
Escalate: notify the operator's corrosion-control engineer, revise the JSA, and do not resume until the AC mitigation and work method are agreed in writing.
2. The trainee argues that because the meter read 41 V, the pipe 'is at 41 V' and the gloves and meter just need to be rated for that. Is that sound reasoning?
Read the worked answer
It is only half sound. A voltage reading gives magnitude, not source strength.
- Induced AC on a well-coated pipe is often a high-source-impedance voltage. A low-impedance or solenoid-type AC tester will make it collapse; a high-impedance digital meter reads something close to open-circuit voltage. Two very different hazards can both read 41 V.
- Shock severity depends on current through the body, which depends on source impedance and on the resistance of the path (wet boots, wet soil). Wet conditions make a 'soft' source far more dangerous than it looks.
Practical test: read the value with both a high-impedance and a low-impedance instrument. If the value holds up under the low-impedance load, treat the pipe as a stiff energized conductor and do not make contact at all. If it collapses, the touch hazard is lower but the measurement itself is suspect and must be taken with a meter/lead combination rated for the environment.
Also check every link in the chain: meter category rating, lead insulation, and the reference-electrode cable are all part of the insulation system. The lowest-rated item sets the protection level.
3. If the crew stops work, what has to be documented and what must be true before they restart?
Read the worked answer
Document: date, time, exact location (station, GPS, chainage), the measured AC and DC values and the instrument/lead IDs used, weather and soil condition, who was present, who was notified and when, and the specific reason for stopping.
Also document the measurement method, because a 41 V high-impedance reading and a 41 V low-impedance reading support different decisions and a reviewer must be able to tell which was taken.
Restart criteria should be objective, not a judgement in the field:
- revised JSA and work method signed off;
- AC mitigation in place (gradient control mats, decoupling, temporary grounding) or an agreed alternative method for the crossing;
- rated PPE and instruments on site, inspected and in date;
- traffic control adequate for the crew size.
Why this matters: a stop-work that is closed out with 'crew proceeded with caution' gives no defensible record if someone is later injured or if the survey data are challenged. The record must show what changed, not just that work resumed.