In substations there are no "small mistakes": medium or high voltage reaching a human body offers no second chance. That's why isolation procedures are built as a strict chain in which no single link may be broken, no matter how simple or urgent the task seems.
The Golden Chain of Isolation
- 1. Permit to Work (PTW): an approved document defining the task, the equipment, the hazards, and the person responsible for isolation — no work without a permit.
- 2. Disconnection from all sources: open the circuit breaker, then isolate with disconnectors for visible isolation, remembering that the equipment may be fed from more than one direction (double busbars, ring main connections, back-feed).
- 3. Lockout-Tagout (LOTO): personal locks and warning tags on every isolation point and its operating mechanisms.
- 4. Verification of absence of voltage: using a device rated for the voltage level, tested on a known live source before and after the measurement.
- 5. Earthing: close the earthing switches or install portable earthing sets on all feed directions — treat anything not visibly earthed as live.
- 6. Defining the work area: barriers and signs separating the safe work area from adjacent live parts — other parts of the substation are operating around you.
Substation-Specific Hazards
| Hazard | Precaution |
|---|---|
| Stored charge in capacitors | Discharge time + discharge and earthing before touching — see Capacitor Discharge |
| Opening a current transformer secondary under load | Strictly prohibited — short-circuit the secondary first |
| Opening a disconnector under load | Disconnectors cannot interrupt current — the circuit breaker first, always |
| Approaching adjacent live parts | Safety clearances based on voltage level, and a safety observer when needed |
| SF6 gas in enclosed spaces | Ventilation and gas detection before entering trenches and vaults |
The interlocking system works in the background as an automatic guard preventing incorrect switching sequences — but it is an additional safety net, not a substitute for the correct procedure.
Sample answer: After obtaining the permit to work: open the feeding circuit breaker, then perform visible isolation with disconnectors from all possible feed directions including back-feed, then apply lockout-tagout (LOTO) at the isolation points, then verify the absence of voltage with a rated device tested before and after, then earth by closing earthing switches or installing earthing sets on all directions, and finally define the work area with barriers and signs separating it from adjacent live parts.
Relying solely on opening the circuit breaker without visible isolation by disconnectors and earthing. The breaker may be re-closed by mistake or remotely via SCADA — safe isolation is based on visible disconnection + lockout + earthing, not on the changeable status of a breaker.
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