Mission Brief

The Isolation You Bet Your Life On

A new 145 kV disconnector and its earth switch sit between the breaker and the line on Bay 5. Nobody interrupts fault current with it — its whole job is to make a gap you can see and an earth you can trust before people work downstream. So it has to close with low-resistance contacts, drive fully home on a weak supply, tell the truth through its aux switch — and, above everything, it must be impossible to operate on load. Prove all of it before the bay is energized.

• Measure main-contact resistance with a micro-ohmmeter (DLRO)
• Confirm the operation time and the 89a/89b aux-switch changeover
• Verify the interlock — it must not operate on load
• Check insulation across the open isolating gap
• Confirm the motor minimum operating voltage and issue the record