Safely discharge HV feeder using load break disconnect switch (procedure)
High voltage feeder cables are capable of storing dangerous capacitive charges depending on various factors such as length and environmental conditions.
During a recent maintenance outage, our crew was tasked with servicing four 35kV 600A load break air switches (with transformers) at a rather large complex, all fed off a single 35kV feeder (~2500') with T-splices coming from the main switchgear located on the other side of the facility.
This scenario instantly reminded me of the time when a co-worker was hospitalized after grounding out a similar feeder that was still holding a significant charge.
Thankfully, he was not seriously injured, but he did experience a great deal of pain that day, and his story serves as the perfect example of why we chose to use the following procedure before doing any work on this freshly de-energized feeder.
Step 1.) Open the switch - Always a good place to start. Circuits are considered energized until grounded. Reference NFPA 70E to determine which level of PPE is adequate.
Step 2.) Use a proximity voltage tester to check for dead - I like to use the most sensitive setting (ex. 240V) on high voltage equipment because it allows for greater distance, but the tester can be set off from other voltage sources nearby. I work up in scale to filter out any interference, careful not to exceed the setting for the operating voltage (in this case 35kV) and always use the three point testing method (live-dead-live) to verify the voltage detector is working correctly. Check the load side first and then the line side.
Step 3.) Install properly rated safety grounds on load side, above the fuses to avoid short circuits that could potentially blow a fuse. Keeping mindful of what is connected to the load side of the switch. If it's connected to a transformer, for example, the windings could be holding a small charge. We use a hot stick with attachments for installing protective grounds and safely tap out any secondary charge below the fuse before attaching the ground above it.
Step 4.) Close the switch. The feeder is now grounded. In our case, we did observe an arc bright enough to light up the switch observation window.
Step 5.) With the switch closed, install a second set of grounds on the line side. The switch can now be opened and it will be grounded on both sides allowing for you to safely do you work. You can also move or remove the load side grounds depending on your situation. In our scenario the transformer secondary main was racked out and locked out, so we decided to remove them.
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