haze10 wrote:
How could it have been the main? The lights were on till the fault so the main was closed. It looked as if the tech was racking out or in the breaker. If he were racking out the main he would have lost his lights. Guess it could have been a double ended sub. The other possibility is that he was on a branch breaker, and the Main took a while to clear. Mains normally don't have Instantenous functions and their short time delay is usually at the highest setting, plus you can often add delay time to the main with dip switches. The other thing I find often is that all the breakers are set too high. People don't realize how much impedence the utility adds for finite bus calculations. Historically, if we did do a correct fault study, we would set the breakers to react to expected 'bolted' fault current levels. Arcing Current is often 50% to 70% of bolted fault. So an arcing fault may be low enough to keep the breaker in its long time delay portion of the curve.
Most plants feed the substation lights from a different part of the system so that dosent happen. The setting s that you find are always high are probally from the electricians turning them up after a breaker trips, some think thats what was wrong "The dang breaker was set too low"
