We had a failure at work due to an arc jump on all three phases 9 inches. Part of my looking into the cause, I wanted to guestimate the amount of voltage required to jump those 9 inches. It may reveal nothing but then again, it might. Lots of things are being tested and so far no answer. The two theories we have at the moment is inductive kickback or over-excitation.The bus-work normally has 13.8kV on it which could not possibly ionize the air to bridge this gap.The day was pretty dry, 60% humidity.Anyway, I don‘t understand the pin-point arc needing less voltage than some other spread out arc. Say it took a million volts to jump these 9 inches. does 1000kV not ionize the same amount of air whether it‘s pin pointed or not?Isn‘t it a potential sitting on some metal and if that metal is 5 feet in length or 3/8, it‘s still the same potential. Why would the ionization be different?Thanks
Fordman was correct in his answer, use 60% front and 40% rear, this saves a lot of wear on the rear brakes and tire. When coming to a stop, let off the throttle, pull the clutch lever towards you and push down on the shifter peg one click and release pressure on it. From there you can either release the clutch slowly and let the engine help slow you down or after a few seconds push the shifter peg down again and release pressure on it, to the next lower gear. This is called down shifting and takes a little practice to coordinate. You do not have to release the clutch each time you shift but you should be in 1st gear before coming to a complete stop. Make sure you have the clutch handle towards you when stopped or the engine will die. When using the brakes, use the least amount of pressure possible but enough to make the stop. I have an HD Ultra Trike and I have come to a complete stop using the front brake only, at about 25mph, so don't be afraid to use it. Hope this helpsdesertrat
As a general rule of thumb, air is a good insulator at about 10kV/inch dielectric strength. 9 inches then would be goo for a nominal 90 kV. This is not however an exact value but a general rule. Many things may affect this value. At 13.8kV phase to phase and 7.97kV to ground you should have been sufficiently insulated. Having said that, an image of the failure area and point of failure would be helpful. Also, a point end say to a switch or sharp point sticking out from a high voltage source creates a dense electric field around the point, i.e., the field intensity becomes high enough that in some cases you may exceed the 10kV/inch dielectric strength of the air. In this case you will generate a corona discharge or ionization of the air. If this is happening, it may not cause a flash over instantly but it will cause the build up of contamination across nearby insulators. This in turn reduces the dielectric strength of the insulator and will eventually flash-over to ground. Also, if you have a corona condition and you also experience the operation of a vacuum or SF-6 gas circuit breaker, for short line faults, you may be experiencing and additional over-voltage due to transient recovery voltage. Was the fault on the line side of a circuit breaker? In a normal substation configuration with most of today's designs, corona discharge is a small possibility. I would suggest you look closely for cracked or contaminated insulation. What was near by the failure location? Any CTs with insulating tubes (for higher BIL ratings) nearby? As to your last statement, Isn't it a potential sitting on some metal and if that metal is 5 feet in length or 3/8, it's still the same potential. Why would the ionization be different?. Shape and size have a direct impact on the electric field intensity and thus the ionization at that point in the circuit. I would like to see the problem area if you could e-mail it to me I may be better able to help you. Hope this helped, Newton1Law
The most basic thing to know is 'set up and squeeze', in other words, dont just grab a handful of the front brake and pull the lever in instantly, ot you'll lock the wheel, and possibly propel yourelf over the front. A setup, where you take up the slack in the lever and a gentle but firm squeeze on the lever will stop you just as fast. The secret here os to practice, where you will come to no harm (an empty car park, for instance). If you haven't already done so, you should go to a rider learner school (compulsory in Australia). As other riders have noted, braking is mostly with the front brake; except for slow speed riding (u-turns, roundabouts, etc), where the rear brake is used so that the front wheel is free to turn. With a little practice, you will be able to come to a quick stop from 50mph without skidding. Build up to that speed from, say 30. It is MOST important to be able to come to a stop safely in traffic. As one writer put it it's OK to ride at 100 mph; providing you've practiced coming to an emergency stop from that speed Another important thing to know is not to grab a handfull of front brake, if you find you are going too fast into a corner. There will be only one result - you'll go straight across the corner; possibly off the side of the road, or into oncoming traffic. So it is important to praxctice cornering technique as well as braking. Finally, remember that the bike will go where you are looking, and if you fix your sight on something (a ditch or tree, for instance), that is where the bike will head (this is known as target fixation).
Corners and points increase the local potential concentration (think of water swirling around a corner vs a nice curved path) so it is far more likely for an arc to start at a corner and any bug or moisture (as mentioned) could cut the air insulation way down. And you consider 60% low humidity?
I am thinking a large insect or a swarm of small insects flew into that space and thus reduced the ionization requirements for an arc. Or maybe a small bird. If the critters had landed on the ground bar they would have reduced the gap between the ground and all the bus bars.
