On Wed, 24 Nov 2004 20:13:21 -0900 Gerald Newton3 wrote:
| 480 volt is deadly when it comes to arc flash. After working for six months | at a 24 year old industrial facility that had a lot of distribution at 480 | volts, I would not assume any thing as safe anymore. Once I turned on a | 480 volt three phase 70 ampere main breaker and got the shock of a life | time. The plastic breaker handle cover had gotten broken and only the metal | part was left. There was 277 volts to ground on the metal. I was wet and | had wet gloves on. At this same facility cables had been stretched very | tight in 1982 when a 120 pound per square foot snow load broke down the | cable tray support structures leaving the cables taut. The cable were never | replaced. You never know what took place in the past. Do not assume that | anything is safe in the electrical world!!! While working next to a | journeyman with over 30 years experience I witnessed his screw driver | exploding while he was in the bucket. The 400 ampere 480 volt breaker that | he had turned off was still hot on the load side!!! The breaker was | defective. A journeyman I know was working some cables in the back of a | distribution center in that was hot. One of the existing cables came out of | its terminal and cross phased. It blew him several feet and burned him | badly. This stuff is dangerous. I know a person who had a bad experience | with arc flash who now never ever stands in front of 480 volt equipment when | he turns it on. He turns it on with a stick while he stands to one side. | Talk about arc flash shy!
I'd agree 480 volts can take the jump across a wider gap that 208 or 240 volts might not. But for a given kVA or kW capacity, the higher voltage system is going to have a higher impedance, reducing fault current capacity. So I would think the arc blast would actually be less due to more of the power being dissipated in smaller wires and windings.
But ... 480 volts also tends to be present when the kVA or kW capacity needs to be higher, or even a lot higher. If you had to decide between 240 and
480 volts for a 300 kW industrial oven, I'm sure you'd go with the 480 volts (if not nore).
I'm not trying to say 480 volts is not as dangerous as you're saying. What I want to say is that 240 volts or even 120 volts can be awfully dangerous itself when high fault currents are available. In a 1500 kVA system, which has the higher current?
My uncle used to do welding with a dozen 2 volt recycled telco lead-acid cells wired in parallel. He had some whopper current there that took days to recharge (he was doing that with solar) and minutes to discharge.