Direction of travel of an Arc across 2 distribution lines

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well at least it looks like i'm crapping in reasonable company...that's something i guess :(

ok..understood..

ok

ok

ok

agreed

ahhh ...ok...so they will in fact lag behind...but my reasoning was wrong. i forgot about the emmisivity with heat factor.

yup. live and learn,,,thx

now i hav to think whether or not i should do a revised presentation to my section....

bummer! :(

regards

Daniel

Reply to
Daniel
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We are wandering somewhat off track now, and if we continue we probably ought to change the subject line.

The essential thing to remember about interrupting dc is that there is no natural current zero, There are then 4 possible approaches :-

(a) Use a tuned circuit to produce a current zero. This is theoretically possible, but I can not quickly think of a practical example. (b) Stretch the arc so that the arc voltage is higher than the supply voltage. (c) Cool the arc so that the arc voltage is higher than the supply voltage

(d) Split the arc into several arcs in series so that the total arc voltage is greater than the supply voltage because of the extra anode and cathode voltage drops. In practice you can use a mixture of these techniques, but as been said the higher the voltage the more difficult it becomes to make a compact circuit-breaker.

John

Reply to
John Rye

I recently replaced an old (original to the house) light switch. It was rated for use at 120V AC/DC. I don't remember the exact ratings, but the DC current rating was less than the AC current rating. It had a definite Snap when switched.

Reply to
Michael Moroney

I recently replaced an old (original to the house) light switch. It was rated for use at 120V AC/DC. I don't remember the exact ratings, but the DC current rating was less than the AC current rating. It had a definite Snap when switched.

The old ones did better than the new ones - that is the cost of silence.However, the old switches were definitely downrated for 120VDC

Don Kelly cross out to reply

Reply to
Don Kelly

We are wandering somewhat off track now, and if we continue we probably ought to change the subject line.

The essential thing to remember about interrupting dc is that there is no natural current zero, There are then 4 possible approaches :-

(a) Use a tuned circuit to produce a current zero. This is theoretically possible, but I can not quickly think of a practical example.

(b) Stretch the arc so that the arc voltage is higher than the supply voltage.

(c) Cool the arc so that the arc voltage is higher than the supply voltage

(d) Split the arc into several arcs in series so that the total arc voltage is greater than the supply voltage because of the extra anode and cathode voltage drops. In practice you can use a mixture of these techniques, but as been said the higher the voltage the more difficult it becomes to make a compact circuit-breaker.

John ..................................................................................... (b),( c) don't work with HV AC simply because the space needed is so large. After all, an uncontrolled arc at 240KV can flare for a length of the order of 40-50 ft, judging by movies of faults on lines. Hence even in the early days work breakers would have short gaps in oil, trying to get enough pressure to sweep the arc away at current zero. Later this was improved upon (in England) by use of "explosion pots" and the bulk oil breaker probably peaked with a Westinghouse breaker (De-Ion?) where the pressure forced transverse flow of oil across the arc path. The history of circuit breakers is quite fascinating. Air blast breakers pretty well put paid to bulk oil breakers in the 50's. and these were modular bringing in your point (d) Typically a 69KV breaker had 2 gaps of the order of 1 inch in series and used a blast of 600psi air (which at heavy currents wasn't going to break the active arc but could blow out the arc products and replace them by clean air at the current zero. Modern breakers still depend on this whether air, minimum oil or SF6 etc.

As for (a) this, with an AC breaker, has been done with some success on HVDC systems. I am not up to date on progress but it appears that it is easier to simply turn off the rectifier inverters on both side of the fault- hence shutting down the line at the supply current zero. check:

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There have been other ones than what is cited. 100ms is pretty slow by today's standard. This one is a thyrister based unit at lower voltages.

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Don Kelly cross out to reply

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Reply to
Don Kelly

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