200 volts DC leccy in the Meadows.


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Am I correct in thinking that DC mains was far more dangerous to humans in the form of electric shock, than AC?

Col

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Phew what........i should cocoa..............

Hi mick2me I'll get my dad to reply to that one he should be getting up around now I think they are about 8 hrs behind us in Vancouver

Ah yes Miss Slack, she taught me to read in one year, I couldn't read up till then because of my disrupted schooling with all the moving around I did. I got a prize for progress at the end of the year

I've heard/read the same thing DJ. Think it's sumat to do with the DC makes your grip tighter so harder to let go, whereas AC tends to throw you off on account of it reversing it's polarity 50 (60 in USA) times a second.

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Take it from me, AC doesn't always throw you off!! I was once feeding a bare wire, which was going to be an earth wire, behind a fuse board inside it's receptacle, it caught one of the phases, if I hadn't started to black out I'd have been dead, I was the perfect path, up one arm and down the other plus though my body to my feet.

Yeah I can see where the circuits were completed now Dave. With the quick connects you have other outlets paralleled causing problems. I changed two in an office that also fed a flat, the renter complained some of her outlets weren't working, the one in the office was pretty hot to say the least, when I pulled it it was charred on the back, on it's way to catching fire. That office burned down a few years back with the flat, was rumoured the owner had set fire to it, my guess was one of the other outlets was the cause.

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Yep! There can be a lot of amps flowing through an unused outlet if something heavy is plugged in further down the line.

I had heard this thing about DC being more dangerous, but I have had little experience with DC. I've seen plenty of instances of AC being pretty dangerous under the right conditions. :-)

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You should see the inside of a flameproof chamber of an 11Kv circuit breaker after it ionized, not a pretty sight. I rebuilt a Joy cutter control chamber once after the two contactors had vaporized! The contactors were reversing contactors, operated a 90HP 1,1Kv motor on the cutter. Contactors were mechanically and electrically interlocked. There was not enough left to diagnose why it had happened..LOL

My DC was limited to overhauling DC motors and testing them, battery charging and a little time on large HP DC winders that operated on 6Kv, very limited, regardless, you wouldn't want to test any theory of shock with 6KV!! I found 11KV very scary too, very little chance of surviving the experience, so was ultra careful when we had to work on circuits at that voltage. As Leading Hand, (Chargehand) I was authorized to isolate 11KV to work on, or for my elecs to work on. A case of check isolation procedure at least THREE times, have another elec check too, as well as earth the circuit to work on, then before going near bare terminals check with a none contact voltage detector, like the "EZScan". There are no chances for the foolish on HV.

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I remember when I was made up to leading hand, and the Eng was going through everything, he gave me a key to the switchyard control room, after going through the equipment in the control room, he took me into the switchyard, voltage 66Kv, first thing he stressed "DON'T under any circumstances, stray from the marked white path, NEVER step over the lines EVER"!! 66Kv can jump a fair distance.

There's some video's on Youtube on maintenance of 500Kv overhead lines, they rarely isolate them and the crews are taken up to the lines by helicopter. The linesmen then clip their safety harness onto the lines and crawl between to lines. Not my idea of making a living, but I bet they get about $100 an hour for that job!!

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Used to scare me climbing a 40 foot tower to install antennas on it, and that was my tower... 100 foot, forget it.

Why would they use high voltage DC for long distance transmission of power??? I thought it was settled science polyphase AC was the most economic and sensible route to take?

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A fall from 40 ft will break your neck just as sure as from a 100. :-)

As regards the DC thing. 500kv would sure seem iffy to me. What if an arc got started at that voltage? Whew! Wouldn't want to be anywhere near it if that happened. How would you transform it down to a usable voltage once you get it to a city? I really don't see much in the way of benefits, but I'm not an electrical engineer.

In home use it might have advantages. All electronics need dc. heating elements don't care, motors would be an issue. Doubt if I'll see it in my lifetime.

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You can get some good arcs with AC too Dave, have a look at some of the HV AC arcs on Youtube, some of them are hilarious, Although I wouldn't want to be near them, very destructive too. I saw the effect of 11Kv when it formed a plasma inside of an FLP chamber. How it started nobody knew, not much evidence left. The circuit breaker was destroyed and there was half inch of black soot on the inside walls of the chamber.

But dropping 500Kv DC down to usable voltages sounds like an engineers nightmare, I think Steve typed a typo and it should have read AC. The only way I know of reducing DC voltages and producing AC is via motor generator sets, and I doubt there is a DC motor made that could withstand 500Kv.

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# 86 Ayup

DC at 500KV not a typo, a long DC HV line is in Western USA supplying LA. The France - UK interconnector cables are HV DC because the french & UK grids are not synchronised so can't use AC to connect them. Some HV DC lines even use Single Wire Earth Return. (SWER) In Japan they use them back to back to connect the 50Hz parts of the country to the 60Hz parts. They use thyristers & IGBT's gubbins to rectify & invert the AC to DC & vice versa. All very clever stuff..

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I did find out from an engineers answer to the same question a little earlier, his opinion to the question was banks and banks of HV SCR's. I'd hate to be around when just one of them failed, would make bonfire night look like a small BBQ...LOL

SCR's were used for speed control of both Boulby's DC winding motors, both operated on 6000V DC. I see there are HV DC brushless motors available, but only low HP at the present time.

I can see where the problem of synchronizing one countries frequency to another would be a problem, not quite as easy as bringing an alternator up to speed and syncing it to the grid.

Like I said, just one failing could be catastrophic and probably 90% of the UK without power. SCR's are just another electronic device that can and do fail, but when you have banks of them at half a million volts....Yowza....LOL

The main north south feeder from Washington state to Southern Cal is AC, it's 500Kv, it did experience a major problem a few years back, a tower failed, bringing many miles of line down and caused hundreds of towers to collapse, must have been spectacular for those that witnessed it. The damaged section was isolated and made safe while crews moved in to clear up the mess, and "plant" new towers and line. Meantime it was bypassed through feeder circuits to keep power on in Southern Cal.

I thought Japan was officially a 50Hz country, seems odd having two different frequencies, makes for a difficult life for power generators.

Oddly enough, single wire earth return systems are common in Australia, I often wondered what the voltage drop was like in mid summer in rural areas at the end of a supply run. My cousin lives in Victoria, his pole mounted transformer is single line fed, neutral is connected to a grounding rod at the base of the pole.

Around here they use two wire, top is 7400V, bottom is neutral return, grounded at EVERY pole to ensure a low impedance to lightning, lines get hit pretty often.

I'd never heard of the other DC lines you mentioned, so can't comment on them.

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Two different line frequencies in the same country (japan). Sounds a bit dicy to me if anything still uses synchronous motors anymore.

We are on underground feeders here. I think the high side is 14kv. 3 phase. Neutral grounded at every house and presumably at the transformer pad. Each transformer feads four or five houses.

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Auto transformers are used on single and three phase rural long circuits Steve, no moving parts, one winding, auto tap changing is generally used at switchyards.

It's all overhead around here Dave, including all the small towns, not sure of the three phase line voltage, but could work it out, 7400 on the single phase, so must be somewhere in the region of 22Kv between phases.

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Well underground certainly didn't protect our transformer from a lighning strike last year. So I guess the odds of a strike are probably quite similar.

This thread has come a long way from the Medda's. :-)

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Costs plus aesthetics Dave, as you're aware, residential areas don't like overhead lines and pole mounted transformers, where as it would cost a fortune to bury cables in the ground, still costs around $400 a pole though to get power installed off the beaten path. Luckily where I live, there used to be a house many years ago, so all the poles were here, a couple needed replacing, as did the old 50 amp transformer. That lot didn't cost us anything, just a new consumer account charge.

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By a DC clock motor?? Started when the TV was powered up??

That would be an expensive TV, would be cheaper to rent a TV, renting and buying on the "tick" were the only ways I knew of getting a TV.

What years were the slot meter TV's around Steve???

The only slot meters that come to mind were the elec meter, gas meter and Dads mantle clock, took half a crown once a week to keep it running, paid for a life insurance.

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I think my Mum rented a TV from Wigfall's when she first had one, around 1956, then later she had Rediffusion, probably bought a colour one on the never never in the early 70's, I don't recall as I'd left the "nest" by then.

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On E post website that there's been two power cuts in the old Meadows area: I'm wondering if the new AC cables they laid 50 odd years ago have worn out? It says "people can't cook their Xmas din-dins." Back then most people had gas cookers but now most seem to be electric, wonder if the cables are getting overloaded as everyone turns on their electric hobs/ovens? When we have a power cut round here we end up boiling pans of water as we're the only ones who have a gas hob...

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