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#216267 - 11/18/15 01:18 PM short circuit??
annemarie1 Offline
Member

Registered: 03/16/15
Posts: 49
Loc: england
I'm sure like me you all think if you put a short across the mains you will get a big flash and a bang and end up with a blow circuit fuse. I saw a video on YouTube where 2 Ukrainian guys apparently shorted out the mains to there shack and although the light went out and you can here the meter growling and buzzing as soon as the short is removed the supply returns as normal so my question is how did they do it and secondly am I aloud to put the name of the video on here not sure about forum rules for this

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#216268 - 11/18/15 03:30 PM Re: short circuit?? [Re: annemarie1]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
I suppose this could happen on a close to terrible supply with extremely high impedance, probably combined with overfusing. The short draws enough current to pull the voltage almost down to zero but doesn't blow any fuses because the short circuit current is never reached. If that shack is say on a long run of 6 mm2 aluminium the max. short circuit current could well be as low as 100 A and that's decidedly not enough to blow something like a 20 or 25 A fuse.

I'm not aware of any forum rules that'd keep you from linking to such a video.

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#216270 - 11/18/15 06:02 PM Re: short circuit?? [Re: annemarie1]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
It's obvious that the available current wasn't enough to clear any protective device(s) OR that short wasn't as much of a short as you were led to believe. You will have to look at the time/current curves for the fuses to see when they should clear. One of my pet peeves is looking at a set of curves that doesn't show the time that the fuse should clear at 105%. I also just recently saw a set where several of the curves crossed.

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#216271 - 11/18/15 06:45 PM Re: short circuit?? [Re: annemarie1]
annemarie1 Offline
Member

Registered: 03/16/15
Posts: 49
Loc: england
The video is at YouTube kreosann look for one called we don't need fireworks we have electricity you may need to try different versions of the same videos as some have English subtitles and speach some only ukranian or russian

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#216275 - 11/19/15 05:22 AM Re: short circuit?? [Re: annemarie1]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
I don't think the lights actually dim that much! The arc flash is so bright that the camera doesn't even register that tiny wall light any more. That's a normal effect with automatic exposure controls.

The fuses are likely gL Diazed but without knowing how many amps it's pointless to speculate. What I can tell you is that gL and gG fuses are fairly slow. Depending on their size they're supposed to trip at 1.6 to 2.1x rated current within an hour! The overload factor is larger with smaller fuses, 1.6 is only from 20 A up I think.

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#216277 - 11/19/15 11:06 AM Re: short circuit?? [Re: annemarie1]
annemarie1 Offline
Member

Registered: 03/16/15
Posts: 49
Loc: england
Thanks Texas ranger I'm guessing that with a combination of a slow ish fuse a high impedance supply you can put a dead short on the mains and it will not blow the fuse I've never come across this situation in real life but I'm guessing if you have a loop impedance of 2 ohms a supply of 220 volts that's 110 amps and a 60 amp fuse you'd get away with it although I thing anyone else on the same phase is going to be a bit annoyed with you

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#216278 - 11/19/15 06:17 PM Re: short circuit?? [Re: annemarie1]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
If he was across service conductors, it is very possible you would never operate an over current device. You are counting on the input to the primary of the transformer. I have seen some very interesting results from faults in service conductors.
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#216283 - 11/20/15 08:31 AM Re: short circuit?? [Re: annemarie1]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
Don't count on such a big supply! I'd vote for 25 A single phase 2-wire max. or maybe 3 ph (the meter looks too small for that though). I suppose they could be on something as bad as a kilometre of 10 mm2 Al wire (the Italians do 6 mm Cu overhead wiring TODAY, feeding entire apartment complexes!).

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#216286 - 11/20/15 10:22 AM Re: short circuit?? [Re: annemarie1]
annemarie1 Offline
Member

Registered: 03/16/15
Posts: 49
Loc: england
I saw in another of there videos that the little shack is on single phase you can see a 2 wire drop from the 3 phase line running nearby. I suppose a 25 amp feed would be OK for a little shack but I think it would be a struggle for a whole house. I'm surprised the Italians still do such things I can imagine the lights dimming every time you put the coffee machine on!

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#216292 - 11/21/15 06:59 AM Re: short circuit?? [Re: annemarie1]
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
The Italians will happily put you on a 15 A single phase supply regardless of the size of your house! And yes, they've got serious voltage drop issues there! I remember my parents' Sony stereo playing cassettes slower and slower and finally shutting off!

You wouldn't believe with how little power you can actually get by!

A German spark who eventually moved to Italy told me the story of the whole new housing estate. He ran something like 95 mm2 because the line was longish. Then came ENEL and installed a full kilometre of 6 mm2 overhead wire! He was a bit peeved I'd say laugh

15 years ago I spent an exchange week in a 1970-ish flat in Torino. One evening my host mom had dinner in the oven and my host sister went to take a shower while I was sitting in my bedroom reading. Suddenly the whole place went dark because the combination of electric oven and water heater were too much for the 15 A mains! There were no individual MCBs or fuses, just a main 15 A RCBO built into the meter base!

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