Author Topic: Interesting and sad case of electrical injury  (Read 4173 times)

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Offline jitter

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Re: Interesting and sad case of electrical injury
« Reply #75 on: May 27, 2024, 06:00:27 am »
I have seen a Fluke 77 that was returned "under warranty" because the arctrician tried to measure the primary side of a 11kV distribution transformer, and  just grabbed the meter, and a long stick ( some thought there) to hold the probe near the connection. Inside the case was copper plated all the way down from display to input jacks, and there was no more copper on the board, other than the copper by components under the soldering. All blown off the soldermask, leaving bare board behind. Meter did not power on at all, the Fluke ASIC was there, missing the top of the package, and the thick film resistor network was cracked, and the resistors in it had marks of flash over on them for the input side one. Fuses were intact, along with the 9V battery, and the display was still working, put into another meter to verify, as that one was there because they had broken the display. Meter was scrapped, and the broken leads were also cut to pieces. New meter sent, along with the bill. I was still in school, visiting the one place that was a RS distributor (when RS catalogues were still the only way to get data sheets or more exotic IC's, and a 2 week wait for them to arrive) and which is still running these days.

By me the distribution transformer (200kVA, is 50m away, 300m cable wise, and the cable itself probably dates from 1900 in places, being the original paper insulated cable, and the feed to me is only around 60 years old, well worn SWA paper insulated and oil filled, complete with lead wiped ends. 400A fuses by the substation, a standard size, though there are still a good number with 200A fuses, that date from the 1950's, still in use, only replaced when they blow, and they will not blow for 400A of load as evening peak, measured that one evening.

Well, that confirms my expectations what will happen when a DMM is subjected to high energy high voltage supplies like the MV part of the grid is. I don't think even a modern day Fluke will behave differently, the creepage distances inside are simply too short.

Off topic: that paper insulated cable, does it look anything like the picture below? If so, then they might not be as old as you think. Even though they have been superseded by XPLE cables, over here, production was stopped as late as 2005.



For hose who don't read Dutch, from top to bottom it says: copper cunductor, conductor isolation, common isolation, lead ply, burlap ply, double iron ply, double burlap ply.
The conductor isolation is made from paper and impregnated with oil. The burlap layers are impregnated with oil, wax and tar. At the termination point, these cables stick out of the soil vertically into the station and and are terminated in enclosures filled with oil that should be kept topped up.
The outer layer of the cable also contains chalk, but has mostly rotted away by now. It's estimated our grid still has about 50% of this old style cable in it and continues to be in service just fine. Medium voltage cables in this style can also be found with aluminium conductors instead of copper.
« Last Edit: May 27, 2024, 06:28:56 am by jitter »
 

Offline jitter

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Re: Interesting and sad case of electrical injury
« Reply #76 on: May 27, 2024, 06:18:39 am »

Not sure why you thought to add to the conversation.

Because you claimed to have measured more than 10X higher at your parents home. 

And why not?

Sorry, why not what?

The answer to that question was given in the same post (please quote it in its entirety, not just this question), 5 kA of short circuit current is perfectly possible in a residential situation but depends on the impedance of the grid at the connection. A 400 V threephase service, which almost everyone has over here by default, is capable of 10 kA short circuit current when there's only 23 milliOhms of impedance between the transformer in the station and the distribution board in the house. Obviously, the house wiring adds impedance, and depending on wire diameters and lengths, this will lower short circuit current some more.

At the other extreme, 283 milliOhms are allowed (at the board) for a 3 x 25 A service which lowers the short circuit current to 0.816 kA. This must not be any lower otherwise protective devices may react too slowly or not at all.
« Last Edit: May 27, 2024, 06:25:36 am by jitter »
 
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Offline SeanB

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Re: Interesting and sad case of electrical injury
« Reply #77 on: May 27, 2024, 10:01:35 am »

Off topic: that paper insulated cable, does it look anything like the picture below? If so, then they might not be as old as you think. Even though they have been superseded by XPLE cables, over here, production was stopped as late as 2005.



For hose who don't read Dutch, from top to bottom it says: copper cunductor, conductor isolation, common isolation, lead ply, burlap ply, double iron ply, double burlap ply.
The conductor isolation is made from paper and impregnated with oil. The burlap layers are impregnated with oil, wax and tar. At the termination point, these cables stick out of the soil vertically into the station and and are terminated in enclosures filled with oil that should be kept topped up.
The outer layer of the cable also contains chalk, but has mostly rotted away by now. It's estimated our grid still has about 50% of this old style cable in it and continues to be in service just fine. Medium voltage cables in this style can also be found with aluminium conductors instead of copper.

Yes, but those I do know the install date, because they are the original ones installed for the long removed tramway, and the one for the building is the original installed 70 years ago when it was built. Though it is very likely the cable buried underground no longer has any of the steel strap wrap left on it, and there is a good chance the lead is the only thing keeping the water out of the cable Just like the paper insulated phone cables, also around a century old, and likely the 200 pair bundle is now down to around 50 usable pairs, though of course finding any place that actually still uses copper phone service is getting rare, as the encumbent telco is officially no longer doing cable repairs, and has retrenched all the tech staff in that division, and they will simply sell you a mobile phone and port the number, "for free", once you sign a 36 month contract for it. If you had DSL tough, unless it was with them as well, in which place you also sign an additional 36 month data contract, with caps on it.
 

Offline joeqsmith

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Re: Interesting and sad case of electrical injury
« Reply #78 on: May 27, 2024, 01:48:04 pm »

Not sure why you thought to add to the conversation.

Because you claimed to have measured more than 10X higher at your parents home. 

And why not?

Sorry, why not what?

The answer to that question was given in the same post (please quote it in its entirety, not just this question),...

You are trying to ask me a question but are not sure how to ask it, and you already have an answer?   :-// 

Offline Kim Christensen

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Re: Interesting and sad case of electrical injury
« Reply #79 on: May 27, 2024, 02:49:32 pm »
For hose who don't read Dutch, from top to bottom it says: copper cunductor, conductor isolation, common isolation, lead ply, burlap ply, double iron ply, double burlap ply.

I can't read Dutch, but in this case I prefer the Dutch words to the English ones.  :D
 


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