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When electrics go wrong!!!

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Darkwood

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Just setting up a new sticky thread that unlike the dodgy trade pics thread is focused on showing the result of failures in electrical installations IE - damage caused which may be due to various reasons. Please do not post pics of poor workmanship if there is nothing else to show like the consequences, use the dodgy pics thread for that.

I will start off below with an example. I am hoping the fresh faced members and inexperienced can get a visual incite of what to expect when you come across such things and how to understand what has occured.
 
May I add this one (I had the offending sink waste disposal replaced by a plumber but I kept the old one).

IMG_6979.jpg


There was a big bang and a breaker tripped. The tenant found this blackened socket. Where did all this black deposit come from? The molded plug looks undamaged - the 13A fuse was fine - can anyone explain what physically blew up here? I'm just interested.
I plugged the unit into an extension cable in the back garden and it blew the fuse in the extension cable (and left the same black deposit). The disposal was free to rotate so I don't know where the problem was.
 
May I add this one (I had the offending sink waste disposal replaced by a plumber but I kept the old one).

View attachment 97391

There was a big bang and a breaker tripped. The tenant found this blackened socket. Where did all this black deposit come from? The molded plug looks undamaged - the 13A fuse was fine - can anyone explain what physically blew up here? I'm just interested.
I plugged the unit into an extension cable in the back garden and it blew the fuse in the extension cable (and left the same black deposit). The disposal was free to rotate so I don't know where the problem was.
Classic dead short between live-neutral or live-earth. The fault is nothing to do with the motor, so its spinning freely.

The black deposit is powdered charred metal. If you can visually inspect the plug pins with a good plug, youll see damage on the pins. The metal has laterally exploded into powder. If not the pins, then the socket youre plugging into.

Best place for that appliance now is the council dump.
 
Classic dead short between live-neutral or live-earth. The fault is nothing to do with the motor, so its spinning freely.

The black deposit is powdered charred metal. If you can visually inspect the plug pins with a good plug, youll see damage on the pins. The metal has laterally exploded into powder. If not the pins, then the socket youre plugging into.

Best place for that appliance now is the council dump.
Thanks for replying.

I had a look at the plug pins before I threw it all out - they looked ok which confused me (easily done...). I was hoping to see the black insulation that covers about half of the pin vaporised, but it looked OK. Although the 13A fuse was intact (electrically/physically), the "live end" connector in the plug that holds the fuse was much blacker than the other end. I thought the fuse should have protected something, but it looks like the short was somehow before the fuse. The disposal had been working OK for about 8 years and then "bang"!
 
My guess would be that a build up of dirt and damp eventually created a track which allowed an arc to form. Normally the distance is such that even when there's lightning induces surges about, you won't get arcing between L & N. But add dirt and moisture, and perhaps something sub-standard in manufacture, and you can get arcing. Once an arc forms, then the ionised air is a good conductor and the arc won't go out until either the power is cut off (the upstream fuse/breaker goes) or the arc is manipulated in such a manner that it gets broken. The energy in the arc can be "a surprising amount", and in all that black there is likely to be as least some it conductive material - so I'd be replacing the socket as well because it definitely doesn't have the arc resistance that it once had.
 
My guess would be that a build up of dirt and damp eventually created a track which allowed an arc to form. Normally the distance is such that even when there's lightning induces surges about, you won't get arcing between L & N. But add dirt and moisture, and perhaps something sub-standard in manufacture, and you can get arcing. Once an arc forms, then the ionised air is a good conductor and the arc won't go out until either the power is cut off (the upstream fuse/breaker goes) or the arc is manipulated in such a manner that it gets broken. The energy in the arc can be "a surprising amount", and in all that black there is likely to be as least some it conductive material - so I'd be replacing the socket as well because it definitely doesn't have the arc resistance that it once had.
Thanks.
The socket is under a sink and although there was no sign of damp/leaks it could have got wet. I'm annoyed I didn't measure L-E resistance etc. of the disposal when I had the chance.
Here's another picture of the extension block I plugged the disposal in to see what would happen - note the burned groove to the left of the N hole and the edge of the extension is burned off and there's a nice image of where the fuse holder fits! (Fuse didn't blow). Powerful stuff.

IMG_0196.jpg
 
MRV2.jpg
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here's the result of a numpty damaging a brand new voltage stabilizer by getting the 3 phase connections on a generator wrong, and its all marked R,S,T,N. Connected a phase output line to the neutral and neutral to a pahse so it put two phases through the master rotavolt controller and left the incoming utility switched on while testing the changeover. By the time a replacement is shipped from UK looking at around 5K. Also took out the 400amp Auto changeover switch, another 3k to replace. Lucky not to lose the generator as well as it shut down within about 5 seconds of power being turned on. Thankfully the main switch in the MDB was off it could have run in to hundreds of thousands to repalce printers, photocopiers, and the data centre.
Lets just say he's not working for us anymore and will not be getting references.
 
IMG_2511 edited.jpg
IMG_2511 edited.jpg

Thought I would share this burnt-out Inverter that I discovered. The customer wondered why it had stopped generating!
This was caused by water ingress & water pooling in the bottom causing a short between the grid connection 3 phases & which must have short-circuited the DC MC4 input before finally giving up. Good ole Samil Inverter 17kw 3 phase, no wonder they went bust. (Chinese) . I think I am going to give Rayliegh Instruments a try from CEF, I would prefer Fronious but there is a back order, due in sept maybe.
 
Had been running like this for 3 years - we got called in as they never got a cert off the original electrician and he told them to stop pestering him a few months back.
When put back correctly we found many isues that caused the RCD's to trip - shared neutrals, pinched neutral to back box in two seperate postions, switch fuse spur with neutral and earth swapped.
 
Saw a safety alert recently in a large engineering business. Someone was doing some gouging*, cable was a bit short to reach where he was working, and it seems the cable wasn't secure in the trailing socket. Socket comes off leaving exposed wire ends which then falls ...
... onto the gas axe (oxy-acetylene cutting torch) he was also using on the job.
The ensuing arcing from the gouging supply cut the gas axe in half - luckily in the bit between valves and torch head. Had it dropped on the valves then it would have released copious quantities of oxygen and/or acetylene in the presence of a 'kin big arc.

What I did find interesting was that the alert didn't mention the fact that the socket shouldn't have come off the cable - only to pull the cable by the cable rather than the connector.

* For those who haven't seen it, gouging involves striking up a 'kin big arc (very much like arc welding) to melt the metal, and using a jet of compressed air to blow the molten metal out. It's a very effective way of cutting thick steel - though to say it lacks finesse would be an understatement. Example
 

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