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#25 Reply
Posted by
GregDunn
on 29 Oct, 2018 17:03
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I didn't witness this, but it was a legend around the EE lab at school. One of the then-current professors had worked with 3-phase generators at a new power plant. These things were built like, bigger than, and probably stronger than, the proverbial tank; if you've seen one, you will get the picture. One of the jobs during new construction involved spinning up a generator and connecting it to the main buss to bring it online. In the olden days, synchronization was done by a set of 3 bulbs wired between the generator and the power buss, such that when the lights all went out there was no voltage differential and the generator could be safely switched in. It's not clear exactly how it happened, but the indicator was mis-wired such that it's display was 180° out of phase with the actual voltage. The operator carefully synced the lights, threw the switch, and...
The best description which came down of that event was that there was a hellacious screeching noise, and the generator ripped itself free of the floor mountings and rolled over. Most of the participants were fleeing in terror at that point, so further descriptions were vague.
One that I personally was involved with? OK. The school's power lab had a lot of very burly motor and generator equipment, which was used to train power engineers in 3-phase systems. A friend and I were hanging out before a class, and trying to make sense of some of the things we were learning. We had a heated discussion about the differences between 3-phase delta and wye systems, where the delta had a system neutral connection. He insisted that in the same distribution block, the neutrals of both systems came from a common source, should both be near ground potential and could be safely connected together (the power leads, of course, should be left isolated). I felt that was naïve, but couldn't refute his argument without further knowledge of how the secondaries were wired.
Sitting right next to us was the distribution panel for the lab's 3-phase power, with both a delta and a wye setup. I suggested, somewhat humorously, that if he was so sure he could just plug the neutrals together right there and prove it! He noted that there were circuit breakers for both systems - just run a big cable between the neutral sockets and flip the breaker, right? If there was current flow the breaker would pop and that would be it. So we switched off the breakers, ran the cable, and I insisted my friend be the one to flip the breakers on since he was SO certain.
I don't know exactly where the big BOOM came from (no, not from the lab where we were), but the lab and most of that wing of the building went dark. My friend, eyes as big as saucers, yanked the cable out and flung it into the box where the cables were stored. People came boiling out of the classrooms nearby, in a panic because the rooms had all lost power - and most of them had no windows. I thought we were going to be in big trouble, but thankfully no one seemed to connect us with the event. The professors were running around checking all the breaker panels, and finally found all the right ones to bring power back. It astonished both of us that a test panel, clearly used for high power experiments, affected so many essential circuits - I would have expected it to be split off near the incoming circuits out at the power drop. Perhaps the ground currents were so unbalanced that all the other breakers saw significant excess current?
There was no apparent damage, either, despite a significant number of circuits popping their breakers and lots of sensitive test equipment being run in the area. But no one who was in the EE department that day will forget the incident!
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#26 Reply
Posted by
Berni
on 29 Oct, 2018 19:20
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I was told in uni about really bad thing happening if you don't synchronize the generator before connecting it to the grid, but they ware never all that specific on what exactly would happen. I did imagine something mechanical would break and grind to a halt and fuses would blow, but i didn't think one of these massive generators would fling themselves across the room!
And yes we also managed to knock out the power on an entire hall in school when a schoolmate wired up some contactors for star and delta switching of a motor. There is the usual procedure of first powering it up with big powerful incandescent lamps in series with all phases so that a short circuit just lights them up. It passed that check but then it was tested with the lamps bypassed. Motor started up fine in star but then when it was switched into delta the contractors for star did not open while the delta contractors closed, effectively shorting all three phases in a bang while everything went dark. The short did trip the breakers in the classrooms distribution box as it should but it also tripped the bigger ones for the whole hall. This could be because there ware classrooms full of PCs in that hall that likley put a significant load on it while the fault current on top of it convinced the big breakers to trip too. The school had its own transformer substation inside the building the short cable runs provided lots of peak current capability in a fault.
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#27 Reply
Posted by
HerbTarlek
on 30 Oct, 2018 03:20
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I don't know exactly where the big BOOM came from
Petition to change the thread name to "I don't know exactly where the big BOOM came from: Test (and Other) Equipment Disasters!
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#28 Reply
Posted by
BradC
on 30 Oct, 2018 07:20
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The best description which came down of that event was that there was a hellacious screeching noise, and the generator ripped itself free of the floor mountings and rolled over. Most of the participants were fleeing in terror at that point, so further descriptions were vague.
I have a photo somewhere (that I have been persistently unable to find) that was given to me by a grizzled old gentleman of an event that occurred well before I was born. It was a beaut photo of a small power house with a generator sized hole in the wall and the remains of a generator and its turbine sitting on the ground outside. Reputedly another 180deg out of phase event.
That's 3 massively destructive ones I've heard of over the years, so they're probably not as uncommon as one might think. They do seem to get hushed up though.
My old man tells one when he was a junior power engineer. One of the guys in the class synced up the demo motor/generator set out of phase and knocked out the power for about 4 blocks in East Perth back in the 60's. Apparently damage to the generator and some upstream popped breakers, but nothing left the floor.
The worst (most expensive) I've done is poked 240V into the USB port of a very expensive laptop. I have melted a screwdriver on a battery, but while spectacular nothing exploded.
I also vote the thread title change proposed above : "I don't know exactly where the big BOOM came from: Test (and Other) Equipment Disasters!".
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#29 Reply
Posted by
vk6zgo
on 30 Oct, 2018 13:55
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Nothing quite so disastrous as some of the previous posters, but I have experienced a few of these.
Imagine, if you will, a large attended Radio Station, just after lunch in the mid 1960s, with the Boss playing cribbage with a couple of cronies in the Lunchroom.
The rest of us took the time to study, work on "foreigners" (private jobs), or to meander through some of the less strenuous daily routines.
A Cadet Engineer had, as his project, to test the low voltage characteristics of some silicon HT rectifiers
being considered as replacements for the existing mercury vapour rectifiers.
He needed a high current low voltage supply, so a lead acid cell suggested itself.
The 24 volt diesel start battery consisted of a lot of individual cells all connected in series, so off he went, trailed by a Technician in Training holding an Avometer, some wander leads, a couple of big resistors for a load, & clipboard for writing down the results.
The Cadet EE decided to slightly loosen the two terminals on a cell, then quickly connect his test set up.
He grabbed his "crescent spanner" (adjustable wrench) & set to.
As these things are wont to, the spanner slipped & shorted the two terminals of the cell.
The rest of us heard a resounding bang, & appeared out of our various workshops, offices, etc, rather in the manner of ants from a disturbed nest.
We were confronted by an ashen faced, but otherwise unharmed "Ginger Beer" & a battery case with a gaping hole where the guts had been.
His "offsider" was also OK, having leapt backwards over the protruding exciter on the big alternators.
The Boss "did his block" --- there was steam coming out of his ears, whilst the forlorn perpetrator cowered.
We ran with one less cell for a few days, & everything seemed to work OK.
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Thankfully all these accidents related so far, did not cause any human tragedy, and thus can be told as a funny experience.
There have been some other electrical accidents which have indeed caused injury or even death. Youtube has several very scary videos of switchboard faults.
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#31 Reply
Posted by
vk6zgo
on 30 Oct, 2018 14:35
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But wait, there's more!
24 volts from the aforesaid battery bank were reticulated around the building .
There were a number of alarm systems, & so on,which required -24 volts, which was normally provided by a Mains powered supply.
When Mains power was lost, these alarms, etc, would automatically switch to the battery.
Yours Truly was told to replace the electrolytic capacitors on this supply, as they were a bit "long in the tooth".
I duly turned the supply off, the battery took up the job, so I pulled the thing out, & made a nice, neat job of
fitting the new ones .
I turned it on----BANG!! followed by what looked like a snowstorm in the Control Room.
Yep! electros back to front!---forgot it was a -24volt supply!
Many years later, at a different employer, a Trainee came to me so I could check his construction of a "Dick Smith" kit he was building for a work application.
A cursory look showed "all good", electrolytically all polarised as per the PCB, etc.
He turned it on & BANG!
That's right! Dick Smith had got the PCB labelling wrong!
Going back in time, at my first employer, we had some tube radio chassis left over from a long finished run of mono "Radiograms".
My job was to test them prior to them being put out on special.
To this end, I grabbed a big 12" Rola loudspeaker, & looked at the radio to see where the speaker connected.
All I could see was a length of "figure 8" cable coming out through a grommet on the side of the chassis.
"Aha!" Thought I, connected it up to the "12OX" speaker, & turned the radio on.
There was a sound like a rifle shot, & the voice coil was no more.
Turns out the length of "figure 8" was Mains to power a record player, so it could be turned off with the radio "on/off"switch.
Strangely, the speaker looked quite intact, apart from the open circuit voice coil
The Boss didn't get as angry as I thought he would, so I still had my job
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#32 Reply
Posted by
Chris56000
on 30 Oct, 2018 19:53
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Hi!
Mods, I'm more than happy to have the Thread Title changed to "I don't know where the big "BOOM!!!!" came from as that's been the story of my life in over thirty years of assorted fixings!!
Please keep the tales coming everyone!!
Chris Williams
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#33 Reply
Posted by
tautech
on 30 Oct, 2018 20:00
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Hi!
Mods, I'm more than happy to have the Thread Title changed to "I don't know where the big "BOOM!!!!" came from as that's been the story of my life in over thirty years of assorted fixings!!
Please keep the tales coming everyone!!
Chris Williams
You don't need Mods help, just edit the OP yourself !
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#34 Reply
Posted by
najrao
on 31 Oct, 2018 09:16
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I too have seen a 110kW 2000rpm dc motor suddenly lose its field winding feed. But i was near enough to a KILL ALL button to stop it in its tracks quickly. It coasted down in several minutes from the high speed. Nothing broke.
Was doing a dead shorting oscillograph test on a 125kVA generator coupled to a dc motor. The cast iron coupling halves were 14" dia and weighed more than 50kg each. When my assistant closed the shorting 600A oil switch for the umpteenth time, something whizzed past my ear, crashed on to shop window some 60 feet away, took the steel window with it and deposited it and self a hundred feet away in the yard. Yes, it was half of a coupling half! Coupling guards became compulsory after. Eye wash ones, I am afraid: were made of 2.0 sheet metal.
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#35 Reply
Posted by
Brumby
on 31 Oct, 2018 09:40
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Hi!
Mods, I'm more than happy to have the Thread Title changed to "I don't know where the big "BOOM!!!!" came from as that's been the story of my life in over thirty years of assorted fixings!!
Please keep the tales coming everyone!!
Chris Williams
You don't need Mods help, just edit the OP yourself !
YES! Just go to the original post, hit "Modify" and then pop up to the subject line and change it yourself.
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#36 Reply
Posted by
Fludo
on 31 Oct, 2018 12:20
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We had a 3 phase 110kVA variac that we used to transform the shop 440 down to various voltages during testing. It was a really large unit, about the size of a refrigerator. The device was ordered with a Wye connected input, but for some reason the engineers connected it as a delta and operated it no problem for 2 years before my time. They had issues with the variac blowing the main 200A circuit breaker and shutting down 1/4 of the machinery on the shop floor, but attributed to a "tired old breaker" and simply moved the device to a different branch. Meanwhile, I called the vendor and he stated that the inrush current peaks at 900A, and the proper startup procedure was to buy a bigger variac to slowly ramp up the voltage on the 110kVA unit, to avoid inrush....
We had a request from another department to troubleshoot a 120VAC 3 phase power supply, and this variac was the only available source of power. For good measure, the output was fused at 30A per phase as the person requesting help was a known idiot who always managed to leave a wake of destruction in his path. After juicing the variac, a single output fuse blew but the variac remained operating, creating an imbalanced load with no neutral as a return. The supply tested was faulty and had apparently destroyed other power sources(much more expensive) around the plant.
After that incident the variac had one phase 40V lower than the other 2 and the device, and I was called down from the office to troubleshoot. Removing the cover to the enclosure showed no signs of damage, but after measuring continuity for each coil we found that one had significantly lower resistance than the other. It was only after we called the vendor requesting service that he reminded us that the variac was Wye input, and stated very clearly that it was in the orignal quote and on the nameplate. He also mentioned that several fires occurred because people often made the same mistake.
Moral of the story:
- Trust nobody
- Make sure you read the name plate
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Fludo:
3. RTFM!
Every Lab has its own ISO9000-certified idiot.
In a lab for a power supply company I used to work for, we had to make sure "universal input" units met specs both at 120V/ 60Hz and 230V/ 50Hz. As such, the lab was equipped with both North America and European type outlets. Foolproof enough? Not quite for a certified idiot.
We had some 120v only units that he required to test. He went to this particular bench, where the US-outlet had something plugged already, and only the European-outlet was available.
You guess what happened next. He found an European to US plug converter, and proceeded to sequentially plug a total of 30 units, one after another, and blow the fuse on every single one of them.
The units like most power supply nowadays, had a non-user replaceable fuse. Which meant that every single one had to be reworked.
This guy, in another occasion, also connected an array of 12VDC fans to 120VAC.
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#38 Reply
Posted by
rsjsouza
on 31 Oct, 2018 15:37
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At university our electrical machines lab was always source of several interesting events, ranging from a yellow glowing 200W variable resistor to blown bakelite ammeters. Each bench had several outlets and, for good measure, they were fitted with 2A glass fuses all around - the replacement rate used to be at least a dozen each class. Anyhow, the most interesting event was not electrically related, but mechanical: one of our colleagues was running a motor/dynamo combination and everything was going great, but some of the meters were behind the machines and we had to stretch ourselves over the running set to take the readings. Well, this colleague was using a loose shirt, which was caught by the axle and started to rip his shirt out. Fortunately someone else was able to cut the power of the bench before injuries happened. We had a good laugh.
On another occasion I needed ~300V
DC and, given our outlets were 220V
AC, what could possibly go wrong by simply fitting a full wave bridge rectifier directly on the outlet? After borrowing four 1N4007 from the lab inventory and a hefty capacitor, I set out to solder everything to a universal board and properly isolate everything. When I plugged the circuit to the outlet on the wall, there was a loud BANG and two of the 1N4007 diodes vapourized. By elimination I realized that one of the 1N4007 diodes was shorted (damn students that blow up devices but are too afraid and return them to the inventory).
Anyhow, I set out to the inventory parts bin again and got two more 1N4007 diodes. This time they were looking quite new and the chances of a second diode being shorted are zero, right? Right? RIGHT? Well, one thing lead to another and the second BANG proved too much for the poor wire: the plug was still in my hands but the wire had been pushed back towards me - no breakers were opened as the wire had become the fuse. Anyhow, I ended up finishing this another day as my friend was cursing me for the scare I gave him twice.
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#39 Reply
Posted by
0culus
on 01 Nov, 2018 17:11
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Some of the stories in this thread are just crazy...I fortunately have never personally done a "Big Boom" (yet). But when I was reading up on multimeter safety a while back, I came across a story (that of course I cannot find now) about someone deciding it would be a good idea to connect a Fluke multimeter across test points on some kV class equipment at a mine, if I recall correctly. Wasn't much multimeter left, as it was massively above the 8 kV pulse rating that they state on the back of the meter. Perhaps someone remembers this or a similar story?
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There is a Youtube video which describes a very similar accident on a paper mill, if I recall correctly.
Unfortunately there was a casualty on this instance, and the people who witnessed the accident mention on the video that the poor guy was essentially on fire.
The strange thing about this particular accident is that the deceased was a *very experienced* and certified electrician. Overconfidence, perhaps? We'll never know.
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#41 Reply
Posted by
0culus
on 01 Nov, 2018 23:04
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There is a Youtube video which describes a very similar accident on a paper mill, if I recall correctly.
Unfortunately there was a casualty on this instance, and the people who witnessed the accident mention on the video that the poor guy was essentially on fire.
The strange thing about this particular accident is that the deceased was a *very experienced* and certified electrician. Overconfidence, perhaps? We'll never know.
Damn. That's sad. Complacency is dangerous though.
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#42 Reply
Posted by
bd139
on 02 Nov, 2018 00:01
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Had a near miss a couple of days ago. This is in the TEA thread but I thought I'd repeat it here.
Getting a bit tired and dropped a scope probe into the live power supply on scope. Flipping loud crack like you've never heard before as one of the caps emptied through the probe, into my TDS210 and back out of the other probe to ground in the scope. Sounded like a firework going off. One dead DUT! Fortunately the TDS210 survived that little incident unharmed.
My nice new Tek probe had a nice burn on it
However fortunately this wasn't a total disaster. Just blew the line fuse but I spent half an hour testing all the rectifiers and transformer taps to make sure I hadn't properly screwed stuff up.
But poo nearly came out of me as it was a not too shabby Tek 250MHz 475A with DM44 I thought I had buggered completely for a few minutes.
Resulting lesson:
No less than a week before this I smoked some nice Pomona leads by being lazy and using black for positive because it was the nearest thing to my hand
... That made a not insignificant fire and stank the place out for about 5 hours
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#43 Reply
Posted by
Brumby
on 02 Nov, 2018 01:08
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No less than a week before this I smoked some nice Pomona leads by being lazy and using black for positive because it was the nearest thing to my hand ... That made a not insignificant fire and stank the place out for about 5 hours
I get really uneasy when I don't have a lead of an appropriate colour at hand, especially for power connections, for exactly this sort of possibility. As a result, if I use an "alternative" colour, I will keep to cool colours for negative and warm colours for positive. Once I did use a black lead for power - but I put a loose knot in the end to remind me it was "knot" what the colour suggested. Even then, that was easy to mistake for a cable that just had a loop in it and I wasn't comfortable.
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My first day at a new job began rather innocently when I was asked to test a large variable speed drive (in the test area) before taking it into the power lab and wiring it up to do a specific test. All the precursor low energy tests went well so I flicked the 3ph ON switch. BANG. (These retailed for about $150k) I sheepishly went back into the office to an eruption of applause...
Same company several months later, a colleague and I were testing an inside-delta configuration on two of the largest drives we made (at the time), we paper-scissors-rocked for the turn-on honours, and as I threw the switch, the inrush bounced the cables in the overhead tray, dislodging a ~1m length of wood that had been thrown up there...
Same company... during the testing of an overspeed algorithm the large, test motor's rotor expanded out and hit it's stator, ripping the motor out, and rolling it across the floor, dragging the VSD off the bench, and leaving us holding the test box (on/off switch and a speed pot) with some dangling wires...
Same company (boy we had some good times there!) - a new staff member had made up a 240Vac mains powered board, and was probing it. I suggested that he may want to tape up the mains connections so he didn't kill himself. I go back to find he had used packing tape. I suggested that he perhaps use some nomex sheet and some insulation tape instead (both readily available in our lab) as any pressure would simply puncture through that tape... He nodded at me, and proceeded to ignore me.
Literally, as I sat back down at my desk, I hear him yelling, and trying to shake the mains wire spike out of his finger, while being electrocuted. At that point, I became incapable of rendering assistance, instead laughing my proverbial off. A colleague, also watching the previous exchange, manages to compose himself just long enough to hit the emergency stop button...
And now I think about it, I have a bunch more...
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#45 Reply
Posted by
GregDunn
on 02 Nov, 2018 02:30
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I get really uneasy when I don't have a lead of an appropriate colour at hand, especially for power connections, for exactly this sort of possibility. As a result, if I use an "alternative" colour, I will keep to cool colours for negative and warm colours for positive.
Same idea I had - I have some test leads which are yellow and blue which are handy when I already have red/black ones connected to a circuit. No chance of confusing them at either end.
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#46 Reply
Posted by
bd139
on 02 Nov, 2018 07:55
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I just bought a few more red and black ones and persuaded myself that I need to be less lazy.
Mid project my bench looks like Jim Williams and Bob Pease had a hacking session on it which doesn’t help. Need to be more organised.
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Don't forget the small silly mistakes......
Back when single sided, thru hole boards were the norm, jumpers were extensively used.
In large operations jumper-wire insertion machines were used, to avoid slowing down the component sequencer and auto insertion machines.
I burned my fingers when someone attached to a spool of wire with some 63/37 solder labels. Nice prank for a newbie!
Had also a couple of large electrolytics blow because they were installed backwards. Your clothes would stink for hours.
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#48 Reply
Posted by
HKJ
on 02 Nov, 2018 14:34
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I burned my fingers when someone attached to a spool of wire with some 63/37 solder labels. Nice prank for a newbie!
When you have both solder and that type of wire on your bench it is easy to pick the wrong one. I have sometimes had some "solder" that would not melt, but got very hot to hold
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#49 Reply
Posted by
vk6zgo
on 03 Nov, 2018 02:05
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I burned my fingers when someone attached to a spool of wire with some 63/37 solder labels. Nice prank for a newbie!
When you have both solder and that type of wire on your bench it is easy to pick the wrong one. I have sometimes had some "solder" that would not melt, but got very hot to hold
It was a common trick to very carefully solder a length of "tinned copper wire" into a section of solder, then wind it back onto the solder spool, & wait for some busy person to get caught out.
I've caught
myself out with nastier things.(Maybe it was Karma for doing the trick described above).
I was trying to patch up a rusty old car by welding new sections of metal in with an oxy acetylene torch.
A problem is, that it is easy to pick up the rod by the end you've just used( the hot end).
A magazine suggested bending a hook on the cold end, so you would always know which was which.
It worked well, until one time, I absent-mindedly picked up the rod by the centre portion.
The "hook" became caught up under the car wheel arch, so I heaved , it came free, & my hand swung through the "outer envelope" of the gas flame!
Ouch!!!, Super Ouch!!!