Most weird-assed Elect. problems I'll probably not see again ??

[GlennSprigg]

In a Factory once, I found a bank of Fluoros that would ONLY work, if another 'bank'
of lights was turned on too....   (All or nothing). Found the 'fault' in the main SwichBoard.
TWO lighting circuits, on different Phases, had their NEUTRALS twisted together, and in
the same TUNNEL (hole) in the Neutral bar. However, this 'termination' was LOOSE !!!
So it only worked when 415v was applied across BOTH 240v circuits in series, at once !!
I said.... "F^&$ING C$#% Sh@#$", but wrote something different in the report....   

[Brumby]

I have my entry.....

One day I was visiting a friend in his home and he showed me a really bizarre result of a number of light switches being operated in various combinations which produced different brightness in 3 different light fittings.  The combinations and symptoms aren't simple to describe (and it was a few years ago), so please excuse me for not detailing them.  After a few cycles of trying various combinations, I worked out that there was likely a problem with a particular connection - but the question was: Where, exactly, was this connection made?

Fortunately, in trying the more easily reachable light fittings, I found the problem on the second fitting I checked.  It turned out that one of the points on this light fitting had been used as a common connection for 3 wires - but these had badly corroded.  This seemed strange for a light fitting in an under-stair storage cupboard - until I noticed that my friend had stored the dry chlorine used for his pool.

Once cleaned up, all lights worked perfectly - and he relocated the chlorine.

[TerraHertz]

This hardly qualifies as 'weird', but it was yesterday, so...
I was recently given a big _old_ 240V to 120V conversion and isolation transformer. Someone had replaced the output socket, and anyway it's a good idea to check mains stuff. First test: ground continuity between input cord and output socket. OK. Then ground to case.... nope, open circuit.

Opened it up. Huh, ground point on one of the transformer compression bolts. Which are sometimes insulated from the transformer laminations, to prevent eddy currents. I visually checked, can't see any insulation at all on the bolt. It _should_ be electrically one with the L-brackets and the case. But it isn't.
There's no other original grounding point to be seen. So since 1953 this thing has been operating with the metalwork not grounded.

In the last pic the ground lead with the yellow lug is my addition. And yes, the paint is scraped off at the hole, and connection checked.

[duak]

In North America we have single phase for residential and 3 phase for commercial/industrial.  In Canada, 3 phase is most commonly available as either 208/120Y  or 600/347Y (delta/wye voltages).  A friend moved his cabinet shop from a building with 600/347Y to 208/120Y.  Some of his machines operated on 600/347Y and so the electrician connected some of my friend's step-down transformers as step-up transformers.

One transformer was quite hot even when unloaded and one day it blew its primary fuses - 100 A and north of $100 each.  I measured the line voltages and currents and saw that the current was a lot higher than it should have been.  On a hunch, I measured the neutral current at 50+ A when the transformer was unloaded.  I disconnected the neutral and the line currents dropped down to a few amps and the transformer cooled right down.

3-phase distribution transformers are usually delta primary & wye secondary.  It allows the secondaries to be re-referenced to neutral to prevent a circuit from floating.  The electrician, true to code, tied the neutral of the transformer secondary to system neutral, but that's a no-no in this case.  The delta-wye transformer configuration also eliminates negative sequence currents - this is where 3rd harmonic currents produce currents within the transformer that run opposite to the fundamental current.  It not only detracts from the power delivered to the load but it causes I^2R losses.  When reversed, the transformer doesn't block this current.  It seems too that any voltage imbalance will also manifest as a neutral current.

The electrician probably knew better but this is what his customer wanted so he did what he was paid to do.  I don't know the code well enough to say if he violated it but, in the end, you canna deny the laws of physics.  I suppose the wiring was safe, it just wasted power and fuses.

I measured the secondary voltages (600/347Y) to earth/ground and saw they were about 350 V.  I figure that if there's primary to secondary leakage, the maximum voltage won't be all that much higher than usual, ie. maybe 800 V instead of 600 V

Cheers,

[ajb]

First test: ground continuity between input cord and output socket. OK. Then ground to case.... nope, open circuit.

I little while ago I was asked to look at some old hand-me-down Lightronics dimmer packs and see if it was worth getting them working again.  These have four dimmed outputs, and either two input pigtails (giving up to 4800W per pack) or a single input inlet (giving up to 2400W per pack).  All the connectors are standard North American NEMA 5-15 plugs/receptacles, and any fixture plugged into an output is theoretically grounded through the pack and back to the input.  Usual repairs for these types of things are fuse holders and maybe an SCR here or there, no big deal.

Except.  There's no direct connection from the input ground to the output ground.  The inputs are mounted in an anodized panel, and the receptacles are mounted in a powdercoated panel.  So the ground pathway is theoretically output receptacle -> screw -> powdercoated panel -> screws -> anodized panel -> screw -> inlet.  So after 20+ years of being kicked around and sitting there oxidizing in the heat of a bunch of incandescent lamps the LOWEST resistance I measured from input ground to output ground was like 100 Ohms.

I contemplated doing a complete rewiring and fitting a new control board that supports DMX instead of the old Lightronics protocol no one uses anymore, but they're really not worth that much effort to fix these days.

[CopperCone]

cracked socket in a motherboard that made the computer work only when it was sitting flat. When I put it back upright in its desk hole it would stop working. Took me a while to figure out when I was in high school.

Eventually I got the bright idea to  power up the computer laying down and measure which angle it failed at (something like 60 degrees). It would fail slightly differently each time too, so I thought I was damaging it with ESD, thermals/interference from other cabling (in its cubby hole) or some other outlandish shit. 

[TimFox]

A good Hewlett-Packard 6209B 300 V lab supply (from eBay).  I unpacked it and connected to a DMM and found normal behavior.  However, when using it in a test setup, with one side grounded, it made very low voltage.
For some reason, -hp- had an old-fashioned rubber grommet between the circuit board and the metal support structure that contacted a trace on the board.  It had charred to charcoal, and effectively shorted that trace to the case.  When the supply was not grounded, it worked fine.
Replacing the grommet with a non-rubber plastic washer solved the problem.

[Red Squirrel]

I only heard the story and was not around to witness it, but we got a 611 trouble come in and the customer said every time their phone rang, the furnace started.  Baffled, and figuring it was some crazy person, we sent a tech.  Tech gets there and sure enough it's really what the customer says.  Turns out the phone line had somehow been connected to the thermostat wiring.   What I'm surprised is that it did not blow the furnace relay or even the whole board.  Those take 24 volts while phone ring voltage is like 170 volts.

[NiHaoMike]

I only heard the story and was not around to witness it, but we got a 611 trouble come in and the customer said every time their phone rang, the furnace started.  Baffled, and figuring it was some crazy person, we sent a tech.  Tech gets there and sure enough it's really what the customer says.  Turns out the phone line had somehow been connected to the thermostat wiring.   What I'm surprised is that it did not blow the furnace relay or even the whole board.  Those take 24 volts while phone ring voltage is like 170 volts.

There's also the case of the dog that would bark every time the phone ringed because it was chained to a bad ground. Might have just been an urban legend, however.

[Brumby]

A true story - not mine.

Someone had put together a computer rig in a tower case and whenever they tried to run it, it would glitch on far too frequent occasions.  Handed to my friend, he went about checking a few things out and found this problem went away completely with the side cover removed - and would reappear when replaced.

After some further testing and getting past the "It couldn't be" wall, his final assessment was that with this particular arrangement, there was some sort of RF tuned circuit that was feeding signals back into the motherboard and simply disrupting normal operation.  With the side panel off, this vanished.  If he wasn't into antenna design and construction, I think he would have been fighting with it for a lot longer.

The rig builder wasn't convinced and it required a couple of demonstrations to show that my friend wasn't bullshitting.

[Nominal Animal]

I once had a motherboard blow one of its CPU voltage regulators.  Not just burn out; it literally exploded, half of it blown clear off, with a clear audible bang, after almost a year of reliable operation (and a week or two of flakyness).

[Rerouter]

Context:
Most vehicles have a hall effect sensor to provide a speed signal for the vehicle, when it measures metal, it pulls its output low

In this weird case, I was asked to look into why a Buses radio stopped working the moment it went under 8kmph,

Turns out that particular model of speed sensor inverted its output, and some water had gotten inside and shorted across the inverter, so after about half a second of it not seeing metal, it would oscillate up and saturate, working as a wide-band FM jammer, broadcast by the 3+ meters of unsheilded wiring carrying the output signal.

The only reason I even though that might be the case is the workshop radio went out during one of our tests, when we moved it forward enough to align with a gap on the sensor gear.

[CJay]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

[X]

Not sure if I could really class this as being "weird" but once I was testing a prototype high current LDO regulator circuit and it seemed to work OK the first time. Then the next time  I plugged it in, one of the two tantalum bead caps I was using almost transformed me into a pirate.

After clearing the solidified tantalum chunk that landed on my face, I realised the polarity was wrong on both caps. Corrected the polarity of the other cap, and replaced the Kentucky Fried Capacitor with a new one. This time, with both caps in the right polarity, I plugged it in yet again, and then the other capacitor tried to transform me into a pirate.

I didn't plan the localised festivities that occurred, but I had a fire extinguisher ready in case the battery or one of the connecting leads to catch fire due to over-current or an accidental short before the fuse.

[Berni]

We had a interesting situation during construction around the house.

A table saw with a 3 phase motor was brought in along with a smaller single phase chop saw. To get power to it a extension cord was brought along, we plugged it all in and started with the work, however we soon noticed that we have a problem as turning on the chop saw caused the ground fault breaker to trip. We thought the saw developed some short internally, started suspecting the extension cord too, yet it just would not run. Starting to run out of ideas we start unplugging all the cords and the problem went away. Then we discovered that whenever the 3 phase extension cord was plugged in the single phase chop saw would trip the ground fault. What does the 3 phase cord  have to do with it when the saw at fault had its own separate single phase cord going to a seperate mains outlet.

Eventually a neutral to earth short was discovered in that 3 phase cord. The reason this was never discovered before is that my grandparents who own that cord have a old school voltage based ground fault broker that does not care for a neural to earth short.

If you have not already figured it out here is an explanation why this happened. Both the single and 3 phase outlet was close to the electrical distribution box and on a branch that had no heavy loads on it. As a result the neutral wire the socket was on didn't have enough current to leak into earth to trip the GFI. Then the 3 phase saw does not need a neutral as it takes identical current from the 3 phases so the Delta/Star switch never bothers to connect neutral to anything even. This means no current in neutral so the saw runs without a problem. Then when you start the single phase saw the current flows from one phase into neutral and joins the neutral in the house, however this neutral had enough common copper in its path to cause a slight rise in voltage of the 3 phase outlets neutral terminal. This allowed the returning current from the single phase saw to flow into the 3 phase extension cord where it went to earth and back into the sockets earth. This made for not all live current returni back to the GFI trough its neural terminals and caused it to trip. Phew this could really give an inexperienced electrician a good run for there money trying to figure out the problem

[NiHaoMike]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

Sounds just like a common legend. But I'm under the impression that area is very sensitive so it would be hard to miss the accidental contact? Closely related to that was a funny story a friend of mine told me about the time in computer repair class when she accidentally knocked over a box of screws...

[rsjsouza]

I was told this story from my Power Electronics teacher at the university.

He was working with other folks at a small remote power station that was experiencing an uncommon higher power consumption even under minimum load conditions. The situation went on for many months and. during an unscheduled disconnect for repair, they noticed that a large metallic plate (part of the structure) that surrounded the output isolators was terribly hot. This uncovered the mystery that went on undetected: the wires going out of the station were the primary and the metallic plate has become the secondary of a power transformer with its outputs shorted. Replacing the structure with a wooden one solved the problem.

Apart from the occasional ISA card that was being lightly disconnected with the motherboard angle, there are some other experiences of mine reported at a different thread, but none of them are particularly weird.

[CJay]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

Sounds just like a common legend. But I'm under the impression that area is very sensitive so it would be hard to miss the accidental contact? Closely related to that was a funny story a friend of mine told me about the time in computer repair class when she accidentally knocked over a box of screws...

If it hadn't been me and a colleague who diagnosed the problem I'd say it was.

I also believe the area is sensitive but I don't think it's quite sensitive enough to be able to tell if it's hitting desk or keyboard, plus the scaffolding holding it all up and in must have been impressive.

It was rather embarrassing having to explain but to her credit she also turned bright red and later sent us a thank you card for our 'patience' which I *may* still have somewhere.

[jmelson]

Not sure if I could really class this as being "weird" but once I was testing a prototype high current LDO regulator circuit and it seemed to work OK the first time. Then the next time  I plugged it in, one of the two tantalum bead caps I was using almost transformed me into a pirate.

After clearing the solidified tantalum chunk that landed on my face, I realised the polarity was wrong on both caps. Corrected the polarity of the other cap, and replaced the Kentucky Fried Capacitor with a new one. This time, with both caps in the right polarity, I plugged it in yet again, and then the other capacitor tried to transform me into a pirate.

I didn't plan the localised festivities that occurred, but I had a fire extinguisher ready in case the battery or one of the connecting leads to catch fire due to over-current or an accidental short before the fuse.

OH, big rule violation here!  If a tantalum capacitor is reverse-polarized, even with just ONE Volt, even for a millisecond, ALWAYS replace it!  If connected to an ample power source, either immediately, or sometime soon, it will either burn or explode.  Even the momentary reverse polarization causes permanent damage.

Jon

[dmills]

Wirewrapped industrial control system back in the day, big card cage full of 4000 series cmos doing what you would do on a micro the size of a credit card today.
The thing was a mess of random logic, multiplexers, flipflops and counters doing state machine type industrial control.

The complaint? Every few days the thing would loose its state and a (separate) safety system would bring production to a crashing halt.

Much digging ensued, as the thing worked perfectly on my bench.

The fault?

Failed power supply connection, but as long as a sufficient proportion of the inputs were a solid logic 1 the thing was powering itself via the input protection diodes! Sweary, just a bit!

Regards, Dan.

[Gregg]

Three phase power but each phase routed through a different metallic conduit.  Things were getting warm but the client didn't want to spend the money or down time to fix this problem.   

[bsudbrink]

Three phase power but each phase routed through a different metallic conduit.  Things were getting warm but the client didn't want to spend the money or down time to fix this problem.   

Ok.  High power AC electrics aren't really my thing.  Would you mind explaining the problem with this?  All of the conduits are "bonded" at the panel, right?  Where's the trouble?

[dmills]

Magnetic fields and an effective shorted turn.... Lot of current flowing in the conduit walls.

Regards, Dan.

[BrianHG]

Here is a computer related one.

The Amiga SCA (computer virus).  This one caught me off guard when I was young.  Most likely the first Amiga Virus, 1987, if not, the first one to gain notoriety...

When rebooting, you expect the computer to reset, pause, cycle through a few shades or grey for around 5 seconds, then the insert boot Disk logo, or auto boot from the HD.  However, if you had the virus, a random coincidence once in around 20 boots, no 5 second reboot pause, instead, instantly, this colored bars appeared, then, the following eerily timed message:

Something wonderful has happened....
Your AMIGA is alive !!!



History: https://en.wikipedia.org/wiki/SCA_(computer_virus)

[bsudbrink]

Magnetic fields and an effective shorted turn.... Lot of current flowing in the conduit walls.

All right.  I think I see.  It's not really the conduits themselves, it the multiple holes in the cabinet?  You could effectively "fold" the holes together and end up with two interlocked rings, one with the AC flow and the other with the induced current, have I got that right?

[CJay]

Here is a computer related one.

The Amiga SCA (computer virus).  This one caught me off guard when I was young.  Most likely the first Amiga Virus, 1987, if not, the first one to gain notoriety...

Good grief, there's a blast from the past. I remember one of the UK Amiga magazines 'gave away' a copy of Lamer to everyone who bought their magazine and used the coverdisk, it might have been Amiga Format magazine I think?

[SeanB]

Magnetic fields and an effective shorted turn.... Lot of current flowing in the conduit walls.

All right.  I think I see.  It's not really the conduits themselves, it the multiple holes in the cabinet?  You could effectively "fold" the holes together and end up with two interlocked rings, one with the AC flow and the other with the induced current, have I got that right?

Yes, you get conduit entries that are insulated so you need a separate grounding wire in the conduit ( or use plastic) to prevent this, though you also need to slit the cabinet between the conduits to leave a gap as well.  Only done where you cannot fit a bigger conduit to get them all in one.

[BrianHG]

Here is a computer related one.

The Amiga SCA (computer virus).  This one caught me off guard when I was young.  Most likely the first Amiga Virus, 1987, if not, the first one to gain notoriety...

Good grief, there's a blast from the past. I remember one of the UK Amiga magazines 'gave away' a copy of Lamer to everyone who bought their magazine and used the coverdisk, it might have been Amiga Format magazine I think?

It spooked me because I never heard of or seen a computer virus at the time.  Yes, I also heard one of the Amiga format magazines who offered a free disk with the magazine had been infected with the virus.  For that to happen, someone in the chain at the magazines HQ must have been playing pirated games, otherwise they would have never had the virus in the first place.  Also, that random, 1 in 20 or 1 in 100 boots before the message appears may have never been seen by them yet.

Even worse, that old simple D-Ram controller in the Amiga which had no mem-test at power up meant, if you powered down your Amiga, it could retain the virus for a few seconds in some cases at next power-up, even without an active d-ram refresh.

[radar_macgyver]

I once had a motherboard blow one of its CPU voltage regulators.  Not just burn out; it literally exploded, half of it blown clear off, with a clear audible bang, after almost a year of reliable operation (and a week or two of flakyness).

I briefly worked at a company that designed server motherboards. We were advised during board bringup to keep our eyes away from any OSCON caps in case they were inserted the wrong way (these were prototype boards that did not go through any QC).

My entry: when diagnosing a Klystron transmitter which would start up OK, but in a few seconds its output pulse would get jittery and phase noise would get really bad. We were almost at the tube replacement stage when we noticed that the triggering signal for the transmitter (not RF, just differential TTL in a shielded twisted pair cable) was dangling a bit too close to the HV modulator (~10 kV, 500A pulses). Re-routing the cable and adding a ferrite clamp solved it. What still puzzles me is why it took several seconds for the failure to show up, and not immediately.

Another one: on our dual-band radar (S- and X-band combined onto a single feed antenna) I noticed the S-band (2.7 GHz) radar would pick up a weak signal (only a few dB above the noise floor) whenever the X-band (9.4 GHz) transmitter fired. I would have expected something the other way around if the X-band picked up harmonics of the S-band, but why would the X-band transmitter produce any energy at 2.7 GHz? It turned out to be EMI from the X-band's HV modulator that was physically close to the radar antenna. Adding some EMI gasket around the X-band enclosure door cured the problem. What was most strange was this problem only showed up years after the X-band radar was installed.

[German_EE]

One-way cable

Go to any audiophool outlet and you'll see one-way speaker cables for sale where all sorts of nasty things will happen if you connect them the other way around, but this was a 2m length of RF cable with a PL-259 on each end, connect the cable one way around and SWR was OK, connect it the other way around and it was about 2:1.

The cause was the plugs, one had been badly soldered and some of the solder had crept onto the center pin increasing the diameter. Over time the SO-239 socket that it was connected to had expanded to cope with this. So.....

Socket A (wide socket) Plug A (wide center pin with solder)___________________________Plug B (normal center pin) Socket B (normal socket)

Works OK

Socket A (wide socket) Plug B (normal center pin)___________________________Plug A (wide center pin with solder) Socket B (normal socket)

Intermittent contact on the left connection and a stressed socket on the right.

I ended up replacing the cable and the damaged socket, things were then fine.

[GlennSprigg]

Ok... here's another Factory.... (huge automated processing plant).
I had a Chief Engineer above me, (he was non electrical), who could not understand (or be told)
why the main 1,500 amp factory feed was grossly overheating. (Split into 2 parallel sets of cables).

It came down to SIMPLE Ohms Law... :-)
Two 800-amp cables, (each phase), piggy-backed together Lugs, to one side of the huge BusBars.
Unbolted, I calculated about .6 Ohm on one run, and about .3 Ohm on the other runs.  Ok....
Sounds like nothing, but ONE had TWICE the current flow as the other !!!!
(1000 amps in one, and 500 amps in the other). Due to a difference of just .3 Ohm...
They either 'get it' or they dont...

[GlennSprigg]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

Sounds just like a common legend. But I'm under the impression that area is very sensitive so it would be hard to miss the accidental contact? Closely related to that was a funny story a friend of mine told me about the time in computer repair class when she accidentally knocked over a box of screws...

I don't know about 'ample breasts', but I was called out to Yatala Labour Prison in Adelaide late one night, to their main control room.
The MainFrame computer had locked up. (Swamped with thousands of calls from a P.C. it seems)...
Turns out someone had a BOOK that was resting on the 'Print-Screen' button on one of the P.C.'s.
The 'MainFrame' had a 'Hissy-Fit' with it's buffers, and locked out the whole system !!!!!! Sigh..

[SeanB]

Ok... here's another Factory.... (huge automated processing plant).
I had a Chief Engineer above me, (he was non electrical), who could not understand (or be told)
why the main 1,500 amp factory feed was grossly overheating. (Split into 2 parallel sets of cables).

It came down to SIMPLE Ohms Law... :-)
Two 800-amp cables, (each phase), piggy-backed together Lugs, to one side of the huge BusBars.
Unbolted, I calculated about .6 Ohm on one run, and about .3 Ohm on the other runs.  Ok....
Sounds like nothing, but ONE had TWICE the current flow as the other !!!!
(1000 amps in one, and 500 amps in the other). Due to a difference of just .3 Ohm...
They either 'get it' or they dont...

Which connection was not crimped properly then? Did the lug glow or not.

[Eka]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

At one of my former work places we had an amply endowed lady that sometimes accidentally activated the 10 minute count down timer for light shutoff when she reached for the paper stored on the shelf above it. They moved the paper, and maintenance added drawer handles on either side of the button that activated the timer so it wasn't possible to brush against it and activate it.

The reason there was a timer is it took about 10 minutes to walk to the normal exit door from where the light shutoff was. Also the system was sequenced so it didn't cut off such a huge load all at once. Once activated, it took 2 minutes to turn off all the banks of lights, and you could't stop it in mid process. Light turn on also took 2 minutes. They originally had it all turned on and off all at once, but if it and two of their large electric kilns all switched off at the same time it would cause an over voltage event at the plant's substation, and the plant's substation would kick out. With all the machinery, etc in the plant shutting off all at once it would also trip the area substation. Power would then go out for about 1/4th of the industrial area it was in. The electric kilns were also rewired so they had banks of heating elements that were never all turned on or off at the same time.

[Nominal Animal]

Before the turn of the century, I had a short IT support gig at an university.  After getting one classroom rewired, the uni network staff noted that both the panel-to-switch and socket-to-computer Ethernet cables were of the crossover type, and wanted to fix that, by swapping both to straight/normal ones.  It was fine otherwise, except that the panel-to-socket connections were mislabeled, and I ended up breaking three or four Ethernet cards on PowerMac 7200's.  It is a triple WTF: first, the mislabeling from panel-to-wall socket cabling; that was just pure evil. Second, that using a crossover cable instead of a straight one, or vice versa, could break the network card on PowerMacs 7200s. Third, I didn't realize what was happening until I broke the third one (or might even have been the fourth).

[duak]

Sean & bsudbrink: about the 3 phase wires in separate conduits.  It seems to me that this situation is a ground loop - the bane of low level signal engineering.   I'm guessing from the size of the conductors that the phase current is a few hundred amps?  I wonder if the NEC covers situations like this.  Surely others have encountered induced currents in metaliic conduits or raceways.  I'd bet there's be a measureable voltage on the conduits.

BTW, I understand that hospitals require all equipment attached to a patient be powered from one outlet.  Don't know if it's true though.  Makes sense because if, say, an ECG monitor probe set is a voltage or two above the motorized reclining bed there could be fault currents passing thru the patient. I think I've seen special medical receptacles that are ungrounded.

Cheers,

[janoc]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

Sounds just like a common legend. But I'm under the impression that area is very sensitive so it would be hard to miss the accidental contact? Closely related to that was a funny story a friend of mine told me about the time in computer repair class when she accidentally knocked over a box of screws...

I don't know about 'ample breasts', but I was called out to Yatala Labour Prison in Adelaide late one night, to their main control room.
The MainFrame computer had locked up. (Swamped with thousands of calls from a P.C. it seems)...
Turns out someone had a BOOK that was resting on the 'Print-Screen' button on one of the P.C.'s.
The 'MainFrame' had a 'Hissy-Fit' with it's buffers, and locked out the whole system !!!!!! Sigh..

I remember a similar problem from the early 90s from dad's workplace (where the young me was moonlighting as an unpaid computer tech/support here and there). They had an SCO Unix server there, with an UPS for an orderly shutdown in case of a power failure. Both the UPS and the server were in tower cases, sitting on the floor under the desk in the office.

At some point the machine started to turn instantly off, losing power (and some data in the process). That was weird, the UPS was there meant to prevent exactly that and it didn't work? After a lot of testing of both the computer and the UPS everything seemed to be OK. Real headscratcher ...

This went on for a while, with several crashes a week. Until one day someone saw one of the office workers sit down at that desk to do some paperwork - and hit the big button on the front panel of the UPS with their knee in the process. POOF, server down. The protruding button on the UPS was the main shut-off switch for the load (really oddball design, IMO, don't recall the brand of the UPS anymore though). So the UPS remained up, beeping furiously about an apparent loss of power and the server was turned off instantly ... The result was lots of red faces and  a makeshift safety cover out of a piece of acrylic taped over that button. No problems since.

[dmills]

See the story behind the term "Molly Guard" for an older example of the same basic problem.

Regards, Dan.

[Gregg]

Sean & bsudbrink: about the 3 phase wires in separate conduits.  It seems to me that this situation is a ground loop - the bane of low level signal engineering.   I'm guessing from the size of the conductors that the phase current is a few hundred amps?  I wonder if the NEC covers situations like this.  Surely others have encountered induced currents in metaliic conduits or raceways.  I'd bet there's be a measureable voltage on the conduits.

BTW, I understand that hospitals require all equipment attached to a patient be powered from one outlet.  Don't know if it's true though.  Makes sense because if, say, an ECG monitor probe set is a voltage or two above the motorized reclining bed there could be fault currents passing thru the patient. I think I've seen special medical receptacles that are ungrounded.

Cheers,

Since I took the picture of the same phase conductors running through separate conduits, I probably should have explained the situation a little better.
The picture taken in 2005 is of a 480 volt transfer switch, the conductors are 350mcm copper THHN and the measured load was between 740 amps and 800 amps depending on how much HVAC was on line.  This was a small telco/data facility in Silicon Valley; which is still in business, so I won’t mention any names.
NEC article 300.20 strictly prohibits wiring parallel runs of conductors in this manner to eliminate the induced heating of the conduits, raceways etc.  310.4 refers to paralleling large conductors in higher power installations.

[jmelson]

Three phase power but each phase routed through a different metallic conduit.  Things were getting warm but the client didn't want to spend the money or down time to fix this problem.   

Oh NO!  Every industrial electrician knows you have to run ALL phases (plus neutral, if it exists) in each conduit.  Otherwise, you get induced currents in the conduits, which can be very substantial.

So, if you need to run 4 conductors for each phase, then you run 4 conduits, and one conductor of each phase ABC and neutral run in each conduit.

Jon

[NiHaoMike]

Oh NO!  Every industrial electrician knows you have to run ALL phases (plus neutral, if it exists) in each conduit.  Otherwise, you get induced currents in the conduits, which can be very substantial.

So, if you need to run 4 conductors for each phase, then you run 4 conduits, and one conductor of each phase ABC and neutral run in each conduit.

Jon

I presume that dedicating a conduit per phase is OK with plastic conduit? Or is plastic conduit not allowed/heavily restricted for such high power levels?

[basinstreetdesign]

Back in 1997 I was working for a small company run by two shysters.  I was planning to quit but one of the things they did to try to convince me to stay on was to give me a "company car" to use.  All I had to do was put gas in it.  The more I drove it the more I found what a turkey this boat was.  The idle on the carburetor was adjusted way too high so that just shifting into drive and NOT giving it any gas would make it pick up speed so that it would get itself into 2nd gear.  The brakes took a beating.

There were electrical problems, too.  The radio would change stations at random times and whenever I signaled left, the dome light would flash.      I'm sure that there was some kind of corrosion issue or lifted common ground connection or some such.  After a while I found that the battery was so weak that I could not start the car twice in close succession.    When I griped to the owners, they just laughed.  I was gone in about a month but they refused to stop paying me (at 50% of normal) to "keep me available". 

[BradC]

BTW, I understand that hospitals require all equipment attached to a patient be powered from one outlet.

Don't know about hospitals, but I played a gig at a pub once where the PA was plugged in on one side of the stage and my amp on the other. After copping a hundred volts (or so) between the guitar strings and my Mic, I now check sockets for a proper earth during set up and *everything* gets plugged into the same socket.

It turns out your lips are really quite sensitive to that sort of voltage, and the rest of the band thought the slowly decaying echo of "fark...fark...fark..." over the PA was quite comical at the time.

[Berni]

Ah yes disconnected grounds.

I had a case where went to the desk of a programmer to help debug the product that I built the hardware for. Tho I had noticed that touching the PCB would sometimes make it do weird things and cause glitches on the display. So I bring a scope over to see what's going on, I start looking for a good place to put my scopes ground clip while holding it in my hand and upon picking up the PCB I got a shock and threw the product on the table. I looked surprised for a second to what happened, the highest voltage in there was a 30V LCD backlight supply switcher, so how did I just get shocked. Then i found that I get shocked by touching the shield of the microUSB cable that was used to power the product.

Turned out there was 110V between his PCs ground and the earthed ground clip of the scope. Turned out that the power strip all of his stuff was plugged into was stuck into a red UPS backed outlet that had no earth.

Upon explaining it to the programmer who sat at that desk, he did not seam at all concerned by this and even argued that this is normal because it's a special UPS outlet. They did fix it some time later.

[TheWelly888]

BTW, I understand that hospitals require all equipment attached to a patient be powered from one outlet.  Don't know if it's true though.  Makes sense because if, say, an ECG monitor probe set is a voltage or two above the motorized reclining bed there could be fault currents passing thru the patient. I think I've seen special medical receptacles that are ungrounded.

Not quite from one outlet but from one phase; in the hospital I work in, each ICU bed has multiple sockets fed from one phase. I have never seen ungrounded medical receptacles ever!

[bsudbrink]

I presume that dedicating a conduit per phase is OK with plastic conduit?

If I understand correctly now (and I think I do), just plastic conduit won't help if you have different phases entering a conductive box through different holes.  You will have induced current (and presumably heating) in the conductive box.  The solution is to make the separate holes into one hole by connecting them with thin slits.

[jmelson]

So, if you need to run 4 conductors for each phase, then you run 4 conduits, and one conductor of each phase ABC and neutral run in each conduit.

I presume that dedicating a conduit per phase is OK with plastic conduit? Or is plastic conduit not allowed/heavily restricted for such high power levels?

Well, I can't imagine running 4160 V in plastic conduit.  And, running thousands of Amps can lead to at least humming of nearby metals, and possible failure over time due to vibration.  Best to balance and thereby null out the fields by not grouping the phase wires.

Jon

[duak]

I mentioned earlier that I had seen an ungrounded hospital receptacle.  I rummaged thru my collection and sure enough, I had one.  In fact, it was an Isolated Ground receptacle where the grounding pin is not connected to the frame but has a screw terminal to allow for a special insulated ground wire to a grounding rod separate from the equipment grounding wire or conduit.  I sort of remember looking it up and thinking it might come in handy to build an isolating transformer to safely work with offline power supplies.  Apparently, these receptacles have orange triangles near the ground pins.  In my case, the whole receptacle is orange.

I looked up sheath current which is the term for currents induced in conduits or other shields.  I had initially thought it was something to do with three phase circuits, and it sort of is but it really occurs any time the shield is ferromagnetic and there is an unbalanced current flowing thru the conductors within it.  In some cases, the conduit may get hot enough to damage or at least reduce the ampacity of the conductors within it.

Learn something new everyday, or in my case, re-learn.

[calexanian]

As far as the lights only working when another light is turned on it jogged a memory.

Here in the states we have split phase for our 240/120 systems and it was allowed to shrink the size of the neutral when 2 120 loads would essentially be in series across the 240 where nothing but the imbalance current would be carried by the neutral. If you lost the neutral or it went high impedance thats when the funhouse lights started. turn the washing machine on and some lights got brighter, and your washing machine motor burns out due to low voltage. things like that. if you totally loose the neutral then the lights would only work in that scenario when both switches are turned on.

[Brumby]

Where I am in NSW, you are allowed to lock your outside meter box/fuse box - but you can only use a padlock supplied by the electricity company.  They give you a key for your padlock while they hold onto a master key.

This stops nuisance attacks by people flipping switches on you while still allowing access to those who require it.

[sibeen]

The phase wiring on that largish switchboard has triggered a distant memory. About 30 years ago was called to a site which had a large (>500 kVA) UPS installed. The upstream breaker was tripping regularly, transferring the UPS to battery operation and it would eventually shut down and transfer to the bypass line after the batteries were drained. I had began happening on a daily basis and the client was rightly pissed.

The problem was a combination of factors. Firstly the electrical contractor had installed the cabling to the UPS in phase groups, as per the photos earlier, rather than the required trefoil arrangement. Secondly the contractor had used steel cable ties to hold down the cables to the earthed cable tray, and thirdly the circuit breaker supplying the UPS had ground fault detection installed and this had been set to the minimum value.

Each cable tie was acting as a mini current transformer and inducing a small amount of current into the earth, as the load on the UPS increased the amount of current induced into the earth also increased. It eventually reached a level that was above the ground fault setting on the circuit breaker and this would then trip.  The cable ties would not have been an issue if the incoming cabling had been in a trefoil arrangement as the balanced phases would have resulted in a cancelled magnetic field around the cables and therefore no leakage currents to earth. Eventually replaced the majority of the cable ties with plastic and the problem went away.

Sounds all quite easy, but it took me about a week to figure this one out.

[GlennSprigg]

Ok... here's another Factory.... (huge automated processing plant).
I had a Chief Engineer above me, (he was non electrical), who could not understand (or be told)
why the main 1,500 amp factory feed was grossly overheating. (Split into 2 parallel sets of cables).

It came down to SIMPLE Ohms Law... :-)
Two 800-amp cables, (each phase), piggy-backed together Lugs, to one side of the huge BusBars.
Unbolted, I calculated about .6 Ohm on one run, and about .3 Ohm on the other runs.  Ok....
Sounds like nothing, but ONE had TWICE the current flow as the other !!!!
(1000 amps in one, and 500 amps in the other). Due to a difference of just .3 Ohm...
They either 'get it' or they dont...

Which connection was not crimped properly then? Did the lug glow or not.

Sorry about the delay mate..... (No, but they were unusable).  There were multiple issues from memory.
The cable runs took different directions/paths due to availability of various concrete 'holes'. (Lengths!!).
Some Lugs were slightly the wrong size for the cable entries !... but here's the main 'NoNo' reason......
Each 'pair' of Phase Lugs were bolted 'Stacked', to one side of the BusBars !!!! (Can't remember 'washers').

I re-routed/shortened some cables, replaced/re-crimped quality lugs with a proper 100-ton tool.....
And re-bolted using appropriate conductive grease with 1 lug "Either-Side" of the BusBars in direct contact.
The problem went away....  I would have discussed alternatives, due to the high requirement/loading, except
that some departments were being moved off site, so the total plant load was diminishing.

[GlennSprigg]

After several visits to a site and workshop investigations on a PC with repeat claims of random characters appearing in a rather comely lady's word processor documents we figured out that moving her copy holder closer to her made the problem disappear as her ample chest wasn't hitting the keyboard when she turned the page.

Sounds just like a common legend. But I'm under the impression that area is very sensitive so it would be hard to miss the accidental contact? Closely related to that was a funny story a friend of mine told me about the time in computer repair class when she accidentally knocked over a box of screws...

I don't know about 'ample breasts', but I was called out to Yatala Labour Prison in Adelaide late one night, to their main control room.
The MainFrame computer had locked up. (Swamped with thousands of calls from a P.C. it seems)...
Turns out someone had a BOOK that was resting on the 'Print-Screen' button on one of the P.C.'s.
The 'MainFrame' had a 'Hissy-Fit' with it's buffers, and locked out the whole system !!!!!! Sigh..

I remember a similar problem from the early 90s from dad's workplace (where the young me was moonlighting as an unpaid computer tech/support here and there). They had an SCO Unix server there, with an UPS for an orderly shutdown in case of a power failure. Both the UPS and the server were in tower cases, sitting on the floor under the desk in the office.

At some point the machine started to turn instantly off, losing power (and some data in the process). That was weird, the UPS was there meant to prevent exactly that and it didn't work? After a lot of testing of both the computer and the UPS everything seemed to be OK. Real headscratcher ...

This went on for a while, with several crashes a week. Until one day someone saw one of the office workers sit down at that desk to do some paperwork - and hit the big button on the front panel of the UPS with their knee in the process. POOF, server down. The protruding button on the UPS was the main shut-off switch for the load (really oddball design, IMO, don't recall the brand of the UPS anymore though). So the UPS remained up, beeping furiously about an apparent loss of power and the server was turned off instantly ... The result was lots of red faces and  a makeshift safety cover out of a piece of acrylic taped over that button. No problems since.

Ha!!!... yea  UPS's are funny things at times....   Some people might not understand that a proper UPS does not 'Switch-In' or 'Change-Over', but is ALWAYS working, and making a new 'Sinewave' from 'DC'. Either from converted AC supply, or from batteries.

This 'generated' Sinewave waveform is 'Synced' to the mains supply, while the Mains is present.  I used to look after large such systems, and while working on, testing, adjusting the UPS, (for prisons/banks etc), I would throw a huge manual change-over switch.
Here's the catch though !!!....  it HAS to be a 'BREAK before MAKE' change-over, as you now cant guarantee it to be in 'PHASE'.
But being 'break before make', you have to throw it REAL FAST, so that all the electronic equipment/computers has to run on their own Capacitors in their own Power Supplies, for a 1/3 or a 1/6th of a second without crashing !!  (Worked 95% of the time  )

[NiHaoMike]

The last idiot who served my house LOCKED the outdoor fuse panel (in my house, the indoor fuse panel handles appliances and sockets, the outdoor unit handles AC/heat pump and emergency heater).

That is of course, illegal. Last time AC technicians came to my house install the new AC, they had to pick the lock.

Locking out means to disconnect power does make it more difficult for thieves to steal the A/C condenser.

[sibeen]

Ha!!!... yea  UPS's are funny things at times....   Some people might not understand that a proper UPS does not 'Switch-In' or 'Change-Over', but is ALWAYS working, and making a new 'Sinewave' from 'DC'. Either from converted AC supply, or from batteries.

This 'generated' Sinewave waveform is 'Synced' to the mains supply, while the Mains is present.  I used to look after large such systems, and while working on, testing, adjusting the UPS, (for prisons/banks etc), I would throw a huge manual change-over switch.
Here's the catch though !!!....  it HAS to be a 'BREAK before MAKE' change-over, as you now cant guarantee it to be in 'PHASE'.
But being 'break before make', you have to throw it REAL FAST, so that all the electronic equipment/computers has to run on their own Capacitors in their own Power Supplies, for a 1/3 or a 1/6th of a second without crashing !!  (Worked 95% of the time  )

Say what? It just doesn't work like that. Maintenance bypass switches are 'make before break' and are interlocked with the UPS so that the UPS must be in bypass before the switch can be activated.  What you have described is just madness, and I have never seen an installation done that way in Australia, and I've been involved with UPS systems for around thirty years.

[rsjsouza]

This stops nuisance attacks by people flipping switches on you while still allowing access to those who require it.

 ? Why? What do they get besides a potentially hefty fine or a few days in police detention?
Well, there are idiots pointing lasers to pilots on duty. And there are idiots went to prison for this.

Blueskull, you underestimate bored teens rummaging around during summer vacation. I had a similar scenario where some passersby simply shut off the water supply to our home - it took a few hours to find out we had no water (in Brazil all houses have a 500 or 1000l water tank) and a few more hours to figure out what exactly happened.

For something that needs $3000 service fee to install that is only worth half of <$2000, I seriously wonder if anyone is willing to steal it, and it would be even harder to sell it. And consider the potential risk of being frostbitten by refrigerant, being shock by electricity and being charged by not only homeowner and PD, but also OSHA and EPA, I don't think it's remotely worth it.

During the foreclosure days of 2008/2009, it was not uncommon to have external HVAC units stolen from foreclosed houses, so there is certainly a market for it (I even had friends have their units stolen as well). In fact, there is a market for everything.

In newer neighbourhoods around where I live, the double-pane windows act as great noise insulators, thus any "hiss" can easily be confused with a running faucet while doing dishes, for example.

[Leiothrix]

? Why? What do they get besides a potentially hefty fine or a few days in police detention?

It's also so potential thieves can figure out which houses are empty.  Turn off the electricity or water and check it again in a day or two.  If it's still off chances are there is no one home.  Especially good in touristy areas where there are lots of holiday homes.

[Cyberdragon]

This stops nuisance attacks by people flipping switches on you while still allowing access to those who require it.

 ? Why? What do they get besides a potentially hefty fine or a few days in police detention?
Well, there are idiots pointing lasers to pilots on duty. And there are idiots went to prison for this.

Blueskull, you underestimate bored teens rummaging around during summer vacation. I had a similar scenario where some passersby simply shut off the water supply to our home - it took a few hours to find out we had no water (in Brazil all houses have a 500 or 1000l water tank) and a few more hours to figure out what exactly happened.

Lol, my dad used to do that shit all the time back in the olden days. Him and his naughty friends would wait untill there was a big party (with drunks usually) then shut off the water. If they didn't notice there went the power.

[rrinker]

Ha!!!... yea  UPS's are funny things at times....   Some people might not understand that a proper UPS does not 'Switch-In' or 'Change-Over', but is ALWAYS working, and making a new 'Sinewave' from 'DC'. Either from converted AC supply, or from batteries.

This 'generated' Sinewave waveform is 'Synced' to the mains supply, while the Mains is present.  I used to look after large such systems, and while working on, testing, adjusting the UPS, (for prisons/banks etc), I would throw a huge manual change-over switch.
Here's the catch though !!!....  it HAS to be a 'BREAK before MAKE' change-over, as you now cant guarantee it to be in 'PHASE'.
But being 'break before make', you have to throw it REAL FAST, so that all the electronic equipment/computers has to run on their own Capacitors in their own Power Supplies, for a 1/3 or a 1/6th of a second without crashing !!  (Worked 95% of the time  )

Say what? It just doesn't work like that. Maintenance bypass switches are 'make before break' and are interlocked with the UPS so that the UPS must be in bypass before the switch can be activated.  What you have described is just madness, and I have never seen an installation done that way in Australia, and I've been involved with UPS systems for around thirty years.

 Of course, you also need to operate the switches correctly - bunch of years ago I was doing work in a client's data center, couple of racks of PC servers plus a pair of HP minis. Electrician was in to do UPS maintenance. Suddenly, the whole room went dark. Someone said "oops". Electrician swore the UPS was in bypass, but pretty obviously it wasn't as when he turned it off it killed power to everything. I ended up spending 24+ hours straight cleaning up the corrupted databases on the Windows servers.