ECN Forum Forums General Topics of the Trade General Discussion Area wrong polarity

does anyone have any stories of consequences of hot/neutral hot/ground neutral/ground reversals? motors? appliances? injuries?
not morbid, but seems like you learn things better from mistakes.
C

No morbidity in this, but I was present when a 10 Hp DC motor coupled to a generator for a lab experiment had its field removed. The DC motor immediately increased it RPM from about the 3,600 it was at to around 15,000 in around 2 seconds. The room shook quite bad. If it wasn't for the professor hitting the emergency kill switch to the supply, the motor would hav blown apart causing massiv injuries to the people in the room. Not to many people realize how dangerous DC motors are if not properly controlled.

On my very first job in the trade there was a 120/208V panel in a kitchen area that got energized with a phase conductor and neutral reversed. Luckily, the only casualty was a couple of sign transformers. Unfortunately I have seen the same thing a few more times since.

The guy that lives three doors down asked me about a tree limb hitting the service drop. I went over to look and the guy wire/neutral was pulled loose at the house. He said it had been that way for a week. I explained the dangers involved and put a temporary jumper on it until the power company could get out to fix the drop. He said every time they took a shower or washed their hands "the water tingled". It still makes me cringe.

[This message has been edited by wolfdog (edited 01-09-2002).]

Cindy,
Last months issue of EC&M magazines Forensic Casebook feature told an incredible story of a series of unfortunate co-incidences that resulted in the death of a mother of young children. Maybe Joe can help you acquire the article.

I remember hearing of a case here a few years ago in which an "electrician" had done some kitchen installation work -- I forget whether it was a washer, dryer, waste disposal or whatever. Somehow he managed to hook the stainless-steel sink up to the hot side of the supply (For crying out loud: How??!! ).

The owners reported tingles and mild shocks, he was called back twice and told them nothing was wrong. Third time was definitely not lucky, as someone touched it barefoot and was killed.

If I recall correctly he was charged with "causing death due to criminal negligence," or whatever the proper legal wording is.

***

Another accident which is sort-of polarity related was reported from a telecoms facility when I was with BT in about 1984. Like many big plants they had their own generators, and the control gear incorporated synchronization circuitry to allow parallel operation.

Apparently someone mis-wired a unit so that two gennies synched up 180 deg. out of phase. I understand they had to pull bits of engine out of the roof.

[This message has been edited by pauluk (edited 01-10-2002).]

most things in a house will run on reversed hot/neutral won't they? just wondering what would happen in a typical home. don't think i want to experiment to find out i mean besides energizing metal.

btw, tonight i was using a tanning bed at a salon and felt what i thot was a sharp edge on the bed frame, felt it on previous occasions too, but tonight realized it was a tingling sensation and not a sharp edge. the beds use 24, 105-watt fluorescent bulbs and it is wired by flex and controlled by a timer in the other room. any ideas on eliminating the tingling? maybe it's not grounded properly? C

If the metal frame isn't grounded, then you may well be getting a practical demonstration of capacitive coupling. (See? It's not just theory! )

Yes, a great many normal household appliances will work perfectly well with hot and neutral reversed. Over in Continental Europe many countries use a standard appliance plug which is reversible (even the grounding type).

Many old radios/TVs (both in the U.S. & U.K.) were "live chassis," meaning that one side of the cord went straight to the chassis. They were designed so that no part was exposed to touch, but it was still best to ensure that the chassis went to neutral. It's also why we use an isolating transformer when servcing this type of equipment. Although live chassis went out of favor, it has made a slight comeback in modern TV switch-mode power supplies.

With swapped connections, any single-pole switch will leave the bulb, element, motor or whatever hot when off. There's also the point that an in-line fuse (as used in mUch line-powered electronic equipment) will end up in the neutral, so the house breaker will be the only protection against ground faults.

how about computers, stereos, printers, monitors? and what would a garbage disposal or dishwasher motor or washer motor do when hot and neutral are reversed?

think i'll slip an amprobe into the tanning room next time and take some readings... not worried really, just curious

As we use non-polarized plugs here in Austria I can say that I never got problems with any kind of appliances. My printer is imported from the US (nice thing: The printer was designed to work on both 120 and 230V, but the power cord was rated for 10A 125V, the previous user just fitted a new plug). So in everyday use it makes no difference.
The only thing I ever saw that wouldn't work if polarized incorrectly was a hardwired gas heater.