Design Con 2015
Breaking News
Blog

Why you should always pay attention to differential power supplies

Blog
10/14/2010 08:02 PM EDT

 18 comments   post a comment
NO RATINGS
View Comments: Newest First | Oldest First | Threaded View
Guru of Grounding
User Rank
Rookie
re: Why you should always pay attention to differential power supplies
Guru of Grounding   10/21/2010 1:35:48 AM
NO RATINGS
It amazes me how many young engineers are ignorant of how AC power is delivered and distributed in buildings. I teach such things to folks who install sound and video systems. The most common myths are 1) it's OK to defeat the safety ground (you know, the 3-to-2-prong adapters you get at the hardware store) ... they think that's what those adapters a for! and 2) that a ground rod will serve the same purpose, which it decidedly will not. Dozens of folks have been killed because they believe these myths!!

Ajax2
User Rank
Rookie
re: Why you should always pay attention to differential power supplies
Ajax2   10/18/2010 8:51:23 PM
NO RATINGS
While in the Navy during the Vietnam era, I was stationed at communication station that had been a WW2 airbase. Our communications buildings were converted airplane hangers and a fair amount of old equipment was still stored in the "attics". Someone found a WW2-era Morse code transceiver in an attic and set it up to see if it would work. They didn't get too far because the key was floating between 200 and 205 Volts and not 0 and 5 Volts. Our division officer was a crusty, profane, impulsive old guy with a bad temper who had used just this type of equipment in WW2. When he saw it, he rushed over to try it out. People were trying to warn him about the high voltage on the key but he ordered them to shut up. He sat down in a steel chair in front of the transceiver, turned it on and hit the key, giving himself a nasty jolt of electricity. After some yelling, followed by a few minutes of discussion, it was decided that a wooden chair might solve the problem and, indeed it did. Happy that he could now probably use the transceiver, our division officer tried the key again did not get shocked. Sighing in contentment, he reached down, picked up the steel headphones and put them on. Before anyone could stop him, he hit the key again. He survived with only his dignity severely injured.

David Ashton
User Rank
Blogger
re: Why you should always pay attention to differential power supplies
David Ashton   10/18/2010 12:45:39 AM
NO RATINGS
Anyone care to hazard a guess as to how many meters have been rendered unserviceable by this technique in the course of human history?? Millions???

WKetel
User Rank
Rookie
re: Why you should always pay attention to differential power supplies
WKetel   10/17/2010 1:00:28 AM
NO RATINGS
At a previous place of employment we had an even more exciting experience with a portable digital multimeter. The lab supervisor grabbed the meter off a technicians workbench and attempted to measure between the phases on a 480 volt circuit. Unfortunately for him, the last person to use the meter had been measuring current, and so the leads were plugged into the 1-Amp current position, which has an internal resistance of about 0.1 ohm, and has a fuse sized to protect the shunt. The fuse had a 250 volt rating, normally adequate for most applications. So when he put a direct short circuit across the 480 volt lines, the fuse popped and kept conducting. The meter leads partly evaporated with a flash and loud bang. The meter ceased to function, and the case was partly melted. The lab supervisor had only a small burn on his hand, as the bang had caused him to jump backward. From that time on, it was official policy to check the meter lead connections whenever you needed to use the meter.

David.Bell
User Rank
Rookie
re: Why you should always pay attention to differential power supplies
David.Bell   10/16/2010 6:59:48 PM
NO RATINGS
Another, very recent, errant current story: This week, several of us from work were on travel, staying in a hotel. Thursday night, our youngest engineer decided to take a swim in the hotel pool. Friday morning, he related the following. After enjoying the pool for a while, he swam under water from the deep (lighted) end to the shallow end and steps. As he approached the steps and submerged railings, he noticed the lighting flickering rapidly, as if someone had jumped into the far end. He looked back, and not only was no-one there, the flickering stopped. When he looked towards the steps again, the flickering restarted, and he realized it was his facial and orbital muscles trembling! He reached out towards a stainless steel railing and felt his arm muscles quivering and skin prickling. He called to a couple other young men in the pool, to see whether they experienced the same thing, or if he was going nuts. When they felt the same tingling and trembling, they all made a quick exit. He reported it to the hotel Friday at checkout. It would be interesting to see the failure analysis! High-, not low-voltage submerged lighting. Defective or NO GFCI. I hope they have a lot of liability insurance! Dave

Etmax
User Rank
Rookie
re: Why you should always pay attention to differential power supplies
Etmax   10/16/2010 8:04:33 AM
NO RATINGS
Electrocution is the stopping of life (determined by a stopped heart) by any type of Courtesy of Wikipedia: electric shock. In the vernacular, the term electrocution is used to mean: * death, murder or suicide by electric shock * deliberate execution by electric shock, usually involving an electric chair; the word "electrocution" is a portmanteau for "electrical execution" Electric shock does not mean electrocution.

Stargzer
User Rank
Rookie
re: Why you should always pay attention to differential power supplies
Stargzer   10/15/2010 11:17:48 PM
NO RATINGS
Wonder how long it would have taken the chain to reach incandescence? Permanent reminders: we had a DVOM with a melted probe from crossing a hot with a neutral while checking an outlet; 110V Hot-Neutral & Hot-Ground, 208V Neutral-Ground! I allowed as how we'd better have them correct it before IBM connected anything up to it. Electricity can take some strange paths when trying to return whence it came. Years ago we had cable TV installed but they didn't bury the line so I just had to be careful when I cut the grass. Another company bought out the first and did their Due Diligence and went to bury the cable. The subcontractor showed up one day and started digging with his power trencher, and my wife, who was home with the children, told him to be careful because the underground power lines were right where he was digging. His reply: "Miss Utility says it's alright to dig because the lines are all aerial." My wife asked him, "Do you see any wires in the air here?" Our power and phone were buried. She went back in and shortly the electric went out. He was trying to connect the cable to the connector and it was drawing sparks. "I could have been killed!" he said; I thought it was sad that he still had a chance to reproduce. The cable company sent Miss Utility to [name] Avenue, about 10 miles away with aerial service; we live on [name] Road. Dimwit should have pondered the implications of the lack of aerial wires. As I said before, it's a shame he's still muddying the gene pool. Root Cause Analysis: he was too deep and the electric company too shallow so he sliced the neutral feed; half our circuits dropped down to 10V and the other half surged to 210V as the power tried to find its way to ground. It blew out many things, including the 24V transformer for the the fossil fuel kit that adapted the new heat pump to our oil furnace. We ended up sleeping in front of the fireplace that night. The electric company took the hit since they were too shallow.

More Blogs
There's never been a better time to buy a big-screened phone. Here's how the Google Nexus 6 and the Apple iPhone 6 Plus stack up.
Google may not give much thanks for the gift Europe is about to hand it on Thanksgiving this year.
The technology that helped resurrect the life of Stephen Hawking after the physicist was stricken by Lou Gehrig's disease.
Today’s supercapacitors are being used to replace rechargeable batteries in applications such as electric/hybrid vehicles, wearables, and more.
Engineers and innovators are central to the slow but ongoing economic recovery, a top economist told a recent gathering of semiconductor executives.
Radio
LATEST ARCHIVED BROADCAST
EE Times Senior Technical Editor Martin Rowe will interview EMC engineer Kenneth Wyatt.
Top Comments of the Week
Like Us on Facebook

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)
EE Times on Twitter
EE Times Twitter Feed
Flash Poll