 Posted: Wed Apr 27, 2011 8:20 am Post subject: NEED OF LABLES AND COLOUR CODES |
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First off and Second off please read the following lines: EARTHING SAVES EARTHLINGS! On a sunday afternoon I got a call from my friend asking if one gets shock if one touches the ground pin on 3 Pin Plug of a power cord which is connected to a printer. Without giving any room for analysis I said it is preposterous! But when he said his employee got a nearly fatal shock I immediately rushed to his shop, where he showed me the damaged equipment which had fallen out of the table when the employee tried retreiving his hand when he experienced the shock. He supposed to have fallen on the floor pulling down the printer along. Somehow I got scared before touching any of the equip- ment and asked if he has a line tester and as expected he did not have one. At that moment I saw a pendant holder with a bulb hanging from false ceiling. I took out that to test the earthing. As a standard procedure I did lamp test between Neutral-Ground and Phase-Ground. Alas! Bulb did not glow in both cases. Now was a testing time for me. I want to find out the correct limbs, Phase & Neutral without taking a risking of touching and went out to shop for a line tester, on return I thought of taking a coffee break and over the coffee I pondered about the situation. Yes! there was lightning bolt of realisation. The problem boiled down to: The printer was connected with a power cord and the printer inturn connected to a DTP PC which was switched off. Why the boy should get such a shock? Yes! The PC though not switched is plugged into the wall socket which has faulty earthing (later on found to be Phase), and as with SMPS signal ground and power ground are shorted and when the machines are interconnected the faulty current of one m/c flows into the other. Hence whatever the boy considered as earth pin turned out to be having Phase. But I wondered who would carry out such dangerous and careless wiring. Anyway I went back to the shop with line tester, located phase pin. Now I took the test lamp and connected between the phase and ground the lamp did not light but as I connected the lamp between neutral and ground there it was, lamp was lit. I wondered if I had mistakenly connected neutral and ground at first and then Phase and ground. I did a double check with the line tester again. It was correct and I had located the phase pin properly. The lamp was lit when I connected it between Neutral and Ground! Can you Imagine? One thing came to mind if the shop is connected with multi phase and got a negative answer from my friend. The line tester tested positive on the supposed to be Ground pin. But I wondered why did this not happen when I did lamp test before going out of the shop. I asked my friend if he switched on/off some switches, he said yes he switched on all the switches in the process of locating switch to his photocopier machine. Now with lamp connected between neutral and ground I asked him to put off the switches one by one. There it is! One of the switches did the magic. The lamp went off. As I had seen enough of wrong wiring in the shop I could not risk myself working with live wires and switched off the mains, opened the panel and saw to my horror the ground pin from the wall socket was landing on this switch which was connected to phase instead of ground. Further investigation I found that whoever did the wiring did not follow any colour code but used yellow, red and blue wires at his will and would have got confused while giving finishing touch. He had used blue wire for phase for some points and for Ground for some points. The entire staff would have been electrocuted had there been earth leakage in any of the equipment. But I wondered why this particular operator alone got the shock. It was obvious, the PC and printer in question alone were on that wall socket with the faulty connection. After isolating the problem I asked my friend to get the whole shop rewired with proper colour coding, but whether or not you call it coincidence he had to shift the shop elsewhere before the wiring was corrected. Whatever happened to the next occupant we should find from Chitragupa's records. My friend was taken aback at this horrible wiring he even forgot to thank me for the job I did, but asked me how much I should paid, but I refused because I was happy that I saved lives of my friend and his staff. |
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| Posted: Thu Jan 29, 2009 6:14 am Post subject: |
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Well, how about if you're out sick one day and some unsuspecting geek comes along and uses it? |
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| Posted: Thu Jan 29, 2009 5:05 am Post subject: |
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| First off, how am I going to die? And second off, who ever said I wasn't going to label it? |
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| Posted: Thu Jan 29, 2009 2:07 am Post subject: |
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| What happens to the next guy if you die? I think that's why it might be good idea to label. |
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| Posted: Thu Jan 29, 2009 1:22 am Post subject: |
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| That did the trick! Not sure why it wasn't listed in the specs, but once I cracked it open I noticed the ground bond, so I disconnected it and it worked perfectly. I'm the only one who uses it and only use it for troubleshooting in my shop, so it shouldn't be a problem leaving it disconnected. Thanks for all your help. | |
| Posted: Fri Jan 23, 2009 10:24 am Post subject: |
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| Yes, if you only are to use the transformer for repair shop/laboratory work, then it is ok to disconnect the ground bond. But in cases where a such isolation transformer is used to protect, for example, the power supply for a server room from transients, then the ground bond should be in place. It might be prudent to mark the transformer with a big sign stating that the ground bond is removed. |
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| Posted: Thu Jan 22, 2009 3:12 pm Post subject: |
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| The transformer I use lists 100% isolation under it's specifications. I have seen this spec on some before: Secondary neutral-to-ground bonding eliminates common mode noise, providing an isolated ground reference for sensitive equipment. I'm assuming that's what you are talking about? If that were case, could the ground bond be safely disconnected? |
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| Posted: Thu Jan 22, 2009 1:42 pm Post subject: |
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| I have come across some isolation transformers where the secondary is not floating, but it has a mid tap that is connected to ground. Beware of those transformers! They are not intended for use in repair shops. | |
| Posted: Thu Jan 15, 2009 3:26 pm Post subject: |
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| Thanks for the quick reply, that's all good info. The scope's ground plug is still intact, the mains wiring is correct, and it is not plugged into the same transformer as the power supply. That still doesn't mean I'm not doing something wrong though. I will still double check my setup just to make sure. The power supply is out of a Sylvania 20" LCD TV, LC200SL8. The replacement parts were pretty much an identical match and are mounted and tied in correctly. The fet is in a full plastic casing, so no chance of an insulator problem there. |
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| Posted: Thu Jan 15, 2009 5:48 am Post subject: Re: The "hot side" |
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| I've seen a similar effect twice. One tech had cut off ground pin of scope's power cord and had plugged it in "backward," and another tech had his scope and the item under test plugged into the SAME isolation transformer. One of the "Variacs" we have at the shop is NOT isolated (must use a seperate very large isolation transformer). I'm presuming appropriate types and mounting on replaced part(s). Of course, there still could be a bad part in SMPS. If I knew what you were working on (Brand/model, etc.) I might be more help. |
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