On Sunday 26 Sep 2004 1:52 am, Erylon Hines wrote: > On Saturday 25 September 2004 03:37 pm, Richard Urwin wrote: > | I think "Which" was told off and had to publish a retraction not so > | long ago for publicly giving that bit of advice. > | > | Some things to bear in mind: > | 1. If there's an earth fault the live wire is connected to you. > | 2. The switch is in the live wire. If, like I did, you have an > | extension cable with the live and neutral swapped, then the live > | parts are still live. It may still be a problem with a properly > | wired connection if your neutral is at a different potential to > | your earth. OK, not such a problem with a PC because the PSU is > | enclosed. > > This in itself is a code violation. Why would anyone swap the hot > and neutral?
In my case it was an honest mistake when building an extension cable, and I told my father off for it. > Anyone that would do this shouldn't be doing anything electrical at > all (no offense meant, but everyone should test their equipment > before they use it--your life depends on it). There's a lot of people who's last words were "what damn fool did that?" > And remember, Never--Ever--switch a neutral. Unless, of course, you also switch the live. > The neutral and ground go to the very same place. Usually yes. In the UK it's possible that they don't. Earth may go to an earth spike closer to you than the one the neutral goes to. That's only likely in a rural location or a big site. > | 3. Anti-static straps have 1MegOhm resistors in them. Without such > | they are considered serious health and safety violations. Since > | every piece of anti-static equipment has that, the actual > | resistance to earth of professional gear is usually several meg > | ohms. > > For the anti-static devices, power supplies, etc.--yes. Electrical > codes (at least in the U.S.) require that the case be bonded directly > to ground. That doesn't help if there is a wiring fault, either accidental or man-made. The case is bonded directly to (what should be) earth; the repair-man should not be. A few megohms keeps him safe from bad wiring or grabbing the wrong wire, but conducts static away safely. > In such a case, an external jumper from a known good ground would > need to be applied to the metal frame of the case when the unit is > unplugged. Otherwise, the box is floating with potential for static > discharge, unless the repairman is floating also. This probably > won't be the way it is outside of the lab. I'd say it was only likely outside the lab. I do my work on the dining-room table with the PC on a blanket and me with rubber-soled shoes and carpet. Great way to pick up static, but no earth connection except for the cable. The best way would be a mains plug with only the earth wired, leading to a junction box. Every wire out of the junction box has its own megohm resistor at both ends. One goes to the case, another to the man. It would cost pennies to build. This is a public forum; you and I know what we're doing, and when we can bend the rules safely enough. Most of the time with PC work a direct wire is safe enough, but I'd be willing to bet that you wouldn't use a direct wire if you were replacing the PSU fan for instance. Safe enough is not safe. With a world full of people, million to one chances come up nine times out of ten. -- Richard Urwin
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