At 10:52 -0700 6/14/09, Ernest L. Gunerius wrote: >I always made my own Power Strips with Four Way Metal Wall Boxes with >two dual Receptacles and Round heavy duty three conductor stranded >#10 AWG outdoor rated Cable.
We did that all the time in a well-equipped physics laboratory that was part of the US Navy. Everyone within a mile or so was thoroughly competent to handle things electric and children could never get by the Marines at the gate. But the lawyers forced all such units to be replaced with smaller commercial power strips. The reason: There was a fear that the Navy would get sued because someone stuck a small conductive item through the unused screw holes in the metal boxes and got electrocuted. Now, on topic. . . Here along the front range in Colorado we do get lightning. You can hear a stroke and still have time to look out the window while the current is still flowing from the sky. Florida has more strokes but I'll bet we have more total electrical current. Perhaps it's because the power company is well aware of the problem but the strokes rarely cause a voltage surge on the power lines. They occasionally cause a sudden and complete power failure when a circuit breaker trips or a big transformer loses a primary wire because it melted but that's no worse than turning off a switch. When the power company turns power back on is when surges are possible. They often do things like feeding a row of dwellings from a different phase while they repair a broken wire. That can cause a voltage drop that sometimes gets over-corrected by a generator that senses the change in load. We learn to take advantage of Apple power supplies that will shut down but not restart until an operator says to. It's best, after a loss of power, to wait until the power goes off and on again at least once before restarting things. The big problem has nothing to do with the power company. It's ground currents in planet earth. Lightning strokes return thousands of amps from the place they strike to what we think of as ground potential. Those currents find regions of damp soil to travel in and it's quite possible to have kilovolt differences between ground rods a few meters apart. The power company goes by a code that says there should be a single ground close to the power input wires but it also grounds the center tap of pole mounted transformers with a wire buried as the pole is set. What happens to that difference in potential between the pole ground and the household ground which can easily be 50 meters apart? Worse is what happens between the power company ground and the shield of that co-axial cable that connects your cable TV and internet connection? What about the ground on your Hughes satellite antenna out in the back yard? It has its own ground rod. What about the telephone ground which is connected to a cold-water pipe and has an effective connection to earth wherever it is buried? (For experts, don't forget that grounding wires of any length have inductance that is more important than their resistance to the rapidly changing ground potential.) We have lost television parts and ethernet hubs because of ground currents but never a computer due to surges. One of these days I'm going to install fiber optic cables for the household network. We don't do it but it would be appropriate to disconnect those CAT-5 ethernet connections all over the house when a storm approaches. A telephone connection with its ground wire probably should not be left plugged in to an otherwise grounded computer. -- --> The message came to Abraham that he would beget a son. Sarah, who was behind the door, laughed. <-- --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed Low End Mac's G3-5 List, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to g3-5-list@googlegroups.com To unsubscribe from this group, send email to g3-5-list-unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/g3-5-list?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
