In message <4.1.19990216125710.0099c1b0@mail>, Gerhard Fiedler writes >At 23:37 02/15/99 +0000, Nigel Goodwin wrote: >>In message <4.1.19990215113455.04664ed0@192.168.0.1>, dave vanhorn >> writes >>>Those of us who want to live past next week, use an isolation transformer >>>for the equipment being tested, and let the scope ground stay right where >>>it is. >> >>It's common practice to disconnect the earth wire to oscilloscopes, it's >>far safer than leaving it connected, if connected you have a potential >>230v (in Europe!) across your hands when you try to connect the earth >>lead from the scope to the item under test. By disconnecting the earth >>lead you remove this serious hazard, this only leaves the unlikely >>breakdown of the scope mains transformer as a potential hazard. > >why do you have a potential across your hands when the item under test is >connected to mains through an isolation transformer? i thought (like dave) >this was the safest way to go, and disconnecting the ground on the scope a >work-around for those not concerned enough to use an isolation transformer... You don't have a potential across your hands when the item is connected through an isolation transformer, but if you should ever have occasion to use the scope on an item where you don't have an isolation transformer it's a lot safer if the earth lead isn't connected. TV/Satellite repairs are particularly bad, as they almost always use switch-mode PSU's, so it's quite common to want to connect the scope to the 'live' side of the PSU - again via an isolation transformer it's no problem with or without the scope earthed. BTW, talking TV's, going back a LONG! time they used to use half wave rectifers (usually a metal rectifier), this meant that when the mains lead was inserted correctly the chassis was connected to the neural wire - however if the lead was inserted incorrectly (50/50 chance with an old two pin plug) the entire chassis of the TV was connected directly to the incoming live connection!. One final point, what does having the scope earthed protect you against anyway?. Is there anything else apart from breakdown in it's internal mains transformer), or some kind of short between incoming live and the case. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : nigelg@lpilsley.demon.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | | Chesterfield | Official site for Shin Ki Ju Jitsu | | England | | \--------------------------------------------------------------/