>> If the chassis is connected directly to live (quite possible on old TV's),
>> and you hold chassis and an earthed scope, you have full mains directly from
>> hand to hand - a path straight across your heart :-(.
>
>Yes.  And "floating a scope" (i.e., removing its ground lead) is not good
>practice.  As soon as you try to use some other instrument like a DVM or
>signal generator and forget that it too is grounded you'll have the same
>problem as with a grounded scope.
>
>The correct solution is to put the TV on an isolation transformer.  This
>should always be done when working on anything that might have a hot chassis.
>

There are lots of instances where connecting CRO ground can cause problems
because the other equipment can't easily be isolated. eg telecom lines etc.

Best solution is use a battery powered CRO if available.
And use an isolating transformer for safety, plus earth leakage detector.

By the way is use of RCD trips mandatory in the US? It is in Australia.
The RCD compares the current flowing in active and neutral and if their
is any imbalance (caused by earth leakage for example) the device trips
within a few ms.


Ray Gardiner, Shepparton, Victoria 3630,  Australia,   ray@netspace.net.au