On Wed, Mar 13, 2019 at 03:17 PM, Nicholas Keller wrote:
Is this what you describe as ¡°floating the 2247A¡±?
Hi Nicholas,
Glad you bought an isolation transformer!
My advice to "float your 2247A" actually was very badly formulated. I meant to say something like "float the hot (primary) circuits of your 2247A", meaning the circuits directly connected to the mains *and* I should have added *not* to float your testing equipment (like your testing 'scope).
Many people disconnect safety ground on their measuring 'scope but that's *very* bad and dangerous practice.
Always make sure all instrument chassis are connected to safety ground if at all possible.
Your 2247A is an example where you can get the protection of your isolation transformer and still connect the chassis of both your testing 'scope and the device under test (DUT) to safety ground. This is because T2204 takes care of isolation of all low-voltage circuits from the mains, *including* the 'scope's chassis, which is connected to low-voltage/ secondary ground.
The isolation transformer makes everything directly connected to the two mains input wires (the hot circuits) float i.e. galvanically disconnected from "the rest of the world".
Since the 2247A's chassis is isolated from the hot circuits, it's safe and actually a good idea (safe) to connect that chassis to safety earth. The ground connection on your isolation transformer always connects that transformer's housing to safety earth and the ground wire you're mentioning is supposed to connect that earth ground through to the 'scope's ground (3rd pin in 3-prong adapter).
If you connect things this way, your testing instruments will remain connected to ground, as will your 2247A's chassis.
Only the hot (primary) circuits of your 2247A will be isolated from mains via the isolation transformer on their input and from the 2247A's chassis through T2204 on their output.
Four final notes:
1. In most instruments, there are one or more capacitors between the mains connection(s) and chassis. These are there for EMC reasons but they may present a high-impedance voltage on the chassis (of Vmains/2) if safety ground is disconnected in that instrument.
2. Providing safety earth continuity to your DUT (where possible) provides protection in case of leaks or one of the capacitors as in 1. short-circuiting.
3. In most (older) instruments with linear supplies, like the 465 'scope, providing this earth continuity while using an isolation transformer won't do much good. In those instruments. By all means use an isolation transformer but skip the earth continuity.
4. Be aware that, though disconnected from mains, hazardous voltages still exist inside hot circuits connected via an isolation transformer!
Good luck with your repair!
Raymond
Raymond Domp Frank