Reply To: Dead DRO

[SillyOldDufferModerator @sillyoldduffer]

The brass earth terminal is almost certainly an EMC earth, not a mains safety earth.   It’s purpose is to shield the electronics, done by grounding internal metal shielding by the shortest possible route to a separate earth.

Using the mains earth for EMC is bad practice because the wiring runs all round the house in parallel with live and neutral.  At EMC frequencies:

• the earth wire is capacitively and inductively coupled to the live and neutral wires, so outgoing muck is transferred into them, not good!  At the same time incoming muck on Live and Neutral is transferred into the machine through the EMC earth.
• the mains earth and the machine are so distant that the earth wire acts an aerial, broadcasting muck rather grounding it.
• If either of the above occur, better to disconnect it.

Bottom line, don’t mix radio and electric supply earths!  If an EMC earthing terminal is used, connect it to a nearby radio earth.  A spike is the bare minimum. Supply earths don’t need to be particularly good, a few ohms at DC / 50Hz AC is acceptable.  Radio earths are in a different league, ideally much lower resistance, and spread over a large area near the surface to sink high frequencies.  As a good radio earth is a lot of bother, EMC earth terminals are often left disconnected, maybe unwisely!

When a radio earth is installed check it doesn’t conflict with that provided by electricity supply company.  The rules vary widely!  French methods differ from the UK, and the US is different again.   When in Rome…

Dave