Quote Originally Posted by brman View Post
Care to explain that? Because, like voicecoil I use an isolation transformer when developing 240V switching devices (in my case 3 phase pmsm drives) specifically to prevent myself getting an electric shock. That and stopping me melting 'scope probe leads anyway.... ;)
Yes it does need qualifying.

If there is no earth reference connection anywhere following the transformer and you touch a live connection then that side of the supply will float to your potential and no current will flow. Same with your scope probes, the scope floats to the same potential as that side of the supply (or vice versa) and there is no problem.

The problem arises when one side of the supply is connected to exposed metalwork either deliberately or by accident such as touching a metal bench or via another piece of equipment plugged into a dis board fed from the transformer. This creates the risk of you contacting both sides of the supply at once.

You are quite right that an isolating transformer can protect you in certain circumstances but most electronic workshops have strict guidelines about the use of isolating transformers including only ever plugging in one piece of equipment at a time, covering metal benches with rubber mats etc. My concern was if the whole CNC machine and other gear was going to be isolated by the transformer something somewhere could be providing an earth reference and that creates a danger.

The primary danger I was concerned with is the fact that a transformer negates the protection provided by an RCD. If you connect yourself between a standard mains supply and earth it's the leakage to earth (which creates a small difference in the live and neutral currents at the RCD) that is picked up by the RCD and that trips the supply. Put a transformer downstream of the RCD and connect yourself across it's output and there is no earth leakage back to the mains supply so the RCD won't protect you.