VFD (?) tripping RCD

[russell]

[*]After the first fault I measured the voltage between machine frame/spindle at various points and earth. Less than a volt AC and DC in all cases. Machine frame and spindle are both earthed.

I would hope so! The IEE recommend (0.1 + cable resistance) ohms earth continuity ,say 0.125 ohms. To get a volt of difference you will need 8 Amps of earth current! If you can, borrow an earth continuity tester and make sure the resistance between parts is less than 0.1 ohm.

[*]Measured the voltage above with the spindle running, same result. If I disconnect the earth wire I get 180VAC between frame and earth...so I didn't do that for long!

Although a potentially unsafe practice this shows no problem. Assuming you are using a digital multimeter as opposed to an old AVO, the input impedance of the meter will be about 10 MΩ. So you are measuring a current of only 18 μA.

something to do with filters in the VFD providing a path to earth - which then trips the RCD

All VFDs produce high levels of RF. In order to comply with the EMC directive they are fitted with RF filters on the mains input. These have relatively high capacitance to earth which results in a high reactive current to earth and can trip RCDs.
Try measuring the leakage current of the VFD on its own. If that is the problem connect it via a separate, lower sensitivity RCD but make sure your earth bonding is good!

Russell.