Sheet support for a plasma machine?

[m_c]

Could you clarify that a bit, are you saying your table is a sheet of metal that is connected to plasma ground or something else?

I only use my plasma manually, but have at times just clamped the earth to whatever sheet I've just cut the end of, then recut bits on top of that sheet. I've never had any issues with it not starting, but then mine has a high frequency/pilot arc start, so may be more tolerant of a less than ideal ground.

[m_c]

The point was, do you connect it to the work or to the slat-bed?

Or both?

Generally the bed, as you don't any leads/clamps in the way of what you're cutting.
Personally, I'd earth to the bed, and then use a jump lead from bed to metal if there were any grounding issues.