https://www.ebay.co.uk/itm/144148210...1&gad_source=1
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The product description says:
"Normally closed specification technical parameters
Output mode: BNC (normally closed)"
Which implies that it doesn't depend on a grounded tool/spindle but has a contact which opens on tool touch. This would make sense as it would be compatible with most other probes/setters and machines. Why it says "BNC" I don't know as the connector is nothing like a BNC being a screw lock 4-pin type.
Thanks for the reply , that makes things a lot clearer to me now , they operate different from a touch plate.