I’ve recently seen a motor wired directly from the local isolator to the motor terminal box in 3 core 1.5mm cable. The gland has been made off into a threaded hole in the motor terminal box and at the isolator end directly connected to the incoming swa from the MCC by means using a flylead. My question is is this a sufficient for earthing the motor. My understanding is you can use the armour as a cpc but wasn’t sure if any additional “fly leads” should be fitted at the motor end?
Threaded holes are usually perfectly good for earth continuity - that's how metal conduit is joined after all. Plain holes in thin walled steel enclosures can be tricker - it can be made to work but needs to be done carefully - e.g. making sure all paint etc is thoroughly removed and is properly tightened etc. so is seen as a bit riskier and so many prefer a banjo, ring crimp (or a proprietary earthing nut) and flylead to make sure in that situation. Doubly so where the cable is glanded onto a removable gland plate where you'd have a second (usually) painted junction (between plate and rest of the enclosure) to contend with too.