HT Motors

Not if the motor can withstand the arc and ensuing destruction caused by an unprotected internal fault.

These things can be very expensive in terms of replacing equipment and catching up with lost production, so it depends on local economics, really. Also, not fitting the protection can be seen as career-limiting behaviour, particularly in the aftermath of the ocurrence of an unprotected fault.

So it's hardly essential to fit them, though it would be irresponsible not to, perhaps?

"... so it depends on local economics, really."

There's the heart of it right there. Differential protection is expensive and runs an increased risk of nuisance tripping if not done correctly, which can cause losses from unnecessary down time. This needs to be evaluated against the risk of additional and/or collateral equipment damage (and associated down time from that) which might have been prevented or ameliorated had DP been used. It requires a cost / risk / benefit analysis to be performed every time.

What's an HT motor?

HT = High Tension

3.3 kV or 6 kV supply voltage

Do a cost-benefit analysis & then decide.

Yes.

I agree with electricalexpert65. Generally motors of size 1500 hp (or 1100 KW) or larger are installed with differential protection.

- MS