This would depend on the type of equipment it was isolating and the specification of the ESD, it must isolate both the phase and neutral conductors of the circuit

Even if the isolation is "correct" my view is that a label is not a secure way of ensuring it remains isolated for the required duration and a lock is essential. It is possible of course that the power source is also locked out further up the line and the E-Stop is a secondary isolation.

I tend to agree that this isn't likely to be a safe method of, or probably not by definition - isolation. One of the main issues is whether the power could be restored 'intentionally' while any maintenance is still ongoing? Some form of robust locking off system would seem appropriate.

Vinod - you need look no further!!!!

Emergency switch is not a short cut to isolation, Lock out tag out should be used on all machinery,

Pressed the wrong button! Sorry. Oh to be able to edit... Also the OP does not say what was being undertaken, why the isolation was required, what the tag was for etc. As my opening line says the EM Stop is NOT isolation of anything. It merely disables certain aspects of the machine control system. If isolation is required then isolation must be done, and as far as electrical works go then that means disconnecting all live conductors from the source of energy. This may not be at the first energy inlet at the machine, i.e. the main machine isolator, it could be at a downstream isolation device which as long as it meets the requirements would be OK. This would depend on the item of equipment being maintained.