May be something like this:
"Any kind of internal failure or grid failure, e.g. a one or three phase short circuit, in any normal operating state does not lead to disproportional damage of the terminal box."
Measuring the frequency change is not enough.
You can / must look also to the vector shift and voltage change.
It depends on the country (Energy board and regulations of that country) how a islanding situation has to be measured. Good example is the G59 regulations in the UK.
Look for...
