Can 5th harmonic block be used as overfluxing protection?

[veritas]

In many modern transformer diff relays, there is a 5th harmonic block incorporated in the diff algorithm. This allows the user to set a block on the diff element operation if there is a large % of 5th harmonic in the diff current. Most often the presence of 5th harmonic is due to overfluxing of the trfr core. I have a relay which has a 5th harm alarm as part of the diff settings. I am wondering if this would suffice to provide a reliable overfluxing alarm? I am aware that there are more specific overfluxing protection functions such as V/Hz. However, if I do not need to deploy these then I'd rather not.

Now I am also aware that it is the 5th harm component of the diff current and not the actual load current that I am looking at here. My understanding is that under normal load, Idiff is close to zero. However, overexcitation will lead to a 5th harm component in the primary current only, thus this will be reflected in the diff current as well. So from close to zero, the diff current will increase in direct proportion to the amount of 5th harmonic current.

Any thoughts?