Check how frequently TCNs are being generated, correlate them with interface error counters (make sure there are no full duplex/half duplex mismatches etc. ) and start to introduce different STP guards as required. When you are dealing with spanning tree it's best to draw a small picture of the network and write down the interface roles for each switch (designated, non-designated/alternate or blocked). Because STP loops can bring down an entire network, it is important to understand exactly what is happening and quickly implement an appropriate solution, as in most cases, all you might have is nothing more than a few minutes. Misconfigurations can lead to performance issues, network instability, and even outages. You can verify this using the “show spanning-tree”. Doesn't the trip unit tell you if it tripped on phase or ground? I'm not familiar with this particular trip unit, but normally, there are lights or targets to indicate cause of trip.
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