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What is the definition of the term OSN on micrologic electronic trip units.

OSN (Over Size Neutral) means that the neutral protection is 1.6 times greater than the phase protection.
 
This type of trip unit is used if there is a high level of third order harmonics and multiples thereof (orders 9, 15, etc.).
These harmonics are added together in the neutral and create a high zero-sequence current.
 
In this case, use of the device should be limited to Ir = 0.63 x In, with a maximum setting on the neutral of 1.6 x Ir.
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