20 November 2020
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.