The over-voltage protection component ground reference must be the same as the PHY ground reference and should not be the same ground reference as a grounded RJ45 connector or connected directly to earth chassis. This would remove the isolation barrier provided by the coupling transformer and not comply with the IEEE 802.3 isolation requirements. This type of application is typically exposed to CDEs (cable discharge events), ESD (electro-static discharge)  and EFT (electrically fast transients) events only, so the suggested protection provides sufficient ESD and EFT protection but minimal lightning surge protection. Since this is not subjected to possible power fault events, no fusing is recommended for the line side.