Hi, i have design an Injector with LTC4279.
The Main Supply is Isolated From Netz with normal and regular Toroidal Transformer.
While the secondary DC/DC converter for have the 55 V Out for Injector Type1, Type2, LTPoE++...., isn't isolated, it's a normal Boost converter with choke.
It can be a proper PoE installation ??? (Attach the drawing)
Injector PSE Controller With dc-dc Not Isolated.PDF
Since you are using the LTC4279 I suggest referring to the IEEE 802.3at standard for isolation requirements.
Per the IEEE 802.3at - PDs and PSEs shall provide isolation between all accessible external conductors, including frame ground (if any), and all MDI leads. Refer to the IEEE 802.3at and IEC 60950-1:2001 standards to read on for further details. You may also refer to the IEEE 802.3bt and IEC 62368-1 for the latest standards.
Where you connect your ground after the transformer in your schematic factor in whether your PoE supply will require additional isolation.