The Transformer's case needs to be bonded to "Both Sides", in case of a Ground Fault.
What I mean here is the case is bonded with the following:
- An Equipment Grounding Conductor brought in with the "Primary Feeders" (whatever Branch Circuit is feeding the Transformer),
- A System Bonding Jumper from the Grounded Conductor on the "Secondary Side" (whatever is leaving the Transformer - AKA: the Separately Derived System, or "SDS")
With the Bond described in "A"
above, a Ground Fault from a "Primary Feeder" to the Transformer's case will have a low Impedance path, and (hopefully) trip the Over Current Protection Device (OCPD) on the Feeders for the Transformer.
With a solid Bond, such as described in "B"
above, a Ground Fault from a "Secondary Feeder" to the Transformer's case will be able to (hopefully) load up the Primary Feeder's OCPD and eventually cause it to trip.
WITHOUT the Bonding described in "B"
above, the Transformer's case may remain energised if a Ground Fault occured internally on a Secondary Feeder.
FYI: Even an Ungrounded System with Ungrounded SDS's will have Transformer cases bonded to a given Grounding Electrode System, plus connected to Equipment Grounding Conductors - only difference is there will be no Bonding Jumper between the SDS and the Grounded Equipment or Grounding Electrode System.
Here are a few Schematics for references:
Fig. 1: Single Phase Transformer - with 3 Wire Center Tapped Secondary.
Center Tapped Conductor is the Separately Derived System's Grounded Conductor.
Fig. 2: 3 Phase 4 Wire Wye Secondary - "X0" originates at the "Star Point" connection for all three Secondary Coils (Windings).
This "Star Point" is tapped, and the Conductor which is derived from the star point is the Common Neutral Conductor.
Grounding that tapped Conductor makes it the Separately Derived System's Grounded Conductor.
Fig. 3: 3 Phase 4 Wire Delta Secondary.
The Grounded Conductor is also a Center Tap, as described in "Fig.1" above.
Hope this is helpful.