Just to add a little explanation to this one. This is a Modified Differential Ground fault system designed by GE Industrial systems for a specific project. It is just an example of how to tie together multiple services and feeds that all have, and are required to have, ground fault protection.
This one has three 4000A services (utility feeds) tied together with tie breakers. In addition to the services there are two 1500KVA generators paralleled and tied directly to one of the sub station bus via a kirk-key system. All 5 sources have ground fault protection and require neutral to ground bonds for it to operate. Not code legal you say? (I sure did at first) Well it works by forcing the neutral current to the appropriate breaker (the one in use) so it's ground fault circuitry senses properly.
It is NEC compliant if approved by the AHJ and apparently is not uncommon where multiple services are utilized.