IT earthing arrangement, where used.

Coal mining is one of the places that use an IT earthing arrangement.
Transformers used underground are of a Delta/Star arrangement, namely DY11. The transformer secondary star point is not connected directly to earth but through a current limiting impedance device.
The secondary side goes through a breaker that from memory contains a protection device that would trip if the resistance to earth drops below 20000ohms. Since the transformed supply could be used to supply a number of different parts of the mine called sections. Each section needs to be protected to afford discrimination and this means that insted of 20 k ohms the value is raised to 40 k ohms. Where final circuits are involved then a value of 60K would create a trip of the protective devices. Notice I said all the protective devices.

From memory three resistors are used to create a false neutral which in turn is connected to earth via another resistor. Should an earth fault on any of the circuits fed from the transformer occur then every one of the protective devices would have a voltage drop across the resistor connecting earth to the false star points. This voltage drop is sensed by electronics and causes the protective device to operate.

Prior to the arrangement above a core balance transformer was used. Each phase would go through the core or ring and thus each in turn would set up a magnetic field in the core. Since the current flowing in the three phases would under conditions be balanced, each magnetic field would cancel out the others thus giving no overall net magnetic field.
Under a fault condition there would be an overall magnetic field due to the imbalance and a this would cause a current to flow in a coil wrapped around the core. This current is sensed and would cause the protective device to operate.