Earth-faults on un-earthed networks are detected either by sensitive earth-fault protection using a CBCT (the ITH7111 and RMS711 are both suitable) or by neutral voltage displacement protection (the TTG7114, TTG7134 and TMS714 are all suitable). 
 
Operation of Neutral Voltage Displacement Relays 
The zero sequence current that flows for an earth-fault on an un-earthed network is determined by the capacitance of the cable to earth. This flow of current leads to a rise in the voltage of the neutral point of the incoming supply (the star-point of either the generator or transformer). It is this rise that is detected by a neutral voltage displacement relay. If the network is quite small (i.e. short cables), then the zero-sequence voltage will be unstable. This is why an instantaneous relay is only generally used to provide an alarm.
In the event of a fault, neutral voltage protection cannot detect the location of a fault, so all sources of supply should be disconnected. 
 
Connection of Neutral Voltage Displacement Relays 
Neutral voltage displacement relays are connected in one of two ways as shown in the wiring diagrams of the relays (contact us if you do not have copies). Our wiring diagrams show that the primary star-point of the VTs should be earthed. Although the relays should operate successfully without the earth, this is how VTs should be connected. Earthing the VTs in this way will not affect the earthing of an isolated neutral network.