There are occasions when a HV bus is supplied from a delta connected source (i.e. there is no earth reference). A zig-zag earthing transformer can be used to provide the earth reference and to limit the earth fault current.
There can be a requirement for electrical and mechanical interlocking of the various incoming circuit breakers with the circuit breaker for the earthing transformer. This is especially important for generator applications.
Earth fault detection can be considered for delta connected generators when running off-line. It is recommended that a risk assessment should be undertaken to determine whether or not the risk is