One of the systems feeding the plant where I work is a three phase ungrounded 240V Delta system. The ground fault indicators are feed directly from the load side of the main circuit breaker. (This is an old plant circa 1930's )
The ground indicators are just three bulbs connected in a wye arrangement with the centre point connected to ground. Under normal conditions all three lamps glow dimly and at equal intensity. If one phase becomes grounded the two lamps on the ungrounded phases glow brightly (they now see a full 240V) and the lamp on the grounded phase goes out. Pretty simple but it works well.
