Many existing systems are delta connected and therefore these source transformers have no neutral available for grounding. However, this neutral point can be obtained by applying a zig-zag grounding transformer to the system. A zig-zag grounding transformer can be used to resistance ground a medium-voltage system, or to solidly ground a low-voltage system, provided the zig-zag transformer is properly rated.
The zig-zag transformer has a 10-second current rating that would normally pass sufficient ground current to trip protective devices, and operation is similar to that obtained by the grounded wye system. However, if the fault persists for more than 10 seconds, a ground-fault relay on the neutral of the zig-zag transformer will open the main breaker. This relay should also trip the grounding transformer if a continuous ground-fault would overheat and damage the transformer.
Some low-voltage delta-connected systems have been grounded by grounding one line of the system, but this method has many disadvantages and is not recommended. The center-tap-grounded delta, however, is frequently used on 240-V four-wire systems. These applications use three single-phase distribution transformers in a delta bank. The center-tapped unit usually has a kVA rating larger than that of the other two; it is expected to supply the single-phase load with 120-V on each side of the tap. It also participates in the three-phase load, along with the other two transformers.
Each of the ungrounded wires for this largest transformer is 120-V from the ground, but the third wire of the three-phase system is approximately 208-V above ground. Any single-phase load connected between this wire and the grounded center tap would be severely stressed. Consequently, this wire is frequently distinguished from the other two line wires by using the color orange for its insulation. This unbalanced voltage from line to ground is a possible source of confusion for this system.