Grounding transformers are used to provide a path to ground for unbalanced load current and for fault currents on systems where a suitable ground is otherwise not available. Grounding transformers are normally constructed with one of the two configurations: Zig-Zag (Zn) with or without an auxiliary winding or a Wye (Ynd) with a delta connected secondary. The Zig-Zag connection is preferred as it is more cost effective and physically smaller than the Delta-Wye unit. For added safety, neutral ground resistors (NGR) are often used in conjunction with the grounding transformer to limit neutral ground fault current magnitude. The rated voltage of the NGR should be equal to the line to ground voltage of the grounding transformer. The current rating and duration should match the grounding transformer ratings. It is important to set the current rating high enough to be above the cable charging current and grounding transformer magnetizing current.
The importance of proper grounding for electrical systems in general and electrical transformers in particular cannot be overemphasized. Under normal conditions, an electrical circuit may continue to operate satisfactorily (that is, deliver power to the utilization equipment) even without proper grounding. It is not until an abnormal condition has occurred and after either someone has been injured, equipment has been damaged, or a fire has been started, that it is realized that improper or faulty grounding was the reason. Electrical systems are grounded to:
Terms used in grounding are similar and may be misinterpreted if care is not taken. Article 100 of the NEC contains important definitions that relate to grounding of electrical systems. These include the terms ground, grounded, effectively grounded, grounded conductor, grounding conductor, equipment grounding conductor, grounding electrode conductor and bonded. Grounded is defined as connection to earth or to some conducting body that serves as earth. Bonded is defined as the permanent joining Df metallic parts to form an electrically conductive path that will ensure electrical continuity and capacity.
In general, an electrical system derived from a transformer is required to have noncurrent-carrying metal parts and equipment grounded the same as any cther part of the complete system. Most transformer installations are considered to be separately derived systems, and their grounding requirements are covered in Section 250.30 of the NEC. According to this section the methods used for establishing a ground system shall be the nearest one of the following: