Magnetic field intensity is affected not only by the conductor current but also by the enclosure current. The latter depends on a grounding system of GIs. To study this effect, we used two test circuits, I and 11, as shown in Fig.3. The circuit I of Fig.3 (a) corresponds to a single-point grounding system with little or no enclosure current, while (b) means the case of a multi-point grounding one with nearly full current flowing in the enclosure. A handhole makes a possible measuring spot of the magnetic field when the field is canceled out outside an enclosure due to the enclosure current nearly equal to the conductor one. Reference [6] reports that although a handhole is usually blinded with a metal cap during operation, most of the enclosure current flows in the surrounding area and hardly inside a handhole due to the elongated path. Furthermore, a handhole makes it sometimes possible to measure field at a position closer to the contact point than on the enclosure surface, as later shown in Fig.13.