portable ground-testing instruments, such as a megger. One procedure for measuring ground resistance with a megger is shown in figure 4-10.

If the ground resistance is too high, it may be lowered by one of the following methods:

1. Drive additional rods. spacing them 6 feet apart, and then connect them in parallel.

2. Use larger rods where low resistance soils are too far below the surface to be reached by ordinary rods.

When you are connecting transformers in parallel or in a three-phase bank, it is important to know the polarity of the transformer terminals or leads. In the manufacturing of transformers, the ends of the windings are connected to the leads extending out through the case. The internal connection determines the direction of current flow in the secondary terminal with respect to the corresponding primary terminal. The current may flow in the same direction or in the opposite direction. When the current flows in the primary and secondary windings in the same direction, the polarity of the transformer is said to be subtractive; and when the current flows in the opposite directions, the polarity is said to be additive.

Polarity may be further explained as follows: imagine a single-phase transformer having two high-voltage and three low-voltage external terminals.

Connect one high-voltage terminal to the adjacent low- voltage terminal, and apply a test voltage across the two high-voltage terminals. If the voltage across the unconnected high-voltage and low-voltage terminals is less than the test voltage, the polarity is subtractive; if it is greater than the test voltage, the polarity is additive. This test is shown in figure 4-11.

Figure 4-11. - Polarity tests.

Figure 4-10. - Measuring ground resistance where the ground wire is interconnected with the ground distribution neutral connector.