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Figure 4-9.—Three-phase 25 kVA transformers mounted on an H-frame platform. | Up
Construction Electrician Intermediate - Electricity handbook for electricians | Next
Figure 4-13.—Polarity markings and directions of voltages when polarity is subtractive. |
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.
4-9
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