spikes of overhead lines, provided that the closest spikes are
opened. However, in this situation there are a number of severe
drawbacks, such as:
. Protection against connection errors; in case of overhead lines,
protection against lightings;
. Impedance of the connection: very capacitive for the cable,
very inductive for the overhead line;
. Coupling with parallel lines, in case of overhead lines.
This is why we do not recommend to use T 2000 in case of
large substations.
The last thing to be considered is the earth resistance of the
auxiliary earth connection, that is realized by the auxiliary spike
provided. The total impedance ZT encountered by the generator is
the sum of:
. Earth resistance of the system under test RP;
. Impedance of the conductors ZC;
. Earth resistance of the auxiliary system RA.
ZT = RP + ZC + RA.
For this test, the T 2000 output can reach a maximum voltage of
80 V. A typical value of the earth resistance to be tested is 10
Ohm or less. If we want to perform the test with a current of 1 A,
so that the voltage drop is 10 V, the value of ZT must be less
than 80 Ohm. As ZC can be neglected, this leaves 70 Ohm for the
auxiliary spike: this value can be obtained only if the soil is very
wet, else a typical value is 100 Ohm or more. During the test, it is
possible to pour water around the spike, so that its resistance is
decreased to the desired value; else, it is possible to fit in the
ground provided, and to connect spikes between them with the
cable provided.
It is possible to select the output to which to connect the T 2000
current output: it can be either the 90 V (10 A) output, or the
250 V output. The characteristics of these outputs are the
followings.
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