5. Measurement theory. Ideal 61-920, 61920

ND 5417-1 61-920 Ins 9/26/06 8:49 AM Page 9

5. Measurement Theory

This instrument can measure the earth resistance to earth in multi-earthed system. Let's regard earth resistance under test as Rx, and the other earth resistances as R1, R2, …Rn.

Of these earth resistances, R1, R2, …Rn can be considered connected in parallel and can be regarded as a combined resistance Rs. The Rs can be regarded small enough against Rx since combined resistance consists of several resistances. Following is an equivalent circuit diagram of this scenario.

Page 9

ND 5417-1 61-920 Ins 9/26/06 8:49 AM Page 10

By applying the Voltage (V) to the circuit from the transformer jaw (CT1), current I is flowing in relation to the earth resistance. R can be determined by calculation after measuring the current with the other transformer jaw (CT2). In this case, R displayed in this meter can be regarded as Rx because Rs can be regarded small enough against Rx.

CAUTION

This meter cannot support the measurement for the locations with the following earth systems.

• Single-earth that is not connected to other earths (Lightning rod, etc.)

• Earth on which a current over 2AAC is measured.

• Earth with earth resistance over 1200Ω.

Page 10