Ponovo PCT200 Series, User Manual

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PCT200 Series CT/PT Test System User Manual
34
The comparison between four-cable and two-cable testing:
The transformer secondary tap is connected by claps and the following four-cable connection is
applied.
Four-cable connection Two-cable connection
Otherwise, the chuck resistance might affect measuring result and the CT tester might list the
incorrect measuring result.
Four-cable connection: CT output terminal and input terminal are connected with tested objects by
different claps.
Every testing cable needs a clap.---CORRECT
Two-cable connection: Single testing cable is applied in CT output terminal and input terminal. But
the testing cable is connected with the transformer by a public clap.
Two testing cables use a public clap.---INCORRECT
Physics background:
While two-cable is applied, if there is contact resistance in the CT clap, there will be big measuring
error between the differential error and other parameters. Because the contact resistance might
have change when the clap connect or disconnect with CT, the measuring result can’t be
repeated.
The max. contact resistance of two claps will be 0.5 and the max. overload would be 12.5VA. For
this, the lower the CT coil resistance, the bigger the contact resistance’s effect. When testing a
5ACT, such contact resistance might cause a completely wrong result. But if a higher resistance
such as 10
Ω
, the error can be igored. So the four-cable conection is absolutely applied in 5ACT
testing.