Instrument Transformers Questions and Answers

Answer: A

Correct answer: increasing the length of the secondary leads

Longer leads mean more resistance and therefore more burden, which increases both the ratio and the phase angle error - as the earlier burden items show, the effect goes as the square of the secondary current. The other three all reduce the errors: a better core needs less magnetising current, removing an unused instrument lightens the burden, and turns compensation cancels the ratio error at the working point.

Answer: B

Long thin leads impose a substantial burden, which raises the core flux and the exciting current and so makes the secondary current fall further short of the nominal ratio. Shorter or thicker leads, or a one ampere secondary, is the remedy.

Answer: C

Beyond the accuracy limit the core saturates and the secondary current no longer follows the primary, so the relay sees far less than the true fault current. The accuracy limit factor must be chosen to exceed the largest fault the scheme has to clear.

Answer: D

For an ammeter direction does not matter, but a wattmeter, an energy meter or a directional relay must know the sense of the current relative to the voltage. The marks establish which secondary terminal corresponds to which primary terminal at each instant.

Answer: A

One element's torque reverses, so instead of the sum of the two readings the meter registers their difference. On a balanced load at unity power factor that would give nearly zero, which is a common sign of a reversed connection in the field.

Answer: B

The class number is the limit on the ratio error at rated current with the stated burden, in this case half of one per cent. A separate table gives the corresponding limit on the phase angle, in minutes, and wider limits at fractions of rated current.

Answer: C

The primary is the conductor itself, so there is nothing to wind and nothing to insulate. The cable must nevertheless be disconnected at one end to thread it through, unless a split core clamp type is used, which is the arrangement of a clip-on ammeter.

Answer: D

The split core lets the instrument be applied without breaking the circuit, which is its whole purpose. The joint in the magnetic path introduces reluctance and so a larger error than a solid core, which is the price of the convenience.

Answer: A

The exciting current at the low flux densities of normal service is what determines the error, and nickel iron alloys need far less of it. They saturate at a lower flux density than silicon steel, which for a metering core is a positive advantage.

Answer: B

Twenty metres out and back of two and a half square millimetre cable is around a third of an ohm, which at five amperes is more than eight volt-amperes, comparable with or larger than the ammeter itself. Lead burden is routinely the dominant term.