Measurements of High Value DC and AC Currents

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Measurements of High Value D.C. and A.C. Currents

Reasons for High Current

  1. Short circuit
  2. Lightning discharge
  3. Switching transients surge currents

Where high current measurement is required ?

  1. For conducting temperature rise, heat run test on power equipment like conductors, cables, circuit breakers
  2. Short circuit current measurements.

Methods of Measuring High Currents (Devices used in High Current Measurements)

  1. Low resistance shunt. (For D.C.)
  2. Halt generator (For D.C.)
  3. Current transformer (High frequency Alternating Current)
  4. Peak reading volt meter with resistive shunt.

High Current (DC) Measurement by Low Resistance Shunt

  • The value of shunt resistance used in the circuit has a very small value in the range 10 µF to 10 mΩ. Voltmeter used is a sensitive voltmeter 0 to 1 volt range.
  • Current to be measure is found by application of simple ohm’s law,

Current 1 = Voltage (read by voltmeter) / Resistance of shunt.

  • See the following simple circuit
  • Resistance used (shunt) has four terminals to avoid contact resistance.

C, C are current terminals

P, P are potential terminals

mV – milivoltmeter

R- low value shunt

I – Current to be measured

Shunt and mV Meter to Measure High Value DC Current of 4-Terminals
  • Sometimes the resistance is of three terminals in which case bottom C and P terminals are shorted.
  • To avoid damage due to excessive heat, the resistance is generally immersed in moil to carry away heat and to restrict temperature.

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