Page 319 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

The core is so designed that there is only one break in the magnetic path. The hinge and the opening both fit
tightly when the instrument closes around the conductor. The tight fit of the instrument ensures minimum variation
in the response of the magnetic circuit.
To measure current with a clamp-on meter, open the jaws of the instrument and place them around the conductor
in which you want to measure the current. Once the jaws are in place, allow them to close securely. Then, read
the indicator position on the scale.
When the core is clamped around a current-carrying conductor, the alternating magnetic field induced in the core,
produces a current in the secondary winding.
This current causes a deflection on the scale of the meter movement. The current range can be changed by
means of a `range switch’, which changes the taps on the transformer secondary (Fig 5).
Safety: The secondary winding of the current transformer should always be either shunted or connected to the
ammeter; otherwise, dangerous potential differences may occur across the open secondary.

Before taking any measurement, make sure the indication is at zero on the scale. If it is not, reset by the zero-
adjustment screw. It is usually located near the bottom of the meter.
Looping the conductor more than once through the core is another means of changing the range. If the current
is far below the meter’s maximum range, we can loop the conductor through the core two or more times (Fig 6).

Fig 5 & 6

Application
1 For measuring the incoming current in the main panel board.
2 Primary current of AC welding generators.
3 Secondary current of AC welding generators.
4 Newly rewinded AC motor phase current and line current.
5 Starting current of all AC machines.
6 Load current of all AC machines and cables.
7 For measuring the unbalanced or balanced loads.
8 For finding the faults in AC, 3-phase induction motors.
Precaution
1 Set the ampere range from higher to low if the measuring value is not known.
2 The ampere-range switch should not be changed when the clamp is closed.
3 Before taking any measurement make sure the indication is at zero on the scale.
4 Do not clamp on a bare conductor for current measurement.
5 Seating of the core should be perfect.

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CITS : Power - Electrician & Wireman - Lesson 50-53