Page 318 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Tong - tester (clamp - on ammeter)

Objectives: At the end of this lesson you shall be able to:
• state the necessity of tong-testers
• state the construction and working of a tong-tester
• state the precautions to be observed while using a tong-tester.

A tong-tester is an instrument devised for the measurement of A.C current, without interrupting the circuit. It is
also called clip-on ammeter, or sometimes a clamp-on ammeter (Fig 1).
Working principle
The instrument can function only when current passes through its deflecting system. It works under the mutual
induction principle.
Electromagnetic induction: When a changing flux is linked with the coil, an emf is induced in the coil. The
current in a coil so produced changes as that of the changing magnetic flux. If an alternating current is flowing
through the coil, the magnetic flux produced is also alternative i.e. changing continuously.(Fig 2)

Fig 1 & 2

Placing another coil (2) in the changing flux of coil (1), an emf will be induced. (Fig 3)

This induced emf will send the current, causing deflection of the meter. Introduction of a magnetic core between
the coils increases the induced emf. The coil (1) is called primary and the coil (2) is called secondary.
Construction: Fig 4 shows a tong-tester (the clamp-on ammeter) circuit. The split-core meter consists of a
secondary coil with the split-core and a rectifier type instrument connected to the secondary. The current to be
measured in the conductor serves as the primary of one turn coil. It induces a current in the secondary winding
and this current causes the meter to deflect.
Fig 3 & 4

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