Clamp Ammeter

Clamp meters are a very convenient testing instrument that permits current measurements on a live conductor without circuit interruption. “A clamp meter” (clamp –on meter) is a type of ammeter that measures electrical current without the need to disconnect the wiring through which the current is flowing. A clamp-on ammeter can have either a digital or an analog readout.
Many clamp meters also measure other quantities (voltage, resistance, and so on) by using test leads rather than the clamp-on mechanism.
Using the clamp meter, however, we can measure current by simply clamping on a conductor as illustrated in fig. 1. One of the advantages of this method is that we can even measure a large current without shutting off the circuit being tested.

 How Do Clamp Meters Operate?
In general AC clamp meters operate on the principle of current transformer(CT) used to pick up magnetic flux generated as a result of current flowing through a conductor. Assuming a current flowing through a conductor to be the primary current, you can obtain a current proportional to the primary current by electromagnetic induction from  the secondary side(winding) of the transformer which is connected to a measuring circuit of the instrument. This permits you to take an AC current reading on the digital display(in the case of digital clamp meters) as illustrated by the block diagram of fig. 2(a)

Fig. 2 (a) Block Diagram Of Digital Clamp Meter

The most common forms of clamp are:

i. Probe for use with a multimeter
ii. Self-contained unit
iii. A buitl-in part of a specialized multimeter used by electricians

In order to use a clamp meter, the probe or clamp is opened to allow insertion of the wiring, and then closed to allow the measurement. Only one conductor is normally passed through the probe, if more than one conductor were to be passed through then the measurement would be a vector sum of the currents flowing in the conductors and could be very misleading depending on the phase relationship of the current. In particular, if the clamp were to be closed around a mains extension or similar cord, no current will be measured at all as the current flowing in one direction will cancel that flowing in the other direction.
In practice, nearly all clamp meters are used by electricians and the meters often include additional circuitry to allow the reading of voltage and, sometimes, resistance.
The meters also often contain a mechanical pointer-locking devices so that a reading can be taken in locations where the meter pointer can’t be seen, the pointer then locked, and the meter brought out to a more-convenient place for reading. For the meter shown in the picture below, the while push-button marked “lock” provides these functions. Fig 2.(b).

Fig 2.(b).

Measurement Principle of AC/DC Clamp Meter
In general hall elements are used as a sensor to detect DC current because it is not possible to employ an electromagnetic induction method as used for dedicated AC clamp meters. As shown in a figure (3) at left, a hall element is placed across a gap created by cutting off part of the transformer jaws. When there occurs a flow of magnetic flux proportional to both AC and DC primary currents in the transformer jaws this hall element detects the magnetic flux and takes it out as an output voltage. Hall element: This is a semiconductor to generate a voltage proportional to the product of bias current and magnetic field on the output terminal when bias current is applied to the input terminal

fig 1.28. 

How to Measure DC Current
clamp on to a conductor just the same way as with
AC current measurement using an AC current
clamp meter. In the case of DC clamp meters the
reading is positive (+) when the current is flowing
from the upside to the underside of the clamp
meter as seen in fig 1.28. 

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