Measurement of Current..
Yeah! the usual meter that comes in mind for current measurement is the ammeter. These meters have to be connected in the circuit to get the reading. Thus, for current measurement using ammeter, we have to disturb the circuit and put the meter in between. Also the prior knowledge of the magnitude of circuit current and it’s nature is necessary. One should also know the proper method and have the means to disconnect and reconnect the circuit.
Clamp-On ammeter is a very handy device which can measure the current flowing through any LT circuit without disturbing the existing connections.
A clamp meter is a more versatile instrument that combines a clamp-on ammeter with a multi-meter. It is usually capable of measuring alternating current, AC/DC voltage, and resistance. Many of these meters are able to test the capacitors, measure frequency, temperature, continuity etc. Continuity is a quick test to check an open circuit. When the circuit is closed, the meter emits a “beep” sound.
The display can be analog or digital; accordingly are called analog or digital clamp meters. Digital meters have several advantages over analog ones and thus are commonly used. Figure 1 shows a digital AC/DC clamp meter.
Fig.1: A digital AC/DC clamp meter
Clamp meters have a movable jaw that can be opened. Open jaw allows the clamp meter to be clamped around a current carrying wire to measure the current flowing through the circuit. Figure 2 shows the digital clamp meter with open jaws.
Fig.2: Digital clamp meter with open jaws.
Working of DC clamp meter:
Conventional clamp meters, capable of measuring alternating current only, do not show the reading when used to measure DC current. DC clamp meters work on the principle of “Hall Effect”. The magnetic field because of DC current produces a small voltage across the Hall Effect sensor. This voltage, which is proportional to the DC current flow, is further amplified for measurement.
DC clamp meters or AC/DC clamp meters are very useful in Solar PV system installation and maintenance, since the output current of a solar PV plant is DC in nature.
Accessories such as pair of red and black probes for voltage measurement and thermocouple with leads (clamp meters with temperature measurement facility) are provided. Figure 3 shows the thermocople with leads. Figure 4 shows how the thermocouple leads are connected to the clamp meter (also shows the temperature).
Fig.3: Thermocople with leads
Fig.4: Thermocouple leads connected to the clamp meter
The digital AC/DC clamp meter is a battery operated (9V) device. When the battery is low, a “low- battery” indication appears on the display. The “Auto power off” feature automatically turns off the meter when no operation is detected for 15 minutes, thus helps to extend the battery life.
Advance Features of Clamp Meter:
Several digital clamp meters offer a wide variety of advanced features which ultimately help an electrical engineer to a great extent. These features are:
1. PC Interface – Clamp meters also come with PC interfacing serial ports to facilitate data transfer to computer for further analysis of data and report generation.
2. Data Logger – Some clamp meters have internal memory capable of holding the measured values to be used later on.
3. Inrush Current measurement – This feature allows the clamp meter to measure the high inrush current usually encountered during motor starting.
4. Autoranging – Automatically sets appropriate measurement range thus, avoids manual selection of range.
Safety Requirements and Standards:
The AC/DC clamp meters are designed accordingly to safety standard IEC61010-1 and IEC 61010-2-032 to meet the safety requirements for electrical measuring instruments and hand-held meters. They also comply the European standards 89/336/EEC for Electromagnetic Compatibility and 73/23/EEC for low voltage
A few things should be followed so as to ensure that these meters last long in the harsh environment encountered in the field. The checks to be carried out are –
1. Keep the meters in their safety cover or carry bag after the use to avoid ingress of moisture, dust etc, and shocks and damage.
2. Do not store these meters in a strong magnetic field to avoid loss of accuracy. Electrical noise, unwanted signals or intense electromagnetic fields in the vicinity may disturb the measuring circuit.
3. All meters are very fragile and hence should be handled with care.
4. Do not expose the meters to extreme hot or cold temperature and moist atmosphere.
5. Always select the proper range and nature of circuit i.e. AC or DC.
6. Always inspect the meter and test leads for any sign of damage or abnormality, such as broken or cracked, leads before use.