Digital Multimeter
As the name says, it displays digital values like that of our digital watch. Multimeter indicates the multipurpose nature of the instrument. A typical DM comprises of selection switch, display, ports and probes as its major parts.
The probes are to be inserted into appropriate ports and are to be connected across the parameter which needs to be inspected. At the same time, ensure that the selection switch is to be kept at the position (such as voltmeter, ohmmeter, ammeter, or continuity check) appropriate for the measurement.
Generally, DMs are used to measure three important parameters viz., current, voltage, and resistance. Apart from these, DMs can be used to perform special functions like diode check, frequency measurement and continuity check.
· As Ammeter-used to measure the current. To accomplish this, insert the red probe to one of the current measuring sockets i.e., mA (to measure low level of current) or 20A (to measure larger current). Connect the meter along the line through which the current is to be measured (but in series connection). If we switch on the power supply, the meter will display the current flowing through the circuit.
· As Voltmeter-used to measure the voltage. To start with, one has to insert the red and the black probes to the sockets market as ‘V’ and ‘COM’ respectively. Select the expected range of the voltage that would be. You can select either AC or DC. The voltage will be displayed (provided one connecv0ts the leads across the component (in parallel fashion).
· As Ohmmeter-To start with, one has to insert the red and the black probes to the sockets market as ‘V’ and ‘COM’ respectively; and the selection made on ‘Ohmmeter’. Now, connect the leads across the two ends of the object whose ‘resistance’ is to be determined.
· As Continuity Check-Used to know whether there exists any low resistance path via two points i.e., to check whether the points are short or not. The probes are inserted into the sockets as that in the case of voltage measurement and selector switch is made on point ‘continuity check’. Then the points to be tested are touched with the leads of the probes. If the DM beeps, then it means that the points are ‘shorted’.
Clamp Meter
A clamp meter is an electrical test tool that combines a basic digital multimeter with a current sensor. Clamps measure current. Probes measure voltage. Having a hinged jaw integrated into an electrical meter allows technicians to clamp the jaws around a wire or a conductor at any point in an electrical system, then measure current in that circuit without disconnecting it. The components (as shown in the picture) are given below:
1. Current-sensing jaw
2. Tactile barrier (to protect fingers from shocks).
3. Hold button-Freezes the display reading. Reading is released when button is pushed a second time.
4. Dial (rotary switch)
5. Display
6. Backlight Button
7. Min Max Button-On first push, display shows max input. On subsequent pushes, min and avg inputs are shown. Works in current, voltage and frequency modes.
8. In-rush current button
9. Zero Button (yellow)-Removes DC offset from DC current measurements. Also, serves as dial’s shift button to select yellow functions scattered around the dial.
10. Jaw release lever
11. Alignment marks-To meet accuracy specifications, a conductor must be aligned with these marks.
12. Common input jack.
13. Volts/Ohm input jack
14. Input for flexible current probe.
These days, clamp meters include most of the functions of a digital multimeter such as the ability to measure voltage, continuity and resistance.
Clamp meters are preferred for measuring high levels of current as opposed to DM which cannot measure 10A of current for more than 30 seconds (as this may damage the DM).
Clamp meters offer a min current range of 0A to 100A. Many models have a range up to 600 A, while others go up to 999A or 1400A. Clamp meter such as iFlex can measure as high as 2500A.
Insulation Resistance (IR)
· IR test should be conducted on regular intervals to detect insulation failures to prevent any electrical accidents.
· Is useful in evaluating the presence of absorbed moisture, contamination or severe cracking in insulators.
· Test voltage of 5kV and values above 1 Giga Ohm for 1 minute are considered good values.
The results shown by megger are reliable as we inject current through the device we are testing. There are two types of Megger viz., Electronic Type (Battery Operated) and Manual Type (Hand Operated). In the following section, Electronic Type will be described.
· Wire Leads-Two numbers of leads for connecting megger with electrical external systems to be tested.
· Selection Switches-Switches used to select electrical parameter ranges.
· Indicators-To indicate various parameters status i.e., On-Off.
Note-The above construction may differ as per the manufacturer.
The general thumb of rule (i.e., the acceptable values for safe energization is 1 MΩ per 1000 V of applied test.
Megger or Megohmmeter is a portable insulation tester used to measure a very high resistance of the order of Mega Ohm.
Contact Resistance Measurement (CRM)
This test is conducted to check i) Measure resistance of electrical contacts; ii) Identify eroded/rusted contacts; iii) Identify loose connections of fixed and moving contacts; iv) Verify proper connection of bolted joints.
· The CRM test kit has C+ and C- for injecting the current.
· Two voltage terminals to measure voltage across two isolator contacts.
· Earthing terminal to prevent the operator/equipment from electric short
· CRM is measured on the basis of Ohm’s Law. For example, a current of 100 A or 200 A is injected and voltage drop across the contacts are measured through R = V / I
· Generally, isolator CRM values should be less than 100 micro-ohms.
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