A Megohmmeter or insulation resistance tester, is a special type of

ohmmeter An analog ohmmeter An ohmmeter is an electrical instrument that measures electrical resistance (the opposition offered by a circuit or component to the flow of electric current). Multi-meters also function as ohmmeters when in resistance-meas ...

used to measure the

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...

insulators Insulator may refer to: * Insulator (electricity), a substance that resists electricity ** Pin insulator, a device that isolates a wire from a physical support such as a pin on a utility pole ** Strain insulator, a device that is designed to work ...

. Insulating components, for example cable jackets, must be tested for their insulation strength at the time of commissioning and as part of maintenance of

high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant sp ...

electrical equipment and installations. For this purpose, megohmmeters, which can provide high DC voltages (typically in ranges from 500 V to 5 kV, some are up to 15 kV) at specified current capacity, are used. Acceptable insulator resistance values are typically 1 to 10 megohms, depending on the standards referenced.

Operation

Resistance to be measured is connected across the terminals i.e. connected in series with the deflecting coil and across the generator. When the current is supplied to the coils then they have

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

in opposite directions. If resistance to be measured is high, no

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (hydr ...

will flow through the deflecting coil, the controlling coil, will, therefore, set itself perpendicular to the magnetic axis and hence set the pointer to the

infinity Infinity is something which is boundless, endless, or larger than any natural number. It is denoted by \infty, called the infinity symbol. From the time of the Ancient Greek mathematics, ancient Greeks, the Infinity (philosophy), philosophic ...

. If the resistance to be measured is small, high current flow through the deflecting coil and the resulting torque sets the pointer to zero. For the intermediate value of resistance, depending upon the torque production, the pointer is set at a point between zero and infinity. The hand-driven generator is of the permanent magnet type and it is designed to generate from 500 to 2500

volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...

s. Testing voltage is produced by the rotation of the crank in a hand-operated megger, or by the battery in the case of an electronic-type megger. For testing a range up to 440V, equipment requires 550V DC is sufficient. The current coil or deflecting coil is series-connected and allows the electric current to flow through the circuit being tested. Control and deflecting coil has a current limiting

resistor A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

connected in series so as to protect the external circuit in case of damages caused due to very low resistance. Testing voltage is produced by

electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force, electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1 ...

in case of hand-operated megger and by a battery in case of electronic type megger. The deflection of pointer increases with the increases in

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

in the external circuit and also decreases with the increase in current. That is, resultant torque is inversely related to current and directly related to voltage. While the

electrical circuit An electrical network is an interconnection of electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e. ...

that is being tested is open, the resultant torque caused due to voltage coil is maximum and the deflection pointer shows the value of ‘infinity’ which means the circuit does not have any shorting present and resistance is the maximum within the circuit being tested. In the case of a

short circuit A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...

, the deflection pointer shows ‘zero’ which indicates ‘no’ resistance in the circuit being tested. A Megger consists of an EMF source and

voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. A ...

. The scale of the voltmeter is calibrated in

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (1 ...

s (kilo-ohms or megohms, as the case may be). In measurements, the EMF of the self-contained source must be equal to that of the source used in calibration.

References

{{Electrical and electronic measuring equipment Measuring instruments

Operation

See also

References