Megger

Megger or insulation tester

 PRINCIPLE 

The instrument works on the principle of ratio-meter/ohmmeter. The required deflection torque is produce by both voltage and current. Due to the interaction between the magnetic field produce by voltage and current, the deflecting torque is produce. The required coil are so positioned that the deflecting torque is proportional to the ratio V/I. 

WORKING 

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

• If the resistance to be measured is high, no current is flowing through the deflecting coil. The controlling coil will therefore, set itself perpendicular to the magnetic axis and hence, set the pointer at infinity.

• If the resistance to be measured is small, a high current is flowing through the deflecting coil and the resulting torque sets the pointer to zero.

• For intermediate values of resistances, depending upon the torque production, the pointer is set at a point between zero and infinity.

• The hand driven generator is of permanent magnet type and it is designed to generate from 500V to 2500V.

• 1) Control and Deflecting coil: They are normally mounted at right angle to each other and connected parallel to the generator. The polarities are such that the torque produced by them is in opposite direction.

• 2) Permanent Magnet: Permanent magnet with north and south poles to produce magnetic effect for deflection of pointer.

• 3) Pointer and scale: A pointer is attached to the coils and end of the pointer floats on a scale which is in the range from “zero” to “infinity”. The unit for this is “ohms”.

• 4) D.C generator or battery connection: Testing voltage is supplied by hand operated D.C generator for manual operated Megger and a battery and electronic voltage charger for automatic type Megger.

• 5) Pressure coil and current coil: Provided for preventing damage to the instrument in case of low external source resistance.

• The voltage for testing is supplied by a hand generator incorporated in the instrument or by battery or electronic voltage charger. It is usually 250V or 500V and is smaller in size.

• - A test volt of 500V D.C is suitable for testing ship’s equipment operating at 440V A.C. Test voltage of 1000V to 5000V is used onboard for high voltage system onboard.

• - The current carrying coil (deflecting coil) is connected in series and carries the current taken by the circuit under test. The pressure coil (control coil) is connected across the circuit.

• - Current limiting resistor – CCR and PCR are connected in series with pressure and current coil to prevent damage in case of low resistance in external source.

• - In hand generator, the armature is moving in the field of permanent magnet or vice versa, to generate a test voltage by electromagnetic induction effect.

• - With an increase of potential voltage across the external circuit, the deflection of the pointer increases; and with an increase of current, the deflection of pointer decrease so the resultant torque on the movement isdirectly proportional to the potential difference and inversely proportional to the resistance.

• - When the external circuit is open, torque due to voltage coil will be maximum and the pointer will read “infinity”. When there is short circuit the pointer will read “0”

Megger (Megohmmeter) 

• The megger, or megohmmeter, is a high range ohmmeter containing a hand operated generator. It is used to measure insulation resistance and other high resistance values. It is also used for ground, continuity, and short circuit testing of electrical power systems. The chief advantage of the megger over an ohmmeter is its capacity to measure resistance with a high potential, or "breakdown" voltage. This type of testing ensures that insulation or a dielectric material will not short or leak under potential electrical stress. 

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The megger (figure 1) consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: (1) A hand driven dc generator, G, which supplies the necessary current for making the measurement and (2) the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of the opposed coil type.  

Coils A and B are mounted on the movable member with a fixed angular relationship to each other and are free to turn as a unit in a magnetic field. Coil B tends to move the pointer counterclockwise and coil A, clockwise. The coils are mounted on a light, movable frame that is pivoted in jewel bearings and free to move about axis 0.

• Coil A is connected in series with R3 and the unknown resistance, Rx, to be measured. The series combination of coil A, R3, and Rx is connected between the + and - brushes of the dc generator. Coil B is connected in series with R2 and this combination is also connected across the generator. There are no restraining springs on the movable member of the instrument portion of the megger. When the generator is not in operation, the pointer floats freely and may come to rest at any position on the scale.

• If the terminals are open circuited, no current flows in coil A, and the current in coil B alone controls the movement of the moving element. Coil B takes a position opposite the gap in the core (since the core cannot move and coil B can), and the pointer indicates infinity on the scale. When a resistance is connected between the terminals, current flows in coil A, tending to move the pointer clockwise. At the same time, coil B tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position at which the two forces are balanced. This position depends upon the value of the external resistance, which controls the relative magnitude of current of coil A.

• Because changes in voltage affect both coil A and B in the same proportion, the position of the moving element is independent of the voltage. If the terminals are short circuited, the pointer rests at zero because the current in A is relatively large. The instrument is not damaged under these circumstances because the current is limited by R3.

• There are two types of hand driven meggers: the variable type and the constant pressure type. The speed of the variable pressure megger is dependent on how fast the hand crank is turned. The constant pressure megger utilizes a centrifugal governor, or slip clutch. The governor becomes effective only when the megger is operated at a speed above its slip speed, at which speed its voltage remains constant.

Megger Ratings 

Army meggers are rated at 500 and 1,000 volts. To avoid excessive test voltages, most meggers are equipped with friction clutches. When the megger is cranked faster than its rated speed, the clutch slips, and the generator speed and output voltage are not allowed to exceed their rated value. When extremely high resistances (for example, 10,000 megohms or more) are to be measured, a high voltage is needed to cause sufficient current to flow to actuate the meter movement. For extended ranges, a 1,000-volt megger is available. Usually, meggers are only used on circuits with a normal voltage of 100 volts and up. When testing insulation, always refer to the appropriate TM or the manufacturer's recommendations. 

Megger Use 

Motor windings and components are tested to ensure that the conductors are not coming in direct contact with their housing, frame, or other individual conductor turns because the insulation has been damaged. The difference in potential, provided by the 9-volt ohmmeter battery, may not be substantial enough to correctly indicate an insulation problem in a 450-volt electrical system. The 9-volt push may not be sufficient to bridge some damaged insulation. There would then bean indication of infinite (maximum ohms) resistance. What appears to be an acceptable insulation reading would, in fact, be inconclusive. The higher voltage of the 450-volt electrical system would have no trouble bridging the gap in the damaged insulation. The megger, available in 500-and 1,000-volt power supplies, would detect this damage in the insulation and measure the resistance required when pushing the current past the damaged section of insulation. The megger provides an accurate indication of electrical insulation under system operating conditions. 

The ohmmeter does not allow a conclusive test for conductor insulation. This is because the small potential in the ohmmeter is not sufficient to force electrons across small distances or high-resistance insulation. For this same reason, the megger is not suitable for testing the continuity of a conduct or. The higher potential of the megger would allow completed circuit readings where the low potential ohmmeter would detect defects in conductor continuity. The megger and the ohmmeter should always be used together when substantiating the condition of electrical components. 

Megger Testing 

Many regulatory texts require the periodic testing of insulation. The Institute of Electrical and Electronic Engineers requires the additional testing of idle apparatus. A log book will be maintained for these megger resistance readings. As equipment ages and becomes contaminated with grease and dirt, the resistance of the insulation decreases. When these decreases in resistance are noted, preventive maintenance can be planned. Sometimes, cleaning alone will restore the insulation dielectric strength and return the component to operational condition. It is recommended that all major electrical components over 100 volts be megger tested every two years. Generators and critical electric motors can be megged before missions to evaluate and project their future operating condition. 

As with the ohmmeter, the megger is never used on an energized circuit. Additionally, the megger is never used on a circuit in which solid state components cannot be isolated. The high potential of the megger will destroy rectifiers, voltage regulators, radio equipment, and other electronic equipment. Make sure that the electrical component undergoing testing is completely isolated from the rest of the circuit. 

One megger test lead is connected to the de-energized conductor. The other megger test lead is connected to the noncurrent-carrying conductive material adjacent to the conductor's insulation. To test a cable, one test lead would go to the de-energized normally current-carrying copper conductor of a cable, and the other test lead would be connected to the noncurrent-carrying armor shielding. In another example, a megger lead could be connected to a motor winding lead, and the other megger test lead could be connected to the motor housing. In both of these cases, there should be no continuity. There should be a great deal of resistance between the current-carrying conductor and the housing with which the engineer is likely to come in contact. 

The megger is then operated for a period of at least 30 seconds. Refer to the component manufacturer's information for the specific results of a test. However, if these specifications are no longer available, any change in the insulation resistance must be considered suspect. 

Megger Safety Precautions 

When using a megger, observe the following minimum safety precautions to prevent injury to personnel or damage to the equipment: 

• Use meggers on high-resistance measurements only, such as insulation measurements. 

• Never touch the test leads when the megger is being operated. 

• De-energize and discharge the circuit before connecting a megger. 

• Disconnect the component being checked from other circuitry before using the megger.