👉EARTH TESTER AND MEGGER👈

✏️EARTH TESTER 

📝Earth tester is an instrument which is used to directly measure the earth resistance.  The diagram of the earth register is shown in the figure below:

📝Earth tester consists of various components like DC generator current reverser rectifier current coin potential coin link and then Earth electrode.  In the figure the total and represent the flow of alternating current and the state line represent the flow of direct current or DC.

📝It consists of a hand driven DC generator and direct reading ohmmeter. There is a potential coil and a current coil which is fixed to the spindle and which are free to rotate about a vertical Axis between the poles North and South Pole of a permanent magnet.  The test voltage generated by the generator is usually 500 Volts 1000 volts, the additional constructional features of the earth  tester are:

1) rotating current reverser: Current reverser is a switch or other contrivance for reversing the direction of current in a conductor.
2) Rectifier: A rectifier converts AC to DC.

📝Both these additional features consist of simple commutator which are made up of L shaped segment which are mounted on the shaft of the hand driven DC generator. Each commutator has four fixer brushes one pair of each set of brushes is so positioned that they make contact alternatively with one segment and then with the other at the commentator rotates, the second pair of each set of brushes are so position on the commutator so that continuous contact is made with one segment whatever may be the position of the commutator. In the figure we can see that the Earth tester has got 4 component that is P1 C1 and P2 C2.  In the figure we can observe that the terminals P1 C1 are shorted whereas the terminal P2 and C2 are connected to the electrodes P and C.  The shorted P1 and C1 terminals is then given to the earth electrode or it is connected to the earth electrode E as shown in the figure above.


✏️WORKING OF EARTH TESTER:

📝when the handle of the DC generator is driven at a constant speed, the alternating current flows through the earth through the dotted line connection and the DC current flows to the coils of the instrument.  The deflection of the pointer depends on the ratio of the voltage across the potential coil and the current through the current coil. The deflection of the pointer directly reads the earth resistance on a calibrated scale.  Earth tester is basically a permanent magnet moving coil instrument which operates on direct current only but by including a reverser and rectifier, it is possible to make measurements with alternating current or AC flowing through the earth. The rectifier is used to convert AC to DC. The sending of an AC through the earth eliminates unwanted effects due to the production of back EMF in the coil on account of electrolytic action when their current flows through the earth.





✏️MEGGER

📝Megger is an instrument used to measure very high resistance of the order of mega-ohms such as the insulation resistance of cables, transformers etc.

CONSTRUCTION: 

📝It consists of a hand driven DC generator and direct reading ohmmeter there is a potential coil/PC and a current coil/CC which are fixed to the spindle and which are free to rotate about a vertical Axis between the poles North and South of a permanent magnet.  The coils are connected through flexible leads also called as ligaments, which exert restoring torque on the moving system. The current coil is connected in series with the resistance R1 connected between the generator terminal and the supply line marked through resistance R1 limits the current through the current coil and controls the range of the instrument. The potential coil is connected in series with the compensating coil and the protection resistance or two and across the generator terminals.  The compensating coil is used to get better scale proportions. A guard ring is provided to shunt leakage current over the test terminals. The terminal G is known as a guard terminal which is used to connect the guard ring to the insulation under test. The test voltage generated by the generator is usually 500 volts or 1000 volts.If the megger shows the value in between 2MEGAOHM to 1000MEGAOHM,tgen we can consider it as a good value and if the megger shows the value below 2Mohm then, it is considered as an bad value. 

For example : for 1000 volts =1 mega ohm,in megger, so

📝If I consider a 2400v motor then the megger needs to give the value as 2.4 mega ohm, then we can say it as it is in good condition. 

WORKING:
📝The high resistance which is required to be measured is connected between the test terminals line (L) and ground (G). The generator handle is then steadily rotated at uniform speed till the pointer gives a steady reading.  This reading gives the value of the resistance. The satisfactory working of the megger may be tested as follows.

📝The test terminal line and ground are kept open. Now the resistance across L and G is infinite. The generator handle is rotated, due to which current flows through the potential coil and there is no current to the current coil. Therefore the pointer rotates in such direction that the pointer deflects and comes to rest at the point market infinite on the scale.

📝The test terminals are short circuited and the generator handle is rotated. At this condition, a very large current flows to the current coil and a small amount of current flow through the potential coil the resultant torque produced deflects the pointer in the opposite direction and makes a pointer to read zero marking on the scale.

📝After checking for the above two extreme readings on the scale, the high resistance to be measured is connected between the test terminals. The generator handle is rotated, now reasonable amount of current flow in both potential coil, as well as in the current coil and torque produced deflect the pointer on the calibrated scale which gives the value of the resistance that is being measured.