.?58 



HILI.I. SYSTIIM I liCIIMC.IL JOLKX.ll. 



scheni.ilic circuit arraiigeinciU of the "Kvershcd" megger which is 

 the commercial form of instrument now used for this work. This 

 circuit is contained in a small portable box so that it can be readiK- 

 used at office frames or carried out on the line. 



Circuit Arrangement or"MESGER" Testing Set 



_Guard 



A- Curreni Coil 

 B - Brushes . 

 C - Compensaiina Con 

 D - Generator 



Line Larin 



P - Pressure Coil 



Q - Current Circuit Ballast Resistance 



R - Pressure Circuit Ballast Resistance 



Fig. 4 



The circtiit of I'ig. 4 consists of a high potential direct current 

 generator, hand dri\en by means of a crank handle. This generator 

 is arranged to provide a maximum potential of 400 volts which is the 

 potential now employed in measuring insulation resistance on cable 

 conductors. The potential furnished by the generator is impressed 

 on two coils in an indicating tlevice, one of these coils being in series 

 with a fixed resistance antl the other in series with the circuit imder 

 test. When the circuit luider lest is not connected to the megger the 

 full current from the generator flows thru the first coil of the indi- 

 cating de\ice whii'li for \h\> cniidition causes the pointer to go to the 

 "Infinite" i)osiiiiin. \\ lun .i circuit is connected to the megger 

 "Line" and "Kartli ' terminals some curreni flows through this circuit 

 due to its leakage and through the second coil of the indicating device. 

 This causes the indicating (le\'ice to move the pointer toward the 

 "Zero" position, the amount of the movement depending on the 

 leakage in the circuit under test. The indicating device is calibrated 

 in megohms no that (he results of insulation tests can be read directly. 



