INSULATION TEST EQUIPMENT FOR LOCAL CROSSBAR SYSTEMS 637 



mask the higher resistance cable leakages the test circuit is grounded 

 and connected to the line with the tip and ring short circuited as shown 

 in Fig. 7. The test circuit measures the leakage current flo^\dng from the 

 pair connected for test to the ring conductors of other subscriber lines 

 which are connected to battery potential in the central office. Leakages 

 outside the cable will not cause leakage currents to flow through the 

 line insulation test circuit. This test is called FEMF because it measures 

 leak in the same way as does the test desk voltmeter when the FEMF 

 test key is operated to connect ground instead of test battery to the 

 voltmeter. Leakages as high as about 2 megohms can be successfully 

 located after the cable has been identified by analysis of pairs affected 

 by leaks. 



r 



LINE 



INSULATION 



TEST 



CIRCUIT 



LINE 

 UNDER TEST 



XH' 



LINE 

 RELAYS 



LEAKS IN CABLE TO OTHER 



RING CONDUCTORS CONNECTED 



TO CENTRAL OFFICE BATTERY 



(FEMF) 



Fig. 7 — Foreign E.M.F. test. 



TEST AND RECORDING CIRCUITS 



The equipment for automatically measuring and recording line-in- 

 sulation resistance consists of three principal parts. First, the device 

 for measuring the low currents involved; second, the means for connect- 

 ing this measuring device to each of the lines in rapid succession; and 

 third, the apparatus required for recording the faults discovered. 



The first of these, kno\vn as the line-insulation test circuit, is capable 

 of detecting insulation resistance as high as ten megohms, with fast 

 enough response, so that a satisfactory rate of line testing (about 10,000 

 to 12,000 lines per hour) is attainable. It is provided wdth a filter to 

 attenuate both 25- and 60-cycle interference, so that leakage faults may 

 be separated from other types such as induction from power lines. As 

 shown in Table I, its sensitivity may be easily changed, so as to be 

 compatible with atmospheric conditions, the kind of test to be made, 

 and the condition of the outside plant. When a particular sensitivity or 



