ELECTRICAL TESTS AND THEIR Al'l'I.ICATIONS 



357 



liMkaK*.' im'.isiiri-mi'iUs and routiiu' li>sts, tluTt'fori', bwoine very useful 

 in indicating whoii ri-ini-dial iniMsuri's, such as line inspwlions and 

 irit: trinuninR work, should he undertaken. 



If o|HMi wire telephone circuits are so siluaicil ili.it cont.ul wiili 

 ft>liai;e j;rowth will occur during the growing season low values ol 

 insulation resistance are certain to result even under dry weather 

 conditions. This is illiistratiil 1)\- the curve of I'ig. 'A which shows 



Jan Feb Mar Apr May 



Sept Oct Nov Dec 



Fig. 3 



re>ult> of nionthh- il.i> time (lr\' weather insulation nieasiirenu-nts 

 on a number of toll circuits over a period of a year under conditions 

 of this kintl. The monthly testing periods are plotted as the abscissa 

 while the ordinates show the percentage of circuits which measure 

 10 megohms per mile or more during these monthly testing periods. 

 This curse indicates the need fcjr periodic insulation resistance tests 

 and the use which can be made of such tests in instigating clean-up 

 work. 



(2) Me^i^er Melhod. The \-oltmeter method is not applicable for 

 accurately testing the higher values of insulation resistance such as 

 are encountered in telephone cables. Conductors in cables require a 

 Very high insulation and in practice values of 500 megf)hms or more 

 per mile are specified. The laboratory galvanometer method of test- 

 ing very high resistances, which is the same in principle as the volt- 

 meter method, can, of course, be used, but is not sufficiently rugged 

 for field testing. To take care of cable testing work in the plant a 

 method known as the Megger niethotl is employed. Fig. 4 gives the 



