76 Electrical Power Systems Technology
same as in any series ohmmeter, with the exception that a DC generator is
used as the voltage source. Digital megohmmeters are also available.
Periodic insulation tests should be made on all large power equip-
ment. As insulation breaks down with age, the equipment starts to mal-
function. A good method is to develop a periodic schedule for checking
and recording insulation resistance. Then, it can be predicted when a
piece of equipment will need to be replaced or repaired. A resistance-
versus-time graph (see Figure 3-13) can be made, and the trend shown
on the graph can be noted. A downward trend (a decrease in insulation
resistance) over a specific time period indicates that an insulation prob-
lem exists.
CLAMP-ON METERS
Clamp-on meters are popular for measuring current in power lines.
This indicator may be used for periodic checks of the current by clamping
it around a power line. It is an easy-to-use and convenient maintenance
and testing instrument.
Figure 3-13. Resistance versus time chart to be used with a megohmmeter (Cour-
tesy Biddle Instruments)