Megger ATEC 210415 Series, Руководство пользователя

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APPLICATION NOTES
43
AVTM21-415Ja Rev. A June 2005
In Section 9 the guard was recommended when testing cable.
In practical application this arrangement can resolve the
problem that the dirty surface path over the pothead may
conduct more current to ground than actually passes through
the bulk of the cable itself. The practical result is that the cable
appears to have a relatively low resistance while in fact the
cable itself is acceptable but the pothead is not.
By making an unguarded, then a guarded test it is possible to
determine if the cable or the pothead is at fault thus indicating
the appropriate action to be taken.
The principle of making more than one measurement with and
without guard can be extended to other apparatus in order to
separate multiple resistance paths and so better analyze the
situation.
In some literature the unguarded measurement is called a two-
terminal measurement while the guarded measurement is called
a three-terminal measurement.
The principle of combining 2 and 3 terminal measurements is
often used when the path of interest does not have the
required ground on one terminal in order to make the
necessary measurement.
MEASURING AN UNGROUNDED TEST ITEM
A two winding transformer in a grounded metal case with the
core also grounded is a common construction having the
typical problem of measurement of an ungrounded resistance
path. Frequently it is desirable to measure the insulation
resistance between each winding and ground and the insulation
resistance between the windings. In Figure 14 this case is
shown schematically.