AEMC 8500 - VT;PT Excitation Current; VT;PT Continuity Test

Digital Transformer Ratiometer DTR
®
Model 8500 13
4.3.4 VT/PT Excitation Current
Excitation Current, displayed in milliamperes, is the RMS current in the H
winding due to test excitation, during negligible loading of the associated
Xwinding.
The DTR
®
uses a maximum 44V excitation signal for testing, thus the
displayed excitation current value will almost always be a small fraction
of the tested transformer’s no-load excitation at full (rated) voltage. Most
transformerswillrequire100mAorless;somewillrequirelessthan1mA.
Note:Ifyouarepresentlyusingaratiometerthatemploys“X”side
“step-up”excitation(e.g.commonhand-crankedinstrument),exci-
tation currents displayed by the DTR
®
will,inmostcases,besigni-
cantly lower. This is normal and results directly from the fact that in
most step-down transformers, the H winding exhibits much higher
impedancethanitsassociatedXwinding.
4.3.5 VT/PT Continuity Test
The Continuity Test function of the DTR
®
Model 8500 serves as a useful
means of identifying open primary and secondary windings, open circuit
breakers, and blown fuses. When selected, the DTR
®
alternately checks
forcontinuitybetweenitsXcablesandbetweenitsHcables.
The Continuity Test is selected when the user depresses the TEST button
at the Press [TEST] to Check Continuity prompt. The DTR
®
will display
the results of continuity checking, then enter a brief degaussing period.
During this time, the message Pre-Test Checks will be displayed.
• TheDTRwillindicateOPENfortestedcircuitsgreaterthan4kΩ, and
CONT for those less than 4kΩ. A small percentage of transformers
(e.g. those with extremely high inductance or high winding resistance)
may appear as open circuits to the DTR
®
.
• AnOPENindicationwillnotcancelthetestcycleinprogressandthe
results of ratio testing will still be displayed.
• Transformerswhose winding arrangementinvolves multiple, parallel
paths (e.g. Delta-connected types) may read CONT, when in fact one
or more windings may be open.