Dielectric Discharge Testing (Dd) - Megger BM25 Guia Del Usuario

To change the time, set the right-hand cursor on the BM25 display to P .I.
Test and then the left-hand cursor is moved to Clock.
To set T3 The default time is 10 minutes, but this may be adjusted using
the right-hand arrows up to 90 minutes if so required.
To set T2 Push the 'DOWN' arrow on the left-hand display once more
and the display will indicate (--2) and the time at which the
second insulation resistance value will be taken for calculating
the P .I. ratio (normally 10 minutes). This may be adjusted
using the right-hand arrows.
To set T1 Push the 'DOWN' arrow on the left-hand display one more
time and the display now indicates (--1) and a time at which
the first insulation resistance value will bew taken for
calculating the P .I. ratio (normally 1 minute). This may be
adjusted if required.
Push the 'UP' arrow on the left-hand display until the cursor indicateds
the appropriate test voltage for the P .I. Test.
The BM25 is now set ready for performing the P .I. Test and at the end of
the test the display will step through the 'LAST TEST' results sequentially,
indicating:
Timer Main Indicator
Test Duration Blank
Test Duration P .I. value
1:00 minute* Resistance
10:00 minute* Resistance
10:00 minute* µF
(* or whatever value has been set)
Voltage Indicator
Final Test Voltage
Final Test Voltage
Test Voltage after 1 min.
Final Test Voltage
Final Test Voltage

Dielectric Discharge Testing (DD)

This test measures the dielectric absorption of an insulator and gives an
indication of the amount of absorbed dirt and moisture. Default settings
are 500 V and 30 minutes but these may be changed as required. The
default settings will reset if the cursor is moved away from the 'DD'
position. The insulator under test is charged at 500 V for 30 minutes (to
attain a stable condition). A quick discharge then follows, during which the
capacitance is measured. The remaining current flow is then measured
after 1 minute, and the 'DD' value calculated from the formula:
Current after 1 minute (mA)
Test Voltage (V) x Capacitance (F)
1. Using the L M Range keys move the right hand cursor to 'DD'.
2. Accept the default time of 30 minutes, or using the L M
Selector keys move the left hand cursor to the
and then set the test duration using the L M Range keys.
Maximum test time is 90 minutes.
3. Accept the default voltage of 500 V , or using the L M Selector keys
move the left hand cursor to the required test voltage.
4. Start the test by pressing the red test button for at least one second.
5. On completion, the 'DD' value is displayed sequentially with the
corresponding insulation resistance, the PI ratio and the capacitance
value. This sequential display repeats until a key is pressed or Auto
switch-of operates. For more details of final display information see
Appendix 1. For more detailed information on 'DD' value
assessment, see 'Application of Test Techniques'.
position
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