Megger LTW315, User Manual

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12
5. Loop testing
Different loop testing options are available depending on the instrument used as below:
Instrum
ent NO-TRIP High
Current
MaxZ
R1+R2
HIGH
Resolution
Result storage
and
download.
50 V
-
440V
100 V
-
280 V
LTW315
  
LTW325
    
LTW335
     
LTW425
     
5.1 Description of test methods
5.1.1 No trip test - (All instruments)
Designed not to trip circuits protected by an RCD
≥
30mA.
Description:
A resistive load (15 K
Ω
) switched and rectified at Mains Frequency ÷6, in order to produce a test current at the
same frequency. The voltage produced by this test current is then used to calculate the loop resistance. If the
loop contains inductance, the inductive reactance will be largely ignored.
When testing in close proximity to the sourcing transformer, a system phase angle of 18
°
will cause an additional
error of 5%.
5.1.1.1 No-trip test modes
Two test modes are available in the [No Trip] loop test mode, providing:
Mode 1 = 10s
Mode 1 performs a 10 second loop test only. At the end of the test a result is displayed. If the noise symbol [ ]
is displayed at the end of a test the test should be repeated, or the test mode changed to mode 2, refer to section
8 SETUP procedure.
Mode 2 = AUTO (Default setting)
In Mode 2 the LTW loop tester can detect noise on an electrical supply that could affect the measurement
accuracy, and automatically extend the loop test from 10 seconds up to 20 seconds as necessary. The [
]
symbol is displayed to indicate an extended test is running.
If the noise symbol [
] is displayed at the end of the extended test it is recommended the test be repeated.
The instrument is shipped with mode 2 selected.
To change the test modes, refer to the SETUP procedure, section 8.
5.1.1.2 Symbols used during a normal loop test:
The [ ] symbol is displayed for the duration of an active loop test.
If electrical noise is detected during the loop test that could affect the accuracy of the answer, the following
additional symbols may appear:
If a low level of electrical noise is detected in mode 2, the [ ] will be displayed to the right of the
[ ] symbol. The test time will be extended up to a further 10 seconds to improve the accuracy of the test
results. This only applies if the tester, is running in mode 2, see section 5.2.1.