The display will show , which indicates the suitability for use on circuits protected by a
Type AC or a Type A RCD, or RCBO.
If the RCD on the circuit is a Type B, press and hold
until the display shows (Type B RCD).
NOTE : If unsure of the RCD type use Type B setting.
3. Press TEST. The display will show the
start of a Loop Impedance confidence
test, followed by an initial value of loop
impedance:
> then
>
Figure 5. Loop Impedance Test Start Screen
NOTE : The test is not finished until the analogue arc reduces to one element, or the TEST button has been
pressed again.
4. If no noise is detected the analogue arc will reduce
to a single element and the display will show a
completed test result. The final value may vary
slightly if very small levels of noise were detected.
Figure 6. Final Result
5. If electrical noise is detected, the instrument analyses it continuously and the analogue arc will reduce as the
electrical noise is filtered from the measurement. Even if there is a lot of electrical noise, the confidence of the
measurement will improve with time and the analogue arc will reduce, so that a final stable value will show at
the end.
(electrical noise symbol) will also show to indicate the presence on electrical noise on the circuit.
If the electrical noise in the circuit varies the analogue arc may fluctuate until the test has completed and the
analogue arc shows a single element.
Start Small noise
Momentarily higher
noise Complete
Figure 7. Analogue Arc Fluctuation
NOTE : The maximum measurement time is three minutes. If after three minutes there is still more than
one element in the analogue arc, the test will stop. More than one element shows that the instrument
has not achieved its desired confidence level (electrical noise is high). The test is still valid, but with reduced
accuracy.
Press TEST at any time to stop the test. A digital test result will show and the size of the analogue arc shows the
degree of confidence in the test result (smaller the analogue arc = more confidence).
www.megger.com MFT1800 series
35
Loop impedance testing
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