Measurement Methods And Sources Of Error; Residual Current Device Testing - Megger MFT1700 series User Manual

.3 Switch the range knob to
The Main readout will show the R1+R2 value (eg.Zs - Zref)
The small readout will show the Zref value.
7.8

Measurement methods and sources of error

Method of measurement
During a loop test the instrument measures the difference between the unloaded and loaded supply voltages. From this difference it is
possible to calculate the loop resistance. The test current will vary from 15 mA to 5 A, depending on supply voltage and the loop resist-
ance value. The volt drop from a 15 mA load is exceptionally small, consequently the instrument performs many measurements auto-
matically. This test takes a long time to complete, typically 20 seconds
Possible sources of error
The reading depends on the stability of the supply voltage during the test. Therefore noise, harmonics or transients, caused
by other equipment during the test could cause an error in the reading. The instrument will detect some sources of noise
warn the user.
It is recommended that more than one test is performed on the circuit to ensure the measured value is repeatable, especially when
performing a 3Lo measurement.
Capacitive loads across the Phase-Earth circuit can affect the accuracy of the Non-trip loop test. For this reason the P-E (non-trip) loop
test should not be used on the P-N circuits.
Errors can be reduced by:-
Use the two-wire lead set with prods and making a firm connection to clean conductors.
n
Make several tests and taking the average.
n
Ensure that potential sources of noise in the installation are isolated (switched off), eg: automatically switched
n
loads or motor controllers
8.

Residual Current Device testing

The MFT1700 series can perform the following RCD tests:
1/2I
Non-tripping test at half the rated RCD trip current for 2 seconds, during which the RCD should not trip
I Tripping test at the rated RCD trip current. The trip time will be displayed
5I Tripping test at 5 x the rated RCD trip current. The trip time will be displayed in milliseconds.
0 or 180° Some RCDs are sensitive to the polarity of the supply, i.e whether the test current is applied with the
instantaneous rising or falling. Tests should therefore be performed with the polarity 0° and 180° and the
maximum time recorded.
RampTest Used to check the trip current of an RCD.
The MFT1700 series can test the following RCD types:
AS, A, S, and Programmable (typically a type A RCD with variable disconnection time
The MFT1730 can also test Type B RCDs.
RCD's are also available with a Selective (Delayed) trip time. These are referred to as Type S. These RCDs do not trip instantly,
allowing ordinary type RCDs to trip first.
22
and make loop impedance tests as previously.
and