Megger MFT-X1 - Loop Test Options; Hot Key 1 - Types of Loop Test; Hot Key 2 - no RCD, RCD or EV RDC Options

14.2 Loop test options
Setting the Loop Impedance test options are best performed by setting up the Loop Impedance test from Hot key 1
to Hot key 5, as this gives a logical sequence to the selection. Each of the Hot key options are detailed below:
14.2.1 Hot key 1 – Types of loop test
 Z
This is the standard Loop Impedance test mode. It displays the basic Loop Impedance result plus the fault current.
Depending on the other Hot key settings it can display all three loop Impedances, L-PE, L-N and N-PE or just a single
result.
 Zmax
This mode displays the maximum Loop Impedance value from a series of tests, removing the need to manually record
results when searching for the highest Loop Impedance in a circuit.
 Zref
This mode stores a Loop Impedance value, such as Ze, for use when using the R1+R2 or Vdrop modes. In many cases
this will be Ze, the external part of the Loop Impedance circuit outside the building. But it could also be the entire
circuit between the supply source to the last distribution board if all you want is the final circuit impedance.
Making a Zref measurement stores the Loop Impedance value automatically and is displayed in the lower left of the
Loop Impedance screen of the R1+R2 and Vdrop test modes.
 R1+R2 (Only available when Hot key 5 is set to L-PE)
Once stored, by selecting the R1+R2 mode, the instrument subtracts the stored value of Zref from each measured
result. For example, if you use Zref to store the external Loop Impedance of a circuit (Ze) and then measure using the
R1+R2 mode, the instrument will subtract the Zref from the measured value and display the result. In this case the
result will be the internal impedance or R1+R2 for the circuit under test (RRefer to 14.11 R1 + R2 mode on page
73.). This option requires Hot key 5 to be set to L-PE as it is only valid when making phase to earth measurements.
 Vdrop
This mode uses the Zref value above plus a stored circuit current value to display the volt drop of a circuit.
Vdrop requires both the Zref value to be measured and the i-vdrop value to be entered (Refer to 14.10 Zref –
making and using the reference impedance for R1+R2 and Vdrop calculations on page 71.)
14.2.2 Hot key 2 – No RCD, RCD or EV RDC options
 No RCD
This method uses a high test current between 2.5 A and 4 A to acquire results very quickly. It is relatively immune to
electrical noise but still uses the Confidence Meter™ for best possible results. This method will trip any RCD in the
circuit so should only be used on phase to neutral (L-N) or phase to phase (L-L) circuits
 RCD
This method uses a much lower test current designed to avoid tripping RCDs down to 30 mA.
 RDC EV
This method is designed specifically for EV charger installations fitted with 6 mA RDCs, but can also be used where
the presence of 10 mA RCDs may be tripped by the RCD option.
NOTE : There is always a very small risk that an RCD may trip during a test. If this is unacceptable, you may
need to consider alternative methods of measuring the Loop Impedance. For further advice, contact the
Megger technical support service.
www.megger.com MFT-X1
63
Earth Loop Impedance testing – TrueLoop™