14. Earth Loop Impedance testing – TrueLoop™
WARNING : Loop impedance tests are live circuit tests and all precautions for live working
must be followed.
Fig 47: Selecting Loop Impedance mode
14.1 General
The MFT-X1 uses the unique Megger TrueLoop™ technology to carry out earth Loop Impedance measurements
(which are often called Loop Impedance test or just loop tests) in single and three-phase installations. They can also
be used to measure Loop Impedance of all line conductors such as Live to Neutral and Live to Live.
Test options include:
No RCD: Two-wire standard and two-wire high-resolution tests for circuits that are not protected by RCDs.
RCD: Two- and three-wire tests for circuits that are protected by RCDs and RCBOs, without tripping an RCD.
EV RDC: Three-wire tests for EV circuits protected by a 6 mA RDC.
The tests for RCD- or RDC-protected circuits are very unlikely to trip the RCD but, as with all loop test instruments,
there may be rare combinations of circumstances in which tripping occurs. Consequently, if the risk of tripping the
RCD is too high, alternative methods of testing the circuit impedance should be considered.
14.1.1 About Loop Impedance tests
Loop impedance tests are carried out on electrical installations for three reasons:
To ensure that, if a fault occurs, a large enough fault current can flow to operate the protective device quickly.
To allow the instrument to calculate the prospective fault current. This must always be less than the safe
interrupting capacity of the protective devices in the circuit.
To allow the instrument to calculate the volts drop in the wiring between the point where the supply enters the
property (usually the consumer unit or main distribution board) and the point where the load is connected.
To measure the Loop Impedance, the instrument connects a load between live and earth or between live and neutral,
depending on the test being performed, and measures the effect this has on the supply voltage at the point where
the instrument is connected.
For testing across phases, the instrument can also be connected from L1 to L2, from L2 to L3 or from L1 to L3.
The instrument uses the change in voltage to calculate the Loop Impedance.
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Earth Loop Impedance testing – TrueLoop™
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