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earth fault loop impedance test

Earth Fault Loop Impedance Test

During an electrical fault on a circuit, a current will flow from the Line conductor towards Earth and in to the Neutral point of the supply company transformer.

This circuit (loop), which consists of all the elements within the loop (supply transformer winding , supply company phase conductor, main fuse, main switch, protective device, Line conductor, the fault, Earth conductor, main earth terminal, supply company earthing conductor) is the Earth Fault Loop. It`s impedance is the measured value in Ohms`s.

In order to ensure the operation of the protective devices within the installation (such as MCB`s), the Earth Fault Loop Impedance value must be measured at the furthest point of the circuit and compared with the requirements found in BS 7671. If the measured value is higher than the it is allowed in BS 7671, than the circuit is not safe.

The External Earth Fault Loop test sequence:

  • Step 1. Use an Earth Fault Loop Tester or select the Earth Fault Loop Test option on a multifunctional tester such as the Megger 1553.
  • Step 2. Test on the incoming side of the installation. Connect one test lead to the Line terminal, the second test lead to the Neutral terminal and the third (usually green) test lead to the incoming Earth conductor.
  • Step 3. Press the TEST button. The measurement should be a low reading ohm value. Do not forget to record this value of `Ze` on the Electrical Installation Certificate.

How to test the highest Earth Fault Loop Impedance (Zs)?

Having obtained the `Ze` value for the installation, the value of `Zs` can be easily calculated for every circuit.  During the City and Guilds 2391 practical assessment it is allowed to test each and every circuit for the value of `Zs`; however, because of the limited time at hand it is advised to calculate the value of `Zs` instead of direct measurement.

The formula for determining `Zs:

Zs=Ze + (R1+R2)

Zs – earth fault loop impedance of the circuit tested

Ze – earth fault loop impedance external to the supply

(R1+R2) – Sum of the resistance of Line and Earth for the tested circuit.

The Earth Fault Loop test sequence:

  • Step 1. Locate the furthest point on the circuit to be tested (such as the furthest socket)
  • Step 2. With the appropriate Earth Fault Loop Tester, connect the test leads to the Line, Neutral and Earth terminals.
  • Step 3. Measure and write down the test results on the Schedule Of Test Results.

Please note that if the circuit is RCD protected than you will have to select the “No trip” function of the Megger 1553 to avoid nuisance tripping of the RCD. If your tester does not have this option then you will have to link out the RCD.

Verification of test results

Having obtained the value of Zs for every circuit, you will be expected to verify that these values are within the accepted limits described by BS 7671.

Values of Zs should be compared with one of the following:

– The values in Appendix B of Guidance Note 3 for Inspection and Testing

– Earth fault impedance figures provided by the designer

– Tabulated values in BS 7671, corrected for temperature (Tables 41.2, 41.3, 41.4)

– Rule-of-thumb figures with tabulated values in BS 7671 (Tables 41.2, 41.3, 41.4).

In our guide for the Inspection and Testing practical assessment we advise to use the fastest method which is the last one of the above described.

Rule-of-thumb method The highest measured Zs value for each circuit should not exceed 0.8 of the relevant value in the BS 7671 tables.

In other words, select the appropriate value from the tables in BS 7671 and multiply it with 0.8. This number should be higher than the highest measured Zs value for the given circuit.

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