Earth Loop Impedance Testing

What is Earth Loop Impedance?

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.

How to test External Earth Fault Loop Impedance (Ze)?

`Ze` is a measurement of the external earth fault impedance of the installation. It is the measured resistance of the supply transformer winding, the supply phase conductor, and the erath return path of the supply.

`Ze` can be obtained by enquiry to the electricity supplier or by direct measurement at the supply. On the C&G 2391 practical assessment you will be required to obtain the value of `Ze` by direct measurement. 

The measurement is made with an Earth Fault Loop Impedance tester with the Main Switch in the OFF (open) position and with the means of earthing disconnected from the installation to avoid parallel paths.

The External Earth Fault Loop test sequence:

This is a live test so extra care is advised!

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:

This is a live test so extra care is advised!

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.

The Inspection and Testing C&G 2391 practical exams

In order to verify the effectiveness of measures for fault protection by automatic disconnection of supply, the earth fault loop impedance value for each circuit must be determined by performing an earth loop test.

Next we move on to Prospective Fault Current testing.

Important things to remember:

DO NOT FORGET to replace the earth connections for the installation before reclosing the Main Switch and energising the circuits!

BS 7671 allows the tester to calculate the value of Zs in the way described above. However in real life situation it is better to test every circuit for Zs individually instead of calculation. The measured value will almost never be exactly the same as the calculated one. This is because other factors such as parallel paths of earthing within the installation. Generally calculating the value is only advised when it is dangerous to perform the live test on the circuit.

During the testing remember to constantly inspect the installation for faults and signs of damages.