Earth Resistance and Continuity Tests
2. Earth Resistance and Continuity Tests
Earth resistance tests are required to confirm the installed earthing system will cause circuit protective devices to operate and maintain cable integrity if there is a fault between live parts and exposed conductive parts. Earthing System Impedance
The resistance from any point of the electrical installation required to be earthed, to the point where the main earthing conductor is connected to the neutral conductor (MEN connection) of the supply system, shall be low enough to permit
the passage of current necessary to operate the circuit protective devices. To confirm this value (which is dependent upon the type and rating of the protective device and the fault-loop impedance of the associated conductors) requires the
following test to be performed:
Test 1: Obtain the resistance value of the protective earthing conductor using a quality ohm meter (see next page for procedure) and compare this value with the maximum allowable resistance as tabulated in Table 1, page 14 of this booklet.
Or, where the electricity supply IS connected:
Test 2: Obtain the fault-loop impedance of each sub circuit using a quality fault loop impedance meter (refer to manufacturer’s instructions and page 15 of this booklet for procedure). Compare the measured fault-loop impedance value with the maximum value provided at Appendix B, Table B4.1 of the Wiring Rules.
1 If the application of a fault-loop impedance test onto an RCD protected sub circuit results in the RCD operating, then:
• the earth resistance and continuity test requirements are considered to be satisfied; and
• the fault-loop impedance value need not be considered further.
2 If the value measured is GREATER than the allowable maximum, then the following steps must be performed:
Test 2.1 Check the MEN connection, active, neutral and earth conductor/equipment connections are all solidly made.
Repeat the fault-loop impedance Test 2, above. If the fault-loop impedance values exceed the requirements
of Table B4.1, then perform Test 2.2
Test 2.2 Perform the earth continuity measurement, Test 1 and confirm results. Repeat the fault-loop impedance Test 2, above. If the fault-loop impedance values do not satisfy the requirements of Table B4.1 then contact the network operator advising them of the discrepancy.
Main Earthing Conductor
The resistance from the earthing electrode to the point where the main earthing conductor is connected to the neutral conductor of the supply system shall not exceed 0.5 ohms.
• A multimeter set to ohms.
• Insulated copper conductor of suitable length (long lead).
1. Ensure that the electricity supply has been disconnected.
2. Connect an insulated copper conductor of suitable length (long lead) to one terminal of the ohm meter.
3. Connect a standard length test lead to the other terminal of the ohm meter (short lead).
4. Connect the two leads together, and
• zero the multimeter or, if this is not possible,
• record the resistance of the test leads.
5. Disconnect the water pipe equipotential bonding conductor and the water heater earthing conductor (if applicable).
Care must be taken that there are no parallel earth paths when conducting this test, ie. the earthing system must not be connected to either the water or gas pipes.
6. Disconnect the MEN link from the main neutral link and connect it to the long lead (ie. long lead now connected to the earth bar).
A: Main Earthing Conductor
• Connect the short lead to the earth electrode and measure the earth resistance of the main earthing conductor.
• The resistance shall not exceed 0.5 ohms for the main earthing conductor.
B: Earth Continuity Test
• Using the long lead and zeroed multimeter, measure, for each circuit, the earth conductor resistance from the circuit extremity to the switchboard.
• Confirm the measured values are LESS THAN those values as provided at Table 1, page 14 of this booklet for each sub circuit.
• This test is applicable to all circuits, including socket outlet circuits, lighting circuits and fixed equipment (eg. water heater supply) circuits.
7. Re-connect the MEN link to the main neutral link.
8. Re-connect the water pipe equipotential bonding conductor.