C&G 2395-01 Level 3 Award in the Periodic Inspection, Testing and Certification of Electrical Installations Continuity of Protective Conductors 1
Outcomes of this Session State the requirements which need to be considered for protective conductors in terms of earthing conductors, protective bonding conductors and circuit protective conductors describe the need for, and methods of verifying, the continuity of protective conductors state the relationship between conductor length, cross-sectional area and resistance state the effect of temperature on a conductor state the effect on measured resistance when cables are connected in parallel describe the need for, and methods of, verifying the continuity of ring final circuit conductors explain how the results of the tests should be analysed.
Continuity testing All protective and bonding conductors must be tested to ensure that they are electrically continuous, electrically safe and correctly connected. Prior to the testing of continuity in either radial circuits or ring final circuits, you must check three things: 1. That the instrument has batteries that are adequate for the task. They must be replaced regularly to maintain the current we require. 2. The connecting leads and the instrument must be nulled/zeroed. 3. A current calibration certificate or a current record of the accuracy of the test instrument.
Continuity of protective conductors
Continuity of protective conductors This test is carried out to ensure that the protective conductor links together all exposed conductive parts to the main earthing terminal (MET). A circuit protective conductor (cpc) must be present at all points in the wiring system (411.3.1.1) In a ring final circuit, the cpc must also be wired in a RING configuration!
Continuity of protective conductors Firstly, it is to ensure that any earthing system is a continuous and a low resistance path to allow the passage of fault currents of sufficient magnitude to operate the protective device during fault conditions within the required disconnection time. Secondly, confirmation of the zero potential between metal parts, both extraneous and exposed-conductive-parts, within an installation to reduce the effects of electric shock for the user of the electrical installation. Thirdly, that the conductors, including all line and neutral conductors, are connected in the correct sequence for safety and for operational purposes.
Continuity of protective conductors The GN3 recommended maximum resistance between accessible exposed conductive parts should be 0.05Ω. (p.35 GN3)
Parallel Paths The continuity test(s) test must be carried out prior to the connection of any supplementary bonding conductor or main bonding conductor. If there are either supplementary bonding conductors or main bonding conductors connected, then you may not be testing just the circuit protective conductor, you may also be testing a parallel combination of earthing conductors. This will lead to a different readings.
Continuity Testing of Radial Circuits Regulation 612.2.1 requires testing for continuity. As far as the continuity test is concerned, the earthing conductor, the main bonding conductors, the supplementary bonding conductors and the circuit protective conductors must all be tested. There are two methods for testing the continuity of protective conductors. These are called Method 1 and Method 2. Method 1 (R1 + R2) is generally used for circuits. Method 2 (long wander lead) is generally used for single conductors, such as bonding conductors.
Method 1 Now for the test itself: Where necessary, first prove that the installation or circuit is dead. If the circuits that are supplied from the same distribution board cannot be made safe (i.e. dead) then you should not carry out this test. Connect the line conductor to the protective conductor at the distribution switchboard using a temporary test link. Test between the line and circuit protective conductor at the remote appliance. The measurement of (R 1 +R 2 ) should be recorded in Ohms (Ω). You don t need to test every point, just the furthest points. If there is a need (although unlikely) to determine the resistance of the circuit protective conductor R 2 as a value then the following can be used; R 2 = R R x A 1 A 1 + A 2 OR R 2 = (R 1 +R 2 ) x CSA line CSA line + CSA cpc
Readings From Test Instruments Regulation 612.1 requires that we compare our results with relevant criteria. It is unacceptable to simply just record a result! It needs to be reasonable! In different circumstances, protective devices must operate within set times. A higher fault current may be needed to make the protective device operate in 0.2 s as opposed to 5 s. The cpc must be capable of withstanding the rise in temperature, due to the fault current for the set time. (Time/Current Characteristics). Current increase = Temperature increase. Will the resistance reading of the continuity test allow you to make a reasonable judgement, in accordance with BS 7671, about the time it takes to clear the fault? Can you guarantee the cpc will not open circuit before the protective device operates? If this last question can be answered with a YES, then the results will be acceptable.
What happens if the test is unsatisfactory? First find the type of fault. Open circuit on a cpc A cable could have been cut by another person A nail could have split the cpc The connections are not tightened up adequately and the cpc could have worked free. Borrowed cpc This could happen if the electrician has used a cpc from another circuit to provide an earth for a circuit. This is not acceptable! Borrowed neutral This is an all too common fault when electricians take a neutral from one circuit to give to another circuit. This breaches Regulation 314.4 and is extremely dangerous practice!
Continuity of ring final circuit conductors This test is carried out to verify the continuity of the line, neutral and protective conductors and correct wiring of every ring final circuit. The test results show if the ring has been broken or interconnected to create an apparently continuous ring circuit which is in fact broken or connected as a figure of eight configuration. (GN3 2.7.6) End of chapter 4a