C&G Level 3 Award in the Periodic Inspection, Testing and Certification of Electrical Installations. Earth Fault Loop Impedance Tests

Transcription

1 C&G Level 3 Award in the Periodic Inspection, Testing and Certification of Electrical Installations Earth Fault Loop Impedance Tests 1

2 Revision Inspections are made to verify that the installed electrical equipment is in compliance with Standards, and in accordance with the regulations Prior to the testing, you must check the instrument is fit for purpose. There are two methods for testing the continuity of protective conductors. It is unacceptable to simply record a result, it needs to be reasonable. The insulation resistance test is used to verify that the insulation of conductors, accessories and equipment is acceptable (has a high resistance). Site applied Insulation test is quite rare and is only carried out when insulation has been applied on site. A polarity test is carried out to determine that the line conductor only is switched, to ensure the correct operation of accessories, and to ensure that in Edison Screw lamps, the centre-contact is connected to the line conductor.

3 Outcomes of this Session describe the earth fault loop impedance paths for TN-S, TN-C-S and TT systems describe the methods of carrying out earth fault loop impedance tests in terms of the tests used for measuring actual earth fault loop impedance and methods of calculation of earth fault loop impedance from given data and measurement of conductor impedance explain how the results should be compared with relevant data such as Table 41.2, 41.3 and 41.4 of BS 7671.

4 Earth Fault Loop Impedance Regulation requires that the effectiveness of the measures for fault protection by automatic disconnection of supply is verified for TN- S, TN-C-S and TT systems. This verification can be determined by measuring or calculating the earth fault loop impedance of the circuits within an installation.

5 Typical TN-S Supply Earth (Terre) and Neutral Separate This is the newer type of TN-S system. This is a more traditional arrangement and is found in many older installations.

6 Equivalent Earth Fault Loop for a TN-S Supply A TN-S system usually has an external earth loop impedance of around 0.8 Ω. quoted by the distributor for both single and three phase supplies. The external earth loop depends on many factors such as length and cross-sectional area of the cable. The external fault loop is labelled Z e and the internal earth fault loop path is labelled (R 1 +R 2 ). When combined give the overall impedance of the fault current path. Z S = Z e + (R 1 +R 2 )

7 Typical TN-C-S Supply Earth (Terre) and Neutral Combined through part of the System This system has a combined neutral and earth in the supply cable, but at the consumers installation they separate, and are separated throughout the rest of the installation. Examples of this type of supply arrangement are PME (Protective Multiple Earthing) or PNB (Protective Neutral Bonding)

8 Equivalent Earth Fault Loop for a TN-C-S supply A typical value of a external loop impedance would be a maximum of 0.35 Ω for single-phase supplies. For three-phase supplies the figures vary.

9 TT Supply Earth to Earth (Terre - Terre) This system has no earth provided by the distributor, just the normal current carrying conductors. The consumer must supply their own earth via an earth electrode and RCD installed to A typical figure for the external earth fault loop would be 21 Ω at the distributors transformer. This value covers the resistance of the neutral to earth, the impedance of the transformer winding and the line conductor.

10 TT Supply TT systems are most commonly found on farms and villages and are fed from an overhead supply system. They can also be found on petrol station forecourts. The loop on the distributor s side passes via the general mass of earth.

11 Testing Earth Fault Loop Impedance The most common form of testing for the earth fault loop impedance is using a earth fault loop impedance tester. You can also determine the earth fault loop impedance by calculation The earth fault loop test makes sure that the protective devices operate within the required time. (anything from 0.05 s to 5 s depending on where and what you are testing) There are a number of ways in which the earth fault loop impedance can be determined. Direct measurement using an earth fault loop impedance tester Measurement of (R 1 +R 2 ) and adding to a known value of Z e Zs=Ze+(R1+R2)

12 Test Points for Earth Fault Loop Impedance Testing When testing sockets, all socket-outlets should be tested, and the worse case result recorded (the highest Zs value). When testing lighting circuits, the end point (furthest point) of the circuit should be tested. Again, all lights on the circuit should be tested, and the worse case result (the highest Zs value) recorded. Be aware of the increased risk to others Earth Fault Loop Impedance Testing is a live test and the requirements of EAWR Regulation14 apply.

13 Two-lead Instrument at a D.B Testing at the end point gives you the total loop impedance of that circuit. This is labelled Z s. You will also need to test at the intake position and at each subsequent distribution board. This is the Earth Loop Test with a two lead instrument such as a Megger. One lead is connected to the incoming feed with the other connected to the Main Earthing Terminal (MET) Earthing and main protective bonding conductors should be disconnected for the test!

14 Three-lead Instrument at a D.B Instrument set for Z S This is the Earth Loop Test with three leads such as a Robin/Fluke. One lead is connected to the neutral or neutral block, one to the line, and the other connected to the MET. Earthing and main protective bonding conductors should be disconnected for the test!

15 Guidance Note 3 does not permit testing at the terminals of a motor. You must test on the supply side of the motor. Zs Motor circuits R1 + R2 The total earth-fault-loop impedance will be a combination of the impedance readings and those from the continuity test (R1 + R2).

16 Testing for Zs at the end of the circuit You should also; Make sure that you are taking the power for the meter from the circuit that you are testing. Remove the main equipotential bonding conductors.

17 Checking the Values Once measured, the earth-fault-loop impedance should be checked; For TN systems there are a number of options: For standard thermoplastic circuits, the values in Appendix B of the On-Site Guide and Guidance Notes 3 can be used. The designers own calculated figures. Table 41.2, 41.3 and 41.4 of BS 7671, after being corrected for temperature. (20 o C 70 o C) 1.2!!!!! Using the `rule of thumb method. (80% of max.values BS7671) Remember: Read the exam paper question carefully and use the method requested as per the given scenario!!!

18 Rule of Thumb The IET Guidance Note 3 provides a rule of thumb to operate if you don t want to work through all the factors. This is now agreed to be 80%. This reduction is an allowance for the fact that a measured Zs value is measured at an ambient conductor temperature of 20 o C. However, when under full load, the conductor temperature may rise to a maximum operating temperature of 70 o C, raising the final actual Zs value. This is done by: Determining the new corrected maximum tabulated value and comparing with the measured value. The new corrected maximum tabulated value must be greater than the measured value 20 o C) in order to satisfy ADS!

19 Earth Loop (Zs) Test problems! An Earth Loop Impedance test instrument may cause an RCD, RCBO, or a 6 A Type B circuit breaker to trip when a circuit is under test. For an RCD, there are a number of solutions to this problem: Use a meter that has No-trip setting or effective D-Lok function Replace the RCD for the duration of the test (not a preferred option) with a circuit-breaker Measure the external impedance (Z e ) and add this value to the measured value of (R 1 + R 2 ) You must not short-out the RCD!! Similar solutions can be put in place to reduce the tripping of a 6 A BS EN type B circuit-breaker.

20 End of chapter 6 Earth Loop Impedance (Zs) An actual measured value of Zs would generally be lower than the calculated value due to the following factors: The presence of parallel paths due to the connection of protective bonding conductors Parallel paths through the steel conduit/trunking containment system The effect of the steel wire armour being in parallel with a conductor used as a cpc.