C&G 2395-01 Level 3 Award in the Periodic Inspection, Testing and Certification of Electrical Installations Phase rotation and verification of voltage drop 1
Outcomes of this Session describe how to assess the direction of phase rotation explain how voltage drop might be assessed describe how to determine voltage drop compliance from given criteria describe the requirements for functional testing describe how to complete a schedule of test results
New requirements In this final session we will briefly consider two new requirements for testing introduced by BS 7671 2008. These are: Phase rotation Verification of voltage drop Additionally, we will consider the requirements for functional testing and the practical ways in which testing might be carried out.
Phase Rotation Regulation 612.12 requires that the phase sequence of a multiphase circuit is maintained. In practice this is a comparison test. The check on phase rotation should be made at the incoming supply and then at each position where it is necessary to assess whether there has been any alteration in the phase sequence. This may occur where, a colour-coded steel wire armoured cable feeds a motor control centre and single colour single-core cables then connect the motor control centre to a motor. The single-core cables may all have the same colour and it would be necessary to assess the phase sequence under such circumstances.
Phase Rotation Sequence Test GN3 2.7.17 pg. 54-55 The two instruments used to carry out this test are: rotating disc type indicator lamp type Instruments containing both of the above types are also available The three leads from the test instrument should be connected to each of the lines on the supply side and the test performed. A second test should then be carried out after there has been any colour change etc. to assess whether any change of sequence has occurred. The test instrument is likely to show up either a rotation of 1 2 3 or of 3 2 1. This simply tells you whether the rotation is different. If there is a difference all you need to do is change over any two lines.
Phase Rotation Sequence Test GN3 2.7.17 pg. 54-55 rotating disc type indicator lamp type
Phase Rotation 1. At the origin Three places where we need to confirm phase rotation 2. At the 3 phase distribution board 3. At any other relevant 3 phase equipment in the installation There is no right or wrong way for phase sequence provided that it is consistent throughout the building/installation!
Verification of Voltage Drop Verification of voltage drop, is normally only carried out on a Periodic Inspection & Test of an existing installation, as on a new installation, this should have already been calculated and verified by the designer, prior to testing!
Verification of Voltage Drop The requirements of BS 7671 for the verification of voltage drop is new. Regulation 525.1 requires that in the absence of other considerations, under normal service conditions, the voltage at the terminals of any fixed current-using equipment, shall be greater than the lower limit corresponding to the product standard relevant to the equipment. Appendix 12 of BS 7671 Table 12A (App.4 table 4Ab BS7671 Amd.1) - Voltage drop (i) Low voltage installations supplied directly from a public low voltage distribution system Lighting Other use 3 % 5 % (ii) Low voltage installation supplied from private low voltage supply * 6% 8% *The voltage drop within each final circuit should not exceed the values given in (i). Where the wiring systems of the installation are longer than 100 m, the voltage drops indicated above may be increased by 0.005 % per metre of the wiring system beyond 100 m, without this increase being greater than 0.5%. The voltage drop is determined from the demand by the current-using equipment applying diversity factors where applicable, or from the value of the design current of the circuit. For a new installation the verification of the voltage drop should have taken place during the design stage so there is no reason why any other test or calculation is performed. Where necessary, determine the likely voltage drop from the measured (R 1 + R 2 ) after amending the figures to get the resistance of the line conductor R 1.
Calculating voltage drop using voltage drop formula method, and circuit impedance and Ohms Law method The 17th edition of BS7671 introduced a new requirement that, where necessary, volt drop for a circuit should be verified to ensure that it does not exceed the limits given in (App.4 BS7671 Amd.1) - 3% for lighting circuits (6.9v) and 5% for other circuits (11.5v) (public supply system). The usual method for calculating volt drop involves the mv/a/m figure for the given cable found in the relevant table and the formula:
However, for the 2395, voltage drop will typically be evaluated, using the measured circuit impedance and Ohms Law. For this reason, typically in the 2394/5 you will not be given the mv/a/m figure, but instead, somewhere on the paper, you will be given the mω/m figure for different size cables, and you are expected to use this value to calculate Zs and Volt drop.
The method for calculating volt drop using circuit impedance involves a similar method as calculating R1 + R2 which you are now familiar with?? But, instead of the c.p.c. (R2 ), we are now looking at the live conductor circuit resistance. (remember, at this level, impedance = resistance) so we need to find R1 + Rn. (we can discount the Ze of the circuit, as BS7671 and GN3 states that The requirements for voltage drop are deemed to be met where the voltage drop between the origin and the relevant piece of equipment does not exceed the values stated in Appendix 4 BS7671 Amd.1) In other words, we assume that we have the full Uo at the intake and it is the resistance of the final circuit which will effect the end voltage.
Example: A circuit is 70m long, wired in 10mm 2 with a design current of 45A. Does it meet the requirements for volt drop? (Assume a 230V public supply). Firstly calculate R1 + Rn R1 + Rn = R1 mω/m + Rn mω/m x L x 1.2 1000 1.83 + 1.83 x 70 x 1.2 = 0.31Ω 1000 Therefore R1 + Rn = 0.31Ω The 1.2 represents a correction factor applied to the tabulated resistance values for conductors, mω/m (GN3Table B1 p.120 & O.S.G. Table I1 p.182), which are at 20 C, to give values for a fully loaded cable at 70 C (GN3 Table B2 p.121 & O.S.G. Table I2 p.183) (Full load current = higher cable temperature = higher resistance = greater volt drop!)
Note that the 1.2 factor is not applied this time, as the tabulated mv/a/m figures in the regs, have already been corrected to 70 o C!
As stated in GN3 p.60, measurement of voltage drop within an installation is not practical as this would mean measuring the instantaneous voltage at both the origin and at the point of interest simultaneously, together with the instantaneous load current (2.7.20). It is usually sufficient to check that voltage drop calculations have been undertaken.
VOLTAGE DROP Points to remember - Calculation methods: 1. Volt drop calculation using formula 2. Circuit impedance and Ohms Law R1 + Rn = R1 mω/m + Rn mω/m x L x 1.2 1000 Then: Ohms Law: V = I x R or in this case Vd = Ib x (R1 + Rn)
VOLTAGE DROP Points to remember Correction factor to be applied when using circuit impedance method to compensate for conductor operating temperature where resistance is measured at 20 o C Eg. 20 o C to 70 o C = 1.2
Functional Testing including RCDs
Functional Testing including RCDs The requirements for functional testing cover both RCD s and assemblies such as switchgear and control gear assemblies, drives, controls and interlocks. It is important, as part of the commissioning process for the person carrying out the testing to ensure that switchgear and control gear functions correctly. The inspector should check, amongst other things, that: Circuit breaker's open and close the circuit that they protect Isolators open and close a circuit Overloads are set correctly Switches control the correct circuits Emergency stop buttons work correctly etc
Sequence of Testing The sequence of tests as laid down in BS 7671, gives a clear route through the tests. The reasons why certain tests are carried out in a set order are as follows: Certain tests are very similar and can be performed at the same time e.g testing the continuity of protective conductors using Method 1 and polarity, and testing ring circuits and polarity. It is important however that continuity is assessed prior to insulation resistance, as this ensures that any breaks in the protective conductors are found. It is also important that the earth loop impedance is found to be acceptable prior to RCD tests being performed, as the RCD test places a fault on the protective conductor which can be dangerous should there be a break in the earth fault loop.
The sequence of tests 2.7.4 GN3 1. Continuity of protective conductors 2. Continuity of ring final circuit conductors 3. Insulation resistance 4. Polarity - Dead 5. Polarity Live voltage test 6. Earth fault loop impedance (Ze and Zs) 7. Prospective fault current earth fault and shortcircuit (ka) 8. RCD testing tripping times (ms) 9. Functional testing including RCDs (test button) 10.Voltage drop
Possible Testing Order Continuity of protective conductors and polarity using Method 1. Test every radial circuit for each distribution board and record your results. In addition use Method 2 to test the bonding and earthing conductors. Continuity of ring final circuit conductors and polarity. Test every ring circuit for each distribution board and record results. Insulation resistance test. Turn every breaker on; disconnect every electronic device; all switches on; all fuses is in/breakers on. Test from the main intake position. This will make sure that every part of the installation is tested without having to carry out the same test at each board. Done once at the right place, and this test should take no longer than 5 minutes. As long as the overall insulation resistance value is acceptable, then this value can be recorded for each separate circuit. Turn all switches off before re-energising the system. This will ensure that when the system is energised, only part of it will become live initially, and you will have control. continued
Possible Testing Order Test for live polarity. This ensures that the distributor has connected the main supply the right way round. Test for external earth loop impedance (Ze). Record the results. Test for prospective short circuit current (PSCC) and earth fault current (PEFC). The highest value of the two should be recorded. Turn each subsequent distribution board on and test for earth fault loop impedance (Zs) and prospective short circuit current (Pfc) at each board. Record the results. Turn each circuit on in turn and test earth loop impedance (Zs) for each circuit. Record the results. Test each RCD and record the disconnection times where relevant. Check each circuit works appropriately, phase rotation is correct, each isolator functions appropriately etc. Functional testing including RCD integral test button. Operate and check for correct mechanical functionality of all switches and manually operated switching devices and isolators including RCD integral test button. Voltage Drop verification of voltage drop if applicable. Give this list consideration, but recognise that it is only one option providing a possible route through the testing process.
Completing a Schedule of Test Results All test results, as with all inspection results, must be recorded on a Schedule of Test Results. Neither an EIC or a EICR is complete without one. Appendix 6 of BS 7671 details a model schedule. All other schedules produced must contain at least this information, although they are permitted to contain more. End of chapter 8