Engineering Technical Report 129 ROEP Risk Assessment For Third Parties Using Equipment Connected To BT Lines 2006 Draft for Approval
2006 Energy Networks Association All rights reserved. No part of this publication may be reproduced, stored in retrieval or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written consent of Energy Networks Association. Specific enquiries concerning this document should be addressed to: Engineering Directorate Energy Networks Association 18 Stanhope Place Marble Arch London W2 2HH This document has been prepared for use by members of the Energy Networks Association to take account of the conditions which apply to them. Advice should be taken from an appropriately qualified engineer on the suitability of this document for any other purpose.
Page 3 CONTENTS Foreword... 5 1 Scope... 5 2 Definitions... 7 3 ROEP Magnitude... 9 4 Phase to Earth Faults... 10 5 Risk Assessment Procedure... 11 6 Calculations... 12 6.1 Telephone... 12 6.1.1 Telephone (without LVAC power supply)... 12 6.1.2 Telephone (with LVAC power supply)... 12 6.1.3 Public Telephone Kiosk... 14 6.2 Fax Machine... 15 6.3 Modem... 15 6.4 Miscellaneous Equipment... 16 6.5 Line Jack Unit... 16 6.6 Summary of Results... 16 7 Conclusions... 18 8 Recommendations... 19 8.1 Reassessment of ROEP contour distances... 19 8.2 Site Specific Risk Assessment... 19 8.3 Earthing System Modifications... 19 8.4 Line Isolation... 19 9 References... 20 Appendix A Risk Assessment Calculations... 21 Appendix B - Distribution System Fault Rates And Durations... 26 Introduction... 26 Fault Statistics... 26 NAFIRS... 26 DNO Fault Records (HV)... 27 Conclusions... 27 References... Error! Bookmark not defined. FIGURES Figure 1a Elevation Illustrating Transferred Potentiala... 5 Figure 1b Plan View of Hypothetical Substation Illustrating ROEP contours... 6 Figure 6.1.1 Touch Potential Telephone (without LVAC power supply)... 12 Figure 6.1.2 Touch Potential Telephone (with LVAC power supply)... 13 Figure 6.1.3 Touch Potential Telephone Kiosk... 14 Figure 6.2 Touch Potential Fax Machine... 15 Figure 6.3 Touch Potential - Modem... 15 Figure B1 - Number of earth faults versus the percentage of theoretical maximum ROEP.. 27 TABLES Table 6.6 Individual Risk Levels... 17 Table 7 Consequences by ROEP level... 18 Table A1 6.1.1 Telephone (without LVAC power supply), domestic premises, ROEP = 1700V... 21 Table A2 6.1.3a Public telephone kiosk ROEP=1150V... 22 Table A3 6.1.3b Public telephone kiosk ROEP=1700V... 23
Page 4 Table A4 6.4 Miscellaneous Equipment (ROEP=1150V)... 24 Table A5 6.5 Line Jack Unit (ROEP =1150V)... 25
Page 5 ROEP RISK ASSESSMENT FOR THIRD PARTIES USING EQUIPMENT CONNECTED TO BT LINES FOREWORD This Report has been prepared by a joint working group of Accenture HR services on behalf of British Telecommunications (BT) and the Energy Networks Association (ENA). This report considers hazards to third parties close to electricity substations and/or generating stations. For the purposes of this report all such sites are referred to as substations. These hazards arise from potential differences (due to ROEP caused by an earth fault in the substation) between the third party property infrastructure and any BT line serving the property. The approach adopts the principles established in ITU-T K33 and is consistent with national health and safety legislation, in particular The Management of Health and Safety at Work Regulations 1999. This risk assessment is solely for use by members of the ENA and BT and no responsibility is accepted for its use by other parties. This document does not seek to address the costs and responsibilities involved with measures taken to mitigate ROEP hazards. Guidance on this aspect is provided in ENA ER S36-1. 1 SCOPE This report considers hazards to third parties with BT services from transferred potentials in the ROEP zone of an electricity substation. Figure 1a illustrates how a transferred potential hazard can arise and Figure 1b illustrates ROEP contours at a hypothetical substation. NOTE The ROEP contours shown in Figure 1b should not be taken as representative of any particular substation. Figure 1a Elevation Illustrating Transferred Potentiala The assessment herein includes both fast fault clearance and slow fault clearance times.
Page 6 This report considers the hazard to persons arising from electric shock. Secondary risks associated with electric shock, i.e. falls are not considered. This risk assessment does not cover risks to BT operators working on services within the ROEP zone. These risks are addressed in a separate document. Figure 1b Plan View of Hypothetical Substation Illustrating ROEP contours
Page 7 2 DEFINITIONS Rise Of Earth Potential or ROEP The voltage set up between substation metalwork and true earth potential due to fault current ROEP Zone The zone within which the ROEP may exceed 650V (fast fault clearance) or 430V (slow fault clearance). Individual Risk Probability of fatality of an individual per annum Electricity Company Companies responsible for electricity transmission, distribution or generation at 33kV and above. Fast fault clearance <200ms on average Slow fault clearance >200ms and on average 500ms Third Party Any individual apart from those working for either BT or an Electricity Company. P FB Probability of heart fibrillation P E Probability of exposure P F Probability of an earth fault which gives rise to significant ROEP R s Source resistance (equal to 0 Ohms) R b Body resistance (equal to 650 Ohms to represent 95% of population) R be Resistance between body and earth R ib Resistance between body and source potential R t R s + R b + R be + R ib V B Circuit breakdown voltage
Page 8 I F Current flowing in exposure circuit D f Decrement factor
Page 9 3 ROEP MAGNITUDE ROEPs quoted are rms values. However, a maximum initial offset of 2.8 times the rms value will occur where the fault is initiated very close to current zero, and where the electrical system inductive reactance is very much greater than its resistance. Statistically, the likelihood of a fully offset waveform is therefore low. However, in this assessment, a maximum electrical system resistance/inductive reactance ratio (X/R) of 14 is used for this assessment. This corresponds to an initial offset current of 2.74 times the rms value, i.e. approaching the maximum value. Insulation breakdown voltages used in this assessment are rms values i.e. the peak withstand is 1.42 times the rms value. Therefore to allow for the offset the quoted breakdown voltages are reduced by a factor 2.74/1.42 = 1.93. Since the probabilities of heart fibrillation are based on the energy content of a symmetrical current waveform, it is necessary to establish an equivalent rms value of the asymmetrical current waveform. IEEE 80 provides a means of calculating a decrement factor Df by which the rms value can be multiplied to account for asymmetry as follows: Df = Ta 1+ tf 2tf [ 1 e ] Ta where Ta is the offset time constant i.e. X/2πfR in seconds tf is the time duration of the fault in seconds For a fault duration of 200ms and an X/R ratio of 14, Df is approximately equal to 1.2. This factor is therefore taken into account in the determination of the probability of heart fibrillation.
Page 10 4 PHASE TO EARTH FAULTS For substations with equipment having fast clearance times, the probability of a fault (P F ) is estimated to be 0.5 faults per year for substations having fast fault clearance times and 1.0 fault per year for substations having slow fault clearance times. It is recognised that fault rates will vary depending on substation size, location, feeding arrangements etc. but it is considered sufficiently representative for the purposes of this assessment. It is also necessary to consider the average fault clearance time. It is estimated that for slow fault clearance the average clearance time is 500ms and for fast fault clearance the average clearance time is 200ms.
Page 11 5 RISK ASSESSMENT PROCEDURE A review of the types of equipment connected to BT lines was undertaken and an estimate of circuit resistances/breakdown voltages was made using data in [5]. The circuit current was determined by dividing the ROEP by the total circuit resistance (breakdown voltages were also taken into account). The probability of fibrillation (P FB ) was determined using IEC 60479-1. The activities of third parties using equipment connected to BT lines was considered and the probability of exposure (P E ) estimated. Using this information along with the probability of a fault (P F ) it was possible to estimate the level of individual risk to third party individuals. The calculations assume the worst case combination of potentials e.g. for an LVAC supplied telephone, the LVAC supply is assumed to be at ROEP. The intolerable risk threshold level used is 1 in 10,000 as advocated by HSE for members of the public.
Page 12 6 CALCULATIONS Equipment types, third party premises and activities have been considered as follows: 6.1 Telephone The risk assessment for telephone use considers the use of the handset only since time in contact with pushbuttons is assumed to be relatively small. The insulation level for the pushbuttons is considered to be no less than that for the handset, based on experimental data. 6.1.1 Telephone (without LVAC power supply) Exchange earth Telephone Metalwork at ROEP Figure 6.1.1 Touch Potential Telephone (without LVAC power supply) The worst case scenario for use of telephone by a third party in domestic premises is considered to be within a high humidity environment e.g. a kitchen. Experimental data indicates a minimum telephone handset breakdown voltage of 3700V rms at 97% Relative Humidity (note that in a low humidity environment the breakdown voltage is in excess of 8000V rms). A value of 3330V rms is used for the assessment i.e. a safety factor of 10% is applied to the lowest test breakdown voltage. Application of the breakdown voltage factor of 1.93 from section 4 reduces this to approximately 1700V rms. This results in no current flow below 1700V rms and hence negligible individual risk. At 1700V and above the individual risk level is assessed in the upper tolerable region, see Appendix 1 Table A1(6.1.1). In addition, at 1700V rms and above there is a likelihood of damage to equipment and loss of service. 6.1.2 Telephone (with LVAC power supply) The LVAC supply is assumed to be at ROEP, since this represents the worst case condition for equipment failure and consequent loss of service (it is unlikely that the LVAC supply will have a different potential compared with exposed metalwork due to equipotential bonding in accordance with BS 7671). It is assumed that the telephone LVAC supply is insulated in accordance with IEC60664-1 to class II insulation levels. This means that the supply is insulated to 2860V rms (application of the offset factor from section 4 reduces this to approximately 1480V rms) and that a protective earth connection is not supplied. Two equipment configurations are considered: A. That an earth connection is provided which is connected to ELV circuitry within the
Page 13 telephone to provide a signalling path e.g. subscribers private metering and B. No earth connection is provided. Exchange earth Telephone Metalwork at ROEP Earth connection (where fitted) L&N LVAC supply at ROEP Figure 6.1.2 Touch Potential Telephone (with LVAC power supply) It is assumed that the telephone is manufactured in accordance with BS EN 60950 [2] and therefore has line isolation with minimum impulse withstand of 1500V. However, experimental data indicates a minimum breakdown voltage of 2500V rms. A value of 2250V rms is used for the assessment i.e. a safety factor of 10% is applied to the lowest test breakdown voltage. Application of the breakdown voltage factor of 1.93 from section 4 reduces this to approximately 1150V rms. Experimental data indicates a minimum telephone handset breakdown voltage of 3700V rms at 97% Relative Humidity (note that in a low humidity environment the breakdown voltage is in excess of 8000V rms). A value of 3330V rms is used for the assessment i.e. a safety factor of 10% is applied to the lowest test breakdown voltage. Application of the breakdown voltage factor of 1.93 from section 4 reduces this to approximately 1700V rms. For configurations A and B, for ROEP less than 1150V rms, breakdown should not occur and the risks are therefore assessed as negligible. For configuration B, breakdown may occur at 1480V, causing equipment damage and loss of service. For configuration A, breakdown may occur at 1150V, with the same consequences. At 1700V rms and above the resultant risks are considered the same as in 6.1.1.
Page 14 6.1.3 Public Telephone Kiosk Public telephone kiosk Exchange earth LVAC supply L & N only Local earth at ROEP Ground at ROEP Figure 6.1.3 Touch Potential Telephone Kiosk It is assumed that an individual will be in contact with metallic enclosure of the telephone when in use through either resting a hand on the wall mounted unit or by holding the cord to the handset or by touching the kiosk metalwork. It is assumed that the wall mounted unit is electrically connected to the kiosk metalwork through its mountings. It is assumed that the telephone is LVAC supplied, but that the supply is insulated in accordance with IEC60664-1 to class II insulation levels. This means that the supply is insulated to 2860V rms and a protective earth connection is not supplied. Application of the breakdown voltage factor of 1.93 from section 3 reduces this to approximately 1480V rms. A local earth is provided which is connected to ELV circuitry within the telephone to provide a signalling path for e.g. subscribers private metering. It is assumed that there is no insulation between the ELV circuitry earth and the metallic enclosure of the telephone. It is assumed that the telephone is manufactured in accordance with BS EN 60950 and therefore has line isolation with minimum impulse withstand of 1500V. However, experimental data indicates a minimum breakdown voltage of 2500V rms. A value of 2250V rms is used for the assessment i.e. a safety factor of 10% is applied to the lowest test breakdown voltage. Application of the breakdown voltage factor of 1.93 from section 4 reduces this to approximately 1150V rms. There is no current flow below 1150V rms and hence negligible individual risk. At 1150V rms and above (slow clearance) the individual risk level is assessed as unacceptable, see Appendix 1 Table A2 (6.1.3a). In addition, at 1150V rms and above there is a likelihood of damage to equipment and loss of service. For fast clearance times the threshold voltage for 0% chance of fibrillation is approximately 1700V. Above 1700V the individual risk level is within the upper tolerable region, see Appendix 1 Table A3 (6.1.3b).
Page 15 6.2 Fax Machine Exchange earth Fax machine Ground at ROEP LVAC supply at ROEP Figure 6.2 Touch Potential Fax Machine The scenario of the fax machine is considered to be much the same as that of the LVAC supplied telephone in 6.1.2. Where the fax machine has a handset the risks associated with using the handset are considered identical to those in 6.1.2. Experimental data indicates a minimum push button breakdown voltage in excess of 5000V, but assuming this as the minimum value, application of a 10% safety factor and the breakdown voltage factor of 1.93 from section 4 reduces this to approximately 2300V rms. Since the assessment in 6.1.2 indicates that mitigation is required at a lower voltage, analysis at 2300V is not worthwhile, and the outcome is considered the same as in 6.1.2. 6.3 Modem Exchange earth Modem PC Keyboard or mouse LVAC supply at ROEP LVAC supply at ROEP Ground at ROEP Figure 6.3 Touch Potential - Modem The scenario of the modem is considered to be similar to that of the LVAC supplied telephone in 6.1.2, other than that a keyboard or mouse is used instead of a handset.
Page 16 Experimental data indicates a minimum keyboard and mouse breakdown voltage in excess of 5000V, but assuming this as the minimum value, application of a 10% safety factor and the breakdown voltage factor of 1.93 from section 4 reduces this to approximately 2300V rms. Since the assessment in 6.1.2 indicates that mitigation is required at a lower voltage, analysis at 2100V is not worthwhile, and the outcome is considered the same as in 6.1.2. 6.4 Miscellaneous Equipment Miscellaneous equipment includes television set top boxes, alarm systems etc, i.e. equipment which has a connection to the BT network, is LVAC powered but where manmachine contact is infrequent. It is assumed that the equipment may be connected to other associated equipment e.g. a television set in the case of a set top box. The scenario for miscellaneous equipment is considered to be similar to that of the LVAC supplied telephone in 6.1.2, other than for the man-machine contact medium e.g. pushbuttons, rotary control knobs etc. At 1150V and above there is a likelihood of power supply or line isolation breakdown i.e. damage to equipment and loss of service. Since this assessment covers a wide range of equipment, experimental data for the manmachine contact medium is not considered representative. Taking a pessimistic approach, breakdown voltage is neglected in which case the individual risk at voltages of 1150V and above is assessed as tolerable, see Appendix 1 Table A4 (6.4). For fast clearance times the threshold voltage for 0% chance of fibrillation is approximately 3500V. Above 3500V the individual risk level remains within the tolerable region. 6.5 Line Jack Unit A person could come into contact with the metallic connections in the LJU or metallic wires when for example fitting a telephone extension. For non-domestic premises it is assumed that the operator will disconnect from the BT network prior to making terminations, consequently eliminating the hazard. For domestic premises the hazard is assessed as negligible, for voltages of 1150V and above see Appendix 1 Table A5 (6.5). 6.6 Summary of Results All the risk level calculations are detailed in Appendix 1 and the results are summarised in Table 6.6.
Page 17 Scenario 6.1.1 Telephone (without LVAC power supply) 6.1.2 Telephone (with LVAC power supply) 6.1.3 Public telephone kiosk Voltage contour (V rms) 0<ROEP<1700 1700 ROEP 0<ROEP<1150 1150 ROEP<17 00 1700 ROEP 1150 ROEP<17 00 1700 ROEP 6.2 Fax machine As 6.1.2 6.3 Modem As 6.1.2 Individual Risk Negligible Upper tolerable (fast clearance) Unacceptable (slow clearance) Negligible Negligible Upper tolerable Unacceptable (slow clearance) Negligible (fast clearance) Upper tolerable (fast clearance) Equipment risk Loss of equipment/ service possible Loss of equipment/ service possible Loss of equipment/ service possible Loss of equipment/ service possible Loss of equipment/ service possible 6.4 Miscellaneous equipment 0<ROEP<1150 1150 ROEP Negligible 6.5 Line Jack Unit (LJU) 1150 ROEP Negligible Lower tolerable (slow clearance) Loss of equipment/ service possible Table 6.6 Individual Risk Levels
Page 18 7 CONCLUSIONS The conclusions are summarised below in Table 7. ROEP 0<ROEP<1150 Consequence Individual None Equipment None 1150 ROEP<1700 1700 ROEP Unacceptable (slow clearance) Negligible (fast clearance) Unacceptable (slow clearance) Upper tolerable (fast clearance) Loss of equipment/service likely Loss of equipment/service likely Table 7 Consequences by ROEP level
Page 19 8 RECOMMENDATIONS It is recommended that BT and the ESI adopt the following thresholds for ROEP contours that could affect BT services within third party properties. 1150 Volts (Slow clearance protection) 1700 Volts (Fast clearance protection) For ROEP voltages above these values mitigation measures should be considered that will reduce these risks. There are a number of options that can be used as noted below. 8.1 Reassessment of ROEP contour distances The accuracy with which ROEP contours are calculated varies with the methodology adopted. For example, the use of advanced software to model an earthing system may potentially produce a more accurate result than that by simple calculation. The simple methods tend to overestimate contour distances since safety factors are normally included to allow for inaccuracy. Therefore, by carrying out a more accurate assessment, the ROEP contour distances may justifiably be reduced. 8.2 Site Specific Risk Assessment The risk assessment undertaken in this document is generic in nature, intending to cover all reasonably foreseeable hazards. However, at some sites not all hazards will arise. For example, there are likely to be few sites where public telephone kiosks are affected by ROEP. Where there are no kiosks the associated risks do not arise, and this could significantly affect the outcome of the assessment. 8.3 Earthing System Modifications The design of an earthing system (along with fault current and ground resistivity) determines the resultant ROEP contour distances and profiles. For example, installing additional earthing on one side of an earthing system may result in the ROEP contour distances reducing on the opposite side. So, where third party property is only affected say on one side of a substation, and there is vacant land available on the opposite side, additional earthing may result in a reduced third party impact. However, care should be taken to determine that the land used for additional earthing is not likely to be subject to future development. 8.4 Line Isolation BT lines may be afforded suitable electrical isolation. By installing line isolation units at affected properties the electrical continuity to the remote (exchange) earth is broken, thereby eliminating any transferred potential hazard. However, this option would raise other issues; for example the need to give property owners advance warning of higher costs for telephone service. Note that these options are not set out in any particular order of preference and the option(s) actually adopted are likely to vary from site to site, taking due account of the most cost effective approach. It is noted that by implementing these recommendations there will be a residual risk of equipment damage at sites having 1150 ROEP<1700 where the ROEP is caused by a fault which would be cleared by high speed protection. However, this is considered acceptable bearing in mind the pessimistic nature of the assessment (e.g. the inclusion of voltage waveform offset from section 3) and the historically low known occurrence of equipment failure.
Page 20 9 REFERENCES ITU-T K33 Limits for people safety related to coupling into telecommunications system from ac electric power and ac electrified railway installations in fault conditions 1996 The Management of Health and Safety at Work Regulations 1999 ETR128, Risk Assessment for telecommunications operators working in a ROEP zone, Joint Working Group of Energy Networks Association and Accenture HR Services (on behalf of British Telecommunications). IEEE Standard 80 Guide for safety in AC substation grounding 2000 IEC 60479-1 Effects of current on human beings and livestock Reducing Risks, Protecting People, HSE 2001. BS 7671 Requirements for Electrical Installations 1992 IEC60664-1 Insulation co-ordination for equipment within low voltage systems 2002 BS EN 60950 2000 Specification for safety of information technology equipment, including electrical business equipment. Electricity Association Engineering Recommendation S36-1. Procedure to Identify and Record Hot Substations. 1988. NGC document - Risk Assessment associated with Ground Potential Rise at National Grid Substation Sites, draft issue 2.
Page 21 APPENDIX A RISK ASSESSMENT CALCULATIONS Third Party Voltage Contour Map Reference P F Activity P E Circuit resistances R s, R b, R ib, R be, R T Details Telephone user 1700V N/A 0.5 (fast clearance) 1.0 (slow clearance) Using telephone in kitchen while simultaneously touching earthed metallic infrastructure e.g. central heating radiator 0.014 x 0.1 x 0.1= 0.00014 R b =650 R ib = 30 R be = 0 R s = 0 R t = 680 Comments Use of phone = 0.014 (20mins/day) Percentage of phone use in kitchen during high humidity est. 10% Percentage of time in contact with earthed metalwork est. 10% V B 1700V Includes breakdown factor 1.93 I F 1.2A P FB 1.0 Individual Risk 1 in 14,000 approx (fc) 1 in 7,000 approx (sc) Includes D f = 1.2 and heart current factor 0.4 Tolerable (upper) Unacceptable Table A1 6.1.1 Telephone (without LVAC power supply), domestic premises, ROEP = 1700V
Page 22 Third Party Voltage Contour Map Reference P F Activity Details Telephone user 1150V N/A 0.5 (fast clearance) 1.0 (slow clearance) Using telephone Comments It is assumed that third party uses the public telephone as their primary telephone P E 6.9E-3 x 0.5 = 3.47x10-3 Use of phone = 10 mins per day Time touching metalwork est. 50% Circuit resistances R s, R b, R ib, R be, R T R b =650 R ib = 0 R be = 5000 R s = 0 R t = 5650 Assume damp leather shoes on hard soil V B I F 0.24A Includes D f = 1.2 P FB Individual Risk 0 0.5 0 @ 200ms 1 in 550 approx @ 500ms @ 200ms @ 500ms Negligible Unacceptable Table A2 6.1.3a Public telephone kiosk ROEP=1150V
Page 23 Third Party Voltage Contour Map Reference P F Activity Details Telephone user 1700V N/A 0.5 (fast clearance) Using telephone Comments It is assumed that third party uses the public telephone as their primary telephone P E 6.9E-3 x 0.5 = 3.47x10-3 Use of phone = 10 mins per day Time touching metalwork est 50% Circuit resistances R s, R b, R ib, R be, R T R b =650 R ib = 0 R be = 5000 R s = 0 R t = 5650 Assume damp leather shoes on hard soil V B I F 0.36A Includes D f = 1.2 P FB 0.05 @ 200ms Individual Risk 1 in 11,000 approx @ 200ms Tolerable (upper) Table A3 6.1.3b Public telephone kiosk ROEP=1700V
Page 24 Third Party Voltage Contour Map Reference P F Activity Details Misc. equipment user 1150V N/A 0.5 (fast clearance) 1.0 (slow clearance) Touching equipment (or other equipment connected to it) controls. Comments Direct contact usually obviated by use of remote controls P E 5.8x10-5 day estimated at 5 seconds Time in direct contact with equipment per Circuit resistances R s, R b, R ib, R be, R T R b =650 R ib = 0 R be = 1500+10000 R s = 0 R t = 12,150 Assume bare feet and dry foam backed carpet and dry concrete floor (no DPC) V B 0 Assume zero since not known I F 0.11A Includes D f = 1.2 P FB Individual Risk 0 @ 200ms 0.05 @ 500ms 0 @ 200ms 1 in 350,000 approx @ 500ms Negligible Tolerable (lower) Table A4 6.4 Miscellaneous Equipment (ROEP=1150V)
Page 25 Third Party Voltage Contour Map Reference P F Activity Details Person fitting extension to LJU in domestic premises 1150V N/A 0.5 (fast clearance) 1.0 (slow clearance) Touching metallic connections/wires Comments P E 1.9x10-6 per year estimated at 1 minute Time in direct contact with connections Circuit resistances R s, R b, R ib, R be, R T R b =650 R ib = 0 R be = 1500+10000 R s = 0 R t = 12,150 Assume foam backed carpet and dry concrete floor (no DPC) V B I F 0.11A Includes D f = 1.2 P FB 0 @ 200ms 0.05 @ 500ms Individual Risk 1 in 10,000,000 Negligible Table A5 6.5 Line Jack Unit (ROEP =1150V)
Page 26 APPENDIX B - DISTRIBUTION SYSTEM FAULT RATES AND DURATIONS Introduction The analysis within this document and Engineering Technical Report 128 uses fault rates of 0.5 and 1.0 faults per year corresponding to fast and slow average fault clearance times of 200ms and 500ms respectively. It is recognised that the latter fault rate of 1.0 per year is not necessarily typical of UK Distibution Networks. It could therefore be argued that the resultant voltage thresholds for Rise of Earth Potential (ROEP) are not strictly applicable to DNO Distribution Systems. However, the following analysis provides justification that the threshold of 1150V determined within the ETRs can be applied to Distribution Networks to provide safe working for BT (Telecom) operatives and third parties using BT (Telecom) equipment to define safe working zones around substations. The rationale for this reasoning is as follows. Fault Statistics NAFIRS NAFIRS fault data shows the overhead line five year average fault rate up to 2003/2004 for all DNOs as HV (up to 11kV) = 8.2 Faults/100km/Year EHV (above 11kV) = 3.3 Faults/100km/Year Ofgem Information and Incentives (IIP) Benchmark Data shows that the average HV feeder length for the UK DNOs = 30km. This information is not available under IIP for EHV circuits but it is reasonable to use the Southern Electric figure which is = 20km. Per feeder this then equates to HV = 8.2x0.3 = 2.5 Faults/Year EHV = 3.3x0.2 = 0.66 Faults/Year We must also account for the possibility that these feeders will go through an auto-reclose sequence before lockout. If we assume on average 3 reclosures per fault the number of faults per feeder become HV = 4x2.5 = 10 Faults/Year EHV = 4x0.66 = 2.64 Faults/Year Of course not all faults will be to earth. DNO figures suggest that as many as 80% of overhead line faults are to earth and this means that the number of faults with possible ROEP impact are HV = 0.8x10 = 8 Faults/Year EHV = 0.8x2.64 = 2.11 Faults/Year For the total earth faults affecting an individual substation we must multiply this value by the average number of feeders connected to it. It is reasonable to assume there are on average 2 EHV Feeders and 3 HV Feeders connected. HV = 8x3 = 24 Faults/Year EHV = 2x2.11 = 4.22 Faults/Year
Page 27 DNO Fault Records (HV) A fault monitor at a typical DNO HV substation recorded 34 HV earth faults during a year, which is of the same order as the foregoing NAFIRS derived result. Analysis of these faults gave an average fault clearance time of 715ms. Although longer than the fault duration used in the analysis in the ETRs, it is not high enough to significantly affect the outcome. Figure B1 shows the number of earth faults versus the percentage of theoretical maximum ROEP. The theoretical maximum corresponds to that which would be calculated by the distribution company following an earthing assessment at the site. Note that in determining the ROEP at a particular substation, the earth resistance or impedance (found by measurement and/or modelling) is multiplied by the theoretical maximum earth return current (often with an additional safety factor). This calculated ROEP will be compared against the new 1150V recommended safety threshold voltage and mitigation applied where this is exceeded. The residual risk therefore arises from ROEP events having voltage magnitudes within a statistical fault distribution which cannot exceed 1150V. From figure B1 it can be seen that 94% (32 out of 34) of faults do not exceed 40% of the maximum theoretical value. The average percentage of the maximum theoretical ROEP is 26%, which corresponds to a 50% Probability of Heart Fibrillation in ETR 128 (see section 6.2.1.2). This means that rather than using a 100% Probability of Fibrillation as pessimistically adopted in the ETR, it is justifiable to use 50%. 14 12 Number of Faults 10 8 6 4 2 0 0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100 % of Theoretical Max ROEP Figure B1 - Number of earth faults versus the percentage of theoretical maximum ROEP Conclusions The risk analysis outcome in the body of this document is not significantly affected by the changes in fault frequency and duration contained herein.
Page 28 It is therefore concluded that the calculated threshold value of 1150 volts for fault clearance times greater than 200ms for the DNO Distribution Networks can be adopted in place of the current 430V threshold.