Engineering Technical Report 128. Risk Assessment for BT Operators Working In a ROEP Zone. Draft for Approval

Transcription

1 Engineering Technical Report Risk Assessment for BT Operators Working In a ROEP Zone Draft for Approval

2 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.

3 Page 3 CONTENTS Foreword Scope DEFINITIONS ROEP Magnitude Phase to Earth Faults Risk Assessment Procedure Risk Assessment Procedure Proportion of Time in ROEP zone working on a cable Calculation for metallic cables serving third parties within a ROEP zone Probability of heart fibrillation (PFB) Exposure hand to feet Exposure hand to seat/knees Exposure Hand to Hand Probability of Exposure (P E ) Probability of Fault (P F ) Individual Risk Level (IR) Conclusions References Appendix A - DISTRIBUTION SYSTEM FAULT RATES AND DURATIONS Introduction Fault Statistics NAFIRS DNO Fault Records (HV) Impact on ETR 128 (BT Operators) EHV Substations HV Substations Conclusions References FIGURES Figure 1a Illustration of transferred potential hazard within ROEP zone... 4 Figure 1b Plan view of hypothetical substation illustrating ROEP contours... 5 Figure 6.2 Cable terminating within a ROEP zone Figure A1 - Number of earth faults versus the percentage of theoretical maximum ROEP.. 16 TABLES Table Probability threshold voltages hand to feet Table Probability threshold voltages hand to seat/knees Table Probability threshold voltages hand to hand... 11

4 Page 4 RISK ASSESSMENT FOR BT OPERATORS WORKING IN A ROEP ZONE FOREWORD This Report has been prepared by a joint working group of Accenture HR services on behalf of British Telecommunications and the Energy Networks Association (ENA). This report considers hazards to BT operators in the vicinity of electricity company 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 metallic service components (conductors, cable sheaths etc) or between these components and ground. 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 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. 1 SCOPE This report considers hazards to BT operators working on BT services to third parties 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 Illustration of transferred potential hazard within ROEP zone Hazards to BT operators within a substation or hazards associated with cables serving a substation are not considered, since these are managed through the use of appropriate

5 Page 5 equipment and work procedures. Hazards to BT operators within an exchange sited within a ROEP zone are also not considered. Hazards to third parties using equipment connected to BT lines are considered in a separate document. This assessment includes both fast fault clearance and slow fault clearance times. This report considers the hazard to persons arising from electric shock. Secondary risks associated with electric shock, i.e. falls are not considered. Figure 1b Plan view of hypothetical substation illustrating ROEP contours

6 Page 6 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 P FB Probability of heart fibrillation P E Probability of exposure P F Probability of an earth fault which gives rise to significant ROEP

7 Page 7 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 = 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.

8 Page 8 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.

9 Page 9 5 RISK ASSESSMENT PROCEDURE A review of the work activities undertaken by BT operators was carried out. However, rather than subject all these to specific assessment, it was decided to chose an activity resulting in the greatest exposure to ROEP hazards, i.e. cable termination. An estimate of circuit resistances/breakdown voltages for the cable termination activity was made. The circuit current is determined by dividing the ROEP by the total circuit resistance. The probability of fibrillation (P FB ) is determined according to IEC The probability of exposure (P E ) is estimated from a knowledge of operator working practices. Using this information along with the probability of a fault (P F ) it is possible to estimate the level of individual risk. The calculations assume the worst case combination of potentials e.g. at a cable joint, remote earth potential is assumed to appear on the cable and the ground local to the cable is assumed to be at ROEP. The intolerable risk threshold level used is 1 in 10,000, which is 10 times lower than that advocated by the HSE for workers.

10 Page 10 6 RISK ASSESSMENT PROCEDURE 6.1 Proportion of Time in ROEP zone working on a cable It is necessary to consider the proportion of time a BT operator may spend working within a ROEP zone (excluding within a substation itself or on cables serving the substation). It is assumed that all substations (33kV and above) have ROEP zones extending to 100m i.e. an area of approximately 31,416m 2. It is also assumed that the total number of substations at 33kV and above is 3,000. The total area affected by ROEP is therefore 3,000 x 31,416m 2 = 9.4 x10 7 m 2. According to the Department for the Environment the total land area for the UK is 24,093,000 hectares (1 hectare = 10,000m 2 ). Therefore the proportion of land affected by ROEP is 32, x = 0.04% (approximately). 6.2 Calculation for metallic cables serving third parties within a ROEP zone ROEP zone Third Party Property Exchange Metallic cable Figure 6.2 Cable terminating within a ROEP zone The situation where a cable terminates within a ROEP zone is illustrated in Figure 6.2. Here the cable supplies a third party property within the zone and an operator is required to work on the cable at or near to the property Probability of heart fibrillation (PFB) Exposure hand to feet It is assumed that the operator is standing and wearing work boots or shoes with elastomer sole. The resistance of dry footwear is taken to be 2MOhms under dry conditions and 30kOhms under wet conditions. The resistance of 30kOhms dominates the exposure circuit and hence no additional resistances need be considered. The threshold levels of voltage corresponding to heart fibrillation can be determined from IEC Figure 14, for a heart current factor 1.0, a circuit resistance of 30kOhms and an offset factor of 20% and these are summarised in Table

11 Page 11 % Probability of fibrillation 200ms fast fault clearance threshold (V) 500ms slow fault clearance threshold (V) 0 8,750 2, ,000 5, ,750 12,500 Table Probability threshold voltages hand to feet The threshold voltages in Table 1 are relatively high and therefore the risks are considered negligible. The exposure scenario hand to feet can therefore be eliminated from the assessment Exposure hand to seat/knees For the current path hand to knees or hand to seat, the threshold levels of voltage corresponding to heart fibrillation can be determined from IEC Figure 14, for a heart current factor 0.7, a circuit resistance of 650Ohms and an offset factor of 20% and these are summarised in Table % Probability of fibrillation 200ms fast fault clearance threshold (V) 500ms slow fault clearance threshold (V) Table Probability threshold voltages hand to seat/knees Since the threshold voltages in Table are relatively low, the probability of heart fibrillation is pessimistically assumed to be 100% irrespective of voltage level. NOTE This analysis assumes no insulating layer between seat/knees and the inducing potential. In reality this is unlikely for two reasons: Firstly many work sites will be on hardstanding e.g. tarmac or concrete which will provide considerable insulation and secondly operators will be using suitable work equipment i.e. wearing appropriate clothing and may be using stools or mats Exposure Hand to Hand For the current path hand to hand, the threshold levels of voltage corresponding to heart fibrillation can be determined from IEC Figure 14, for a heart current factor 0.4, a circuit resistance of 650Ohms and an offset factor of 20% and these are summarised in Table % Probability of fibrillation 200ms fast fault clearance threshold (V) 500ms slow fault clearance threshold (V) Table Probability threshold voltages hand to hand The threshold voltages in Table 3 are significantly higher than those in Table 2 indicating a lower likelihood of fibrillation for this condition. Furthermore, for hand hand exposure to occur there would have to be ROEP on one side of the termination and remote earth on the other which is unlikely to happen. This is because of the inherent isolation provided by third

12 Page 12 party equipment between the incoming line and local LVAC supply. For more information on this aspect see BS EN Probability of Exposure (P E ) The probability of exposure (P E ) is therefore equal to 4 x 10-4 x 0.25 x 7.6 x 10-3 = 7.6 x Probability of Fault (P F ) 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 (see Section 4) Individual Risk Level (IR) The individual risk level is determined as follows: IR = P F x P E x P FB 0.5 x 7.6 x 10-7 x 1.0 = 3.8 x 10-7 or 1 in 2,600,000 approx (fast clearance). 1.0 x 7.6 x 10-7 x 1.0 = 7.6 x 10-7 or 1 in 1,300,000 approx (slow clearance).

13 Page 13 7 CONCLUSIONS The calculated risk level for a BT operator working on a metallic cable serving a third party within a ROEP zone is approximately 1 in 2,600,000 (fast clearance) and 1 in 1,300,000 approx (slow clearance). These risk levels lie in the Acceptable Region as defined by the HSE and therefore additional control measures are not justifiable. There are however a number of additional aspects which should be taken into account: Where an operator has to undertake a task requiring a long period of time spent in a ROEP zone, for example to repair a trunk cable passing through a zone, the exposure may be increased. However, the number of metallic trunk cables are few and reducing in number which helps to offset the increased exposure. Also, the proportion of land affected by ROEP may, in some geographical areas, be greater than that calculated in Section 6.1. For example, high ground resistivity in areas such as North Wales may result in larger ROEP zones, which could increase operator exposure. On the other hand, the risk assessment is pessimistic in that the protective effect of insulation between seat/knees and the ground provided by work equipment has not been included in the assessment. This would normally be some form of waterproof clothing, waterproof sheet or stool etc. Use of this equipment is expected to substantially reduce the individual risk level.

14 Page 14 8 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 ETR129, ROEP risk assessment for third parties using equipment connected to BT lines, Joint Working Group of Energy Networks Association and Accenture HR Services (on behalf of British Telecommunications). ISIS Practice Cabling and Wiring at Electricity Stations EPT/PPS/B013, issue 2, British Telecommunications, 1996 IEEE Standard 80 Guide for safety in AC substation grounding 2000 NGC document - Risk Assessment associated with Ground Potential Rise at National Grid Substation Sites, draft issue 2. IEC Effects of current on human beings and livestock Reducing Risks, Protecting People, HSE Digest of Environmental Statistics, Department for Environment, June 2003 ISIS Practice Working Practices at Electricity Stations EPT/PPS/B014, issue 1, British Telecommunications, 1992 BS EN Specification for safety of information technology equipment, including electrical business equipment.

15 Page 15 APPENDIX A - DISTRIBUTION SYSTEM FAULT RATES AND DURATIONS Introduction The analysis within this document and Engineering Technical Report 129 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

16 Page 16 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 this document and ETR 129, it is not high enough to significantly affect the outcome. Figure A1 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. It should be borne in mind 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 A1 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 ). This means that rather than using a 100% Probability of Fibrillation as pessimistically adopted in the ETR, it is justifiable to use 50% Number of Faults % of Theoretical Max ROEP Figure A1 - Number of earth faults versus the percentage of theoretical maximum ROEP Impact on ETR 128 (BT Operators) In section the determined quantified Individual Risk (IR) level is 1 in 1,300,000 which is based on a Probability of Heart Fibrillation of 100% and a fault frequency of 1 per annum.

17 Page 17 EHV Substations Assuming a similar statistical fault distribution for EHV substations as for HV substations results in a reduction in the probability of heart fibrillation to 50% rather than 100%, which reduces the IR level to 1 in 2.6m. This partly counteracts the increased fault frequency of 4.22 faults per year compared with 1 fault per year used in the ETR, and results in an IR level of 1 in 600,000. This IR level lies in the lower ALARP region, where mitigation measures are unlikely to be justifiable. HV Substations A reduction in the probability of heart fibrillation to 50% rather than 100%, reduces the IR level to 1 in 2.6m. The fault frequency determined from the statistics is 24 (using the NAFIRS data as opposed to the DNO data as being more representative of the distribution system), which is well in excess of that used in the analysis in the ETR. Application of this factor to the assessment results in an Individual Risk level of 1 in 108,000. However, this IR level is within the ALARP Region as defined by HSE, and importantly below the 1 in 100,000 action threshold and therefore mitigation measures are unlikely to be justifiable. Note Conclusions This is still likely to be a pessimistic outcome, since the beneficial effect of an insulating layer provided by work equipment is not wholly taken into account. The fault rate for DNO EHV substations (4.22 faults per year) compared with that used in the body of this document and ETR 129 (1 per year) does increase the determined Individual Risk levels, and these now lie in the lower ALARP region where mitigation measures are unlikely to be justifiable. The fault rate for HV substations (24 faults per year) compared with that used in the body of this document and ETR 129 (1 per year) does increase the determined Individual Risk levels, but these remain below the 1 in 100,000 action threshold where mitigation measures are unlikely to be justifiable. 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. References (1):- Energy Networks Association National Fault and Interruption Reporting Scheme 2003/2004.