10. Dependability of an electrical installation
10. Dependability of an electrical installation
10. Dependability of an electrical installation
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783<strong>10.</strong> <strong>Dependability</strong> <strong>of</strong> <strong>an</strong> <strong>electrical</strong><strong>installation</strong>Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
784<strong>10.</strong> DEPENDABILITY OF AN ELECTRICAL INSTALLATIONThe voltage at the terminals <strong>of</strong> a load is affected by phenomena originating either from theutility's network or the disturbed user's <strong>electrical</strong> <strong>installation</strong>.Abnormal operation <strong>an</strong>d the cost <strong>of</strong> damage undergone depend on the type <strong>of</strong> loads <strong>an</strong>d thecritical level <strong>of</strong> the industrial process. Thus, a momentary critical load cut c<strong>an</strong> have seriousconsequences on the industrial process operation without the load itself being affected. Inother cases, the same disturb<strong>an</strong>ce c<strong>an</strong> damage loads <strong>an</strong>d be tolerated by the industrialprocess.For the design <strong>of</strong> <strong>an</strong> <strong>electrical</strong> network, a precise <strong>an</strong>alysis <strong>of</strong> the effects <strong>of</strong> failures <strong>an</strong>ddisturb<strong>an</strong>ces which may occur must be carried out. Provisions must generally be taken in orderto limit their consequences.Harmonic effects, flicker, voltage unbal<strong>an</strong>ce, frequency variations <strong>an</strong>d overvoltages c<strong>an</strong> bereduced by implementing network structures <strong>an</strong>d equipment suitable to each case (see § 3;§ 5 <strong>an</strong>d § 8).Power cut immunity also requires specific equipment such as uninterruptible power supplies<strong>an</strong>d <strong>electrical</strong> energy generator sets (see § 4). This equipment is generally not sufficient tosolve all the problems. The network structure, the automatic supply restoration systems, thereliability level <strong>of</strong> equipment, the presence <strong>of</strong> a control <strong>an</strong>d monitoring system, as well as themainten<strong>an</strong>ce policy, all play <strong>an</strong> import<strong>an</strong>t role in reducing <strong>an</strong>d eliminating power cut times. Acomplete <strong>an</strong>alysis <strong>of</strong> the problem requires dependability studies taking into account all <strong>of</strong> thesedata as well as the occurrence <strong>an</strong>d duration <strong>of</strong> power cuts allowed by the industrial process.These studies c<strong>an</strong> be used to determine the structure <strong>an</strong>d equipment most suitable for theneeds <strong>of</strong> the pl<strong>an</strong>t. They generally require loads to be classed in relation to their sensitivitylevel <strong>an</strong>d the following distinctions to be made:- loads allowing prolonged power cuts: 1 hour or more- loads having to be re-supplied after several seconds- loads unable to withst<strong>an</strong>d <strong>an</strong>y power cuts.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
785Figure 10-1 shows a network example for which this distinction has been made:- vital loads unable to tolerate <strong>an</strong>y power cuts are fed by <strong>an</strong> uninterruptible power supply- essential loads are re-supplied several seconds after utility network loss once the voltage<strong>an</strong>d frequency <strong>of</strong> the generator set have been stabilised- priority loads are reconnected once the essential loads have been fully started again(generator set c<strong>an</strong>not inst<strong>an</strong>t<strong>an</strong>eously accept the whole load)- non-priority loads allow long power cuts <strong>an</strong>d are only re-supplied when the utility networkhas been restored.Replacement source sizing with respect to operating requirements c<strong>an</strong> be optimised if the rightstructure <strong>an</strong>d automatic controls are chosen.<strong>Dependability</strong> studies c<strong>an</strong> be used to determine the "minimum" structure meeting the powersupply requirements <strong>of</strong> critical loads.It must also be remembered that the choice <strong>of</strong> neutral earthing system is also <strong>an</strong> import<strong>an</strong>telement. Thus, for loads requiring high availability, it is strongly advisable to choose <strong>an</strong>isolated neutral system (see § 2).Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
786utility supplynetworkGmains /st<strong>an</strong>dbyc<strong>an</strong> be shedc<strong>an</strong>notbe shedc<strong>an</strong>notbe shedc<strong>an</strong> be shedfor severalminutesUPS. non priority loads. power cut <strong>of</strong>several hours. essential loads. power cut <strong>of</strong>several seconds. vital loads. no power cut. priority loads. loads that c<strong>an</strong> be shedfor several minutesFigure 10-1: network with loads classified in relation to their sensitivity to power cutsPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
787<strong>10.</strong>1 definition <strong>of</strong> terms relating to dependability (according to IEC 50 § 191)FailureThe terminaiton <strong>of</strong> the ability <strong>of</strong> <strong>an</strong> item to perform a required function.Note: - after failure the item has a fault- "failure" is <strong>an</strong> event, as distinguished from "fault", which is a state- this concept as defined does not apply to items consisting <strong>of</strong> s<strong>of</strong>tware onlyAvailabilityThe ability <strong>of</strong> <strong>an</strong> item to be in a state to perform a required function under given conditions at a giveninst<strong>an</strong>t <strong>of</strong> time or over a given time interval, assuming that the required external resources are provided.Note: - this ability depends on the combined aspects <strong>of</strong> the reliability perform<strong>an</strong>ce, themaintainability perform<strong>an</strong>ce <strong>an</strong>d the mainten<strong>an</strong>ce support perform<strong>an</strong>ce- required external resources, other th<strong>an</strong> mainten<strong>an</strong>ce resources, do not affect the availabilityperform<strong>an</strong>ce <strong>of</strong> the itemReliability (perform<strong>an</strong>ce)The ability <strong>of</strong> <strong>an</strong> item to perform a required function under given conditions for a given time interval.Note: it is generally assumed that the item is in a state to perform a required function at the beginning <strong>of</strong>the given time intervalMaintainability (perform<strong>an</strong>ce)The ability <strong>of</strong> <strong>an</strong> item under given conditions <strong>of</strong> use to be retained in, or restored to, a state in which itc<strong>an</strong> perform a required function, when mainten<strong>an</strong>ce is performed under given conditions <strong>an</strong>d usingstated procedures <strong>an</strong>d resources.Mainten<strong>an</strong>ce support perform<strong>an</strong>ceThe ability <strong>of</strong> a mainten<strong>an</strong>ce org<strong>an</strong>ization, under given conditions, to provide upon dem<strong>an</strong>d, the resourcesrequired to maintain <strong>an</strong> item, under a given mainten<strong>an</strong>ce policyNote: The given conditions are related to the item itself <strong>an</strong>d to the conditions under which the item isused <strong>an</strong>d maintained.Preventive mainten<strong>an</strong>ceThe mainten<strong>an</strong>ce carried out at predetermined intervals or according to prescribed criteria <strong>an</strong>d intendedto reduce the probability <strong>of</strong> failure or the degradation <strong>of</strong> the functioning <strong>of</strong> <strong>an</strong> item.Corrective mainten<strong>an</strong>ceThe mainten<strong>an</strong>ce carried out after fault recognition <strong>an</strong>d intended to put <strong>an</strong> item into a state in which itc<strong>an</strong> perform a required function.FaultThe state <strong>of</strong> <strong>an</strong> item characterized by inability to perform a required function, excluding the inabilityduring preventive mainten<strong>an</strong>ce or other pl<strong>an</strong>ned actions, or due to lack <strong>of</strong> external resources.Note: a fault is <strong>of</strong>ten the result <strong>of</strong> a failure <strong>of</strong> the item itself, but may exist without prior failure.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
788<strong>10.</strong>2 application areas <strong>of</strong> dependability studiesThe studies c<strong>an</strong> be carried out on all network types <strong>an</strong>d on their control <strong>an</strong>d monitoringsystem:- single fed network (see fig. 1-17)- dual fed network (see fig. 1-18 <strong>an</strong>d 1-19)- loop network with automatic reconfiguration system (see fig. 1-20a)- double busbar network (see fig. 1-14, 1-15 <strong>an</strong>d 1-16)- network with uninterruptible power supplies (see fig. 1-30, 1-31, 1-32, 1-33, 1-34 <strong>an</strong>d 1-35)- network with automatic ch<strong>an</strong>geover systems (see § 12.2.1) <strong>an</strong>d load-shedding <strong>an</strong>drestoration systems (see § 12.2.3)- ...They c<strong>an</strong> be customised in relation to the requirements:- detail <strong>of</strong> the study- types <strong>of</strong> dependabiltiy criteria to be assessed- type <strong>of</strong> <strong>an</strong>alysis• detail <strong>of</strong> the study (see table 10-1)- a rapid study or pre-study is generally used to make a rapid structure choice- a very detailed study takes into account as m<strong>an</strong>y factors as possible; all the operatingconfigurations, detailed <strong>an</strong>alysis <strong>of</strong> possible faults <strong>an</strong>d their consequences, network modelas close as possible to its behaviour during a fault.Detail <strong>of</strong> hypothesesDetail <strong>of</strong> modelPre-studyA single type <strong>of</strong> fault with averageduration <strong>an</strong>d occurrenceThe consequences <strong>of</strong> faults aregrouped into large familiesDetailed studyFaults divided into familiesdepending on their effects on the<strong>installation</strong>The consequences <strong>of</strong> faults are<strong>an</strong>alysed in detailTable 10-1: example illustrating the differences between a pre-study <strong>an</strong>d a detailed studyPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
789• types <strong>of</strong> dependability criteria to be assessed- the average number <strong>of</strong> operating hours before the first failure(MTTF: Me<strong>an</strong> Time To Failure)- the average number <strong>of</strong> hours between two failures (MTBF: Me<strong>an</strong> Time Between Failures)- the availability <strong>of</strong> power supply to a load. This is the probability <strong>of</strong> correctly supplying a load.This probability takes into account the frequency <strong>of</strong> faults <strong>an</strong>d the duration <strong>of</strong> repairs- the average number <strong>of</strong> faults per year- the average repair time (MTTR: Me<strong>an</strong> Time To Repair)- the average time during which the system is unavailable during a fault (MDT: Me<strong>an</strong> DownTime). This includes the fault detection time, the mainten<strong>an</strong>ce service travel time, the faultyequipment procurement time <strong>an</strong>d the repair time- the optimum frequency <strong>of</strong> preventive mainten<strong>an</strong>ce operations <strong>an</strong>d the availability <strong>of</strong> the<strong>installation</strong> during these operations- the optimisation <strong>of</strong> replacement sets for mainten<strong>an</strong>ce.For example, for the <strong>electrical</strong> network in figure 10-1, we c<strong>an</strong> calculate:- the probability that the generator set does not start on loss <strong>of</strong> utility supply- the optimum frequency <strong>of</strong> generator set tests- the average number <strong>of</strong> priority load power cut minutes per year- the average number <strong>of</strong> hours before vital loads undergo a power cut.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
790• types <strong>of</strong> <strong>an</strong>alysis- help with structure design by assessing dependability criteria (see fig. 10-2).simpleststructuredependabilitycriteria assessmentdependabilitycriteria metnonewstructureyeschosenstructure- comparison <strong>of</strong> structures (see fig. 10-3)Figure 10-2: help with structure designstructureAstructureBdependabilitycriteria assessmentdependabilitycriteria assessmentstructure Adependabilitycriteria nearerto objectivesnostructure Bchosenyesstructure AchosenFigure 10-3: comparison <strong>of</strong> two structures- determination <strong>an</strong>d <strong>an</strong>alysis <strong>of</strong> structure dependability criteria.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
791<strong>10.</strong>3 carrying out a dependability studyCarrying out a study requires a pr<strong>of</strong>ound knowledge <strong>of</strong> the dependability area. Abnormaloperation <strong>an</strong>alysis s<strong>of</strong>tware (Adelia, S<strong>of</strong>ia) <strong>an</strong>d modelling s<strong>of</strong>tware (Supercat, MOCA-RP) helpexperts to assess chosen dependability criteria.<strong>10.</strong>4 exampleA drinking water production pl<strong>an</strong>t supplies 100 000 m 3 /day at low power 300 days/year <strong>an</strong>d200 000 m 3 /day at high power 65 days/year.The water production is ensured by four production sections, each able to supply100 000 m 3 /day.Six types <strong>of</strong> load L1, L2 , L3, L4, L5<strong>an</strong>d L 6 (pumps, disinfectors, etc.) which must operatesimult<strong>an</strong>eously to ensure production, are associated with each section.We c<strong>an</strong> thus mark the loads ensuring operation <strong>of</strong> each section as follows:L1a, ... , L6a→ section n° 1L1b, ... , L6b→ section n° 2L1c, ... , L6c→ section n° 3L1d, ... , L6d→ section n° 4Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
792• current networkThe current wiring diagram <strong>of</strong> the network supplying the loads used for production is shown infigure 10-4.Figure 10-4: current wiring diagram <strong>of</strong> the network supplying the loads used for prodcutionPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
793During normal operation, the switchboard MV1 is fed by "utility 1", the switchboard MV2 by"utility 2".In case <strong>of</strong> loss <strong>of</strong> one utility incoming feeder, the other feeder ensures the entire power supply.In case <strong>of</strong> loss <strong>of</strong> both utility incoming feeders, the generator sets start <strong>an</strong>d are able to ensurethe entire power supply.In case <strong>of</strong> loss <strong>of</strong> the tr<strong>an</strong>sformer T1 , the tr<strong>an</strong>sformer T2 ensures back-up <strong>an</strong>d vice versa.In case <strong>of</strong> loss <strong>of</strong> tr<strong>an</strong>sformers T3, T4or T5, the tr<strong>an</strong>sformer T6 ensures back-up <strong>of</strong> the nonsuppliedbusbar.To ensure low power operation (100 000 m 3 /day), a single section is necessary, <strong>an</strong>d for highpower operation (200 000 m 3 /day), two sections are necessary.The pl<strong>an</strong>t has four production sections with numerous redund<strong>an</strong>cies in the <strong>electrical</strong> supply. Itshould thus have a sufficient level <strong>of</strong> availability.Now, <strong>electrical</strong> <strong>installation</strong> faults have occurred, causing insufficient production.Indeed, the distribution <strong>of</strong> load power supply is such that availability is not very good. Thus, afault on the busbar BB3 puts all L6 <strong>an</strong>d L5 type loads out <strong>of</strong> service <strong>an</strong>d no productionsection c<strong>an</strong> operate.Similarly, a fault on busbar BB4 or BB5 puts the entire production system out <strong>of</strong> service.• proposed solutionTo improve availability, we propose to group the loads differently so that the loss <strong>of</strong> a lowvoltage busbar only puts one production section out <strong>of</strong> service.Thus, the solution proposed is the network shown in figure 10-5.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
794utility 1 G1 G2 G3utility 2switchboard MV1switchboard MV220 kV5.5 kVT120 kV5.5 kVT25.5 kV400 Vsection n°1. . .L1a L2a L6a5.5 kV400 Vsection n°2. . .L1b L2b L6b5.5 kV400 Vsection n°3. . .L1c L2c L6c5.5 kV400 Vsection n°4. . .L1d L2d L6dFigure 10-5: wiring arr<strong>an</strong>gement proposed to supply loads used for productionPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
795However, this requires investment in equipment as eight extra 5.5 kV switches <strong>an</strong>d two extra5.5 kV circuit-breakers are necessary (6 LV circuit-breakers are taken away).This extra cost will be largely compensated by the very considerable gain in availability, asshown by what follows.The dependability study carried out has allowed us to compare the fault frequency <strong>an</strong>d theavailability <strong>of</strong> the new network <strong>an</strong>d the old network.Furthermore, two specific studies have been carried out on the new network to optimise thefrequency <strong>of</strong> generator set tests <strong>an</strong>d compare the contribution <strong>of</strong> the MV part <strong>an</strong>d LV part t<strong>of</strong>aults.o comparison <strong>of</strong> availabilities <strong>an</strong>d fault frequencies <strong>of</strong> the two networksThe results are presented in the form <strong>of</strong> a comparison between the two networks. The oldnetwork serves as a reference <strong>an</strong>d the availabilities <strong>an</strong>d fault frequencies associated with it aretaken to be equal to 1.The comparison <strong>of</strong> fault frequencies is shown in table 10-2.Old networkNew networkLow power op. 1 1/22High power op. 1 1/21In total 1 1/21Table 10-2: comparison <strong>of</strong> the fault frequencies <strong>of</strong> the two networksOverall, the faults causing insufficient production will be 21 times less frequent with the newnetwork.The comparison <strong>of</strong> availabilities is shown in table 10-3.Old networkNew networkLow power op. 1 110High power op. 1 55In total 1 105Table 10-3: comparison <strong>of</strong> availabilities <strong>of</strong> the two networksPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
796Overall, the availability is 105 times better with the proposed new network.The parameters determining availability are the fault frequency <strong>an</strong>d repair time. A fault causesa reduction in availability which is all the greater the longer the repair time is. Non-availabilitythus reflects the loss <strong>of</strong> production.The gains made on availability are much greater th<strong>an</strong> on the fault frequency since:- the fault repair times for the new network are shorter- during preventive mainten<strong>an</strong>ce operations on the new network, the availability is lessdegraded.We c<strong>an</strong> also see that the availability difference is much greater for low power operation th<strong>an</strong>for high power operation as the probability <strong>of</strong> losing the four sections on the new network isvery low. However, the contribution <strong>of</strong> the low power operation availability is preponder<strong>an</strong>t overthe overall result as the pl<strong>an</strong>t operates mostly at a low power operation (300 days to 65 days).o optimisation <strong>of</strong> generator set testing frequencyThis study has only been carried out on the new networkThe curve in figure 10-6 shows the impact <strong>of</strong> the generator set testing frequency on theprobability that the generator sets are unavailable when they are required.This probability takes into account all <strong>of</strong> the events which may occur during starting. It hasbeen fixed at r<strong>an</strong>dom at 1 for a test frequency <strong>of</strong> one month.The curve in figure 10-6 shows that there is a minimum unavailability point for a test frequency<strong>of</strong> 6 months.We c<strong>an</strong> see that a higher test frequency does not improve the availability <strong>of</strong> the sets but, onthe contrary, degrades it as, during testing, the generator sets are not available.We c<strong>an</strong> thus advise the customer to carry out tests on the sets every 6 months.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
797generator set unavailabilityprobability54.543.532.54.64.921.5<strong>10.</strong>50.91.5test frequency1 month 6 months 1 year 2 years 5 yearsFigure 10-6: probability that the sets are unavailablein relation to the testing frequencyo calculation <strong>of</strong> the MV part <strong>an</strong>d LV part contribution to fault eventsThese calculations have only been carried out on the new network. They have shown that theMV part has a much greater contribution th<strong>an</strong> the LV part, roughly 99.9% to 0.1 %, <strong>an</strong>d asmuch for low power operation as for high power operation.Preventive mainten<strong>an</strong>ce must therefore be particularly thorough on the MV part <strong>of</strong> the network.Finally, it is advisable to implement a control <strong>an</strong>d monitoring system on the MV part in order tomonitor the state <strong>of</strong> the <strong>installation</strong> <strong>an</strong>d check <strong>electrical</strong> equipment mainten<strong>an</strong>ce.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE
798CHAPTER 10 BIBLIOGRAPHY• st<strong>an</strong>dardso IEC 50 (191) (1990): international electrotechnical vocabulary. Chapter 191: dependability<strong>an</strong>d quality <strong>of</strong> serviceo IEC 812 (1985): <strong>an</strong>alysis techniques for system reliability - Procedure for failure mode <strong>an</strong>deffects <strong>an</strong>alysiso IEC 863 (1986): presentation <strong>of</strong> reliability, maintainability <strong>an</strong>d availability predictions• Schneider cahiers techniqueso Introduction to dependability design, Cahier Technique n° 144, P. Bonnefoio High availability <strong>electrical</strong> power distribution, Cahier Technique n° 148, A. Longchamp,G. Gatineo <strong>Dependability</strong> <strong>of</strong> MV <strong>an</strong>d HV protection devices, Cahier Technique n° 175, M. Lemaireo Industrial approach to dependability, Cahier Technique n° 134, H. Krot<strong>of</strong>fPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, tr<strong>an</strong>slation <strong>an</strong>d reproduction, either wholly or partly, <strong>of</strong> this document are not allowed without our written consent.Industrial <strong>electrical</strong> network design guide T & D 6 883 427/AE