SIST IEC 60805:1999

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Guide pour la réception, l'exploitation et l'entretien des pompes d'accumulation et des pompes-turbines fonctionnant en pompeGuide for commissioning, operation and maintenance of storage pumps and of pump-turbines operating as pumps23.080ýUSDONHPumpsICS:Ta slovenski standard je istoveten z:IEC 60805SIST IEC 60805:1999en01-april-1999SIST IEC 60805:1999SLOVENSKI
STANDARD



SIST IEC 60805:1999



CODE PRIXPRICE CODETNORMECEIINTERNATIONALEIECINTERNATIONAL60805STANDARDPremière éditionFirst edition1985-09Guide pour la réception, l'exploitation etl'entretien des pompes d'accumulation etdes pompes-turbines fonctionant en pompeGuide for commissioning, operation andmaintenence of storage pumps and ofpump-turbines operating as pumps© IEC 1985 Droits de reproduction réservés — Copyright - all rights reservedAucune partie de cette publication ne peut être reproduite niNo part of this publication may be reproduced or utilized inutilisée sous quelque forme que ce soit et par aucunany form or by any means, electronic or mechanical,procédé, électronique ou mécanique, y compris la photo-including photocopying and microfilm, without permission incopie et les microfilms, sans l'accord écrit de l'éditeur.writing from the publisher.International Electrotechnical Commission3, rue de Varembé Geneva, SwitzerlandTelefax: +41 22 919 0300e-mail: inmail@iec.chIEC web site http: //www.iec.chCommission Electrotechnique InternationaleInternational Electrotechnical CommissionMemtnyHapoluae 3nenrporexHHVecnaR KOMHCCHHPour prix, voir catalogue en vigueur•For price, see current catalogueIEC•SIST IEC 60805:1999



805 ©IEC1985— 3 —CONTENTSPageFOREWORD 5PREFACE 5ClauseSECTION ONE — GENERAL1.Scope and object 72.Terms and definitions 73.Data on operating conditions 11SECTION Two — COMMISSIONING4.General 155.Test co-ordinator 156.Pre-start tests 157.Initial run 198.Test runs 19SECTION THREE — OPERATION9.General 2510.Test service period 2711.Commercial service 29SECTION FOUR — MAINTENANCE12.General 3113.Basis for maintenance activities 3314.Restoration to optimum condition 3515.Re-starting 35APPENDIX A — Measurements
37SIST IEC 60805:1999



805 © IEC 1985— 5 —INTERNATIONAL ELECTROTECHNICAL COMMISSIONGUIDE FOR COMMISSIONING, OPERATION ANDMAINTENANCE OF STORAGE PUMPS AND OFPUMP-TURBINES OPERATING AS PUMPSFOREWORD1)The formal decisions or agreements of the I E C on technical matters, prepared by Technical Committees on which all the NationalCommittees having a special interest therein are represented, express, as nearly as possible, an international consensus of opinionon the subjects dealt with.2)They have the form of recommendations for international use and they are accepted by the National Committees in thatsense.3)In order to promote international unification, the I E C expresses the wish that all National Committees should adopt the text ofthe I E C recommendation for their national rules in so far as national conditions will permit. Any divergence between the I E Crecommendation and the corresponding national rules should, as far as possible, be clearly indicated in the latter.PREFACEThis standard has been prepared by IEC Technical Committee No. 4: Hydraulic Turbines.The text of this standard based on the following documents:Six Months' RuleReport on Voting
4(CO)414(CO)43Further information can be found in the Report on Voting indicated in the table above.The following IEC publications are quoted in this standard:Publications Nos. 198 (1966): International Code for the Field Acceptance Tests of Storage Pumps.497 (1976): International Code for Model Acceptance Tests of Storage Pumps.545 (1976): Guide for Commissioning, Operation and Maintenance of Hydraulic Turbines.609 (1978): Cavitation Pitting Evaluation in Hydraulic Turbines, Storage Pumps and Pump-turbines.SIST IEC 60805:1999



805 ©IEC 1985— 7 —GUIDE FOR COMMISSIONING, OPERATION ANDMAINTENANCE OF STORAGE PUMPS AND OFPUMP-TURBINES OPERATING AS PUMPSSECTION ONE — GENERAL1. Scope and objectThe purpose of this guide is to establish, in a general way, suitable procedures for commission-inga)non-reversible hydraulic machines without movable wicket-gates, i.e. storage pumps;b)reversible hydraulic machines with fixed blade impellers and without movable wicket-gates,operating as pumps;c)reversible hydraulic machines with fixed blade impellers and with movable wicket-gates, oper-ating as pumps;d)reversible hydraulic machines with adjustable impeller blades and movable wicket-gates, oper-ating as pumps.and associated equipment.This guide also indicates how such machines and equipment should be operated and main-tained.This guide applies to storage pumps and reversible pump-turbines of all types, especially to largeunits coupled to electrical motor-generators.The guide excludes matters of purely commercial interest, except those inextricably boundup with the conduct of commissioning, operation and maintenance.It is understood that a document of this type will be binding only if, and to the extent that, bothcontract parties have agreed upon it.Wherever the guide specifies that documents, drawings or information are to be supplied by amanufacturer (or by manufacturers), each individual manufacturer shall be required to furnish theappropriate information for his own supply only.The guide is not concerned with water conduits, gates, valves, drainage pumps, cooling waterequipment, generators, etc., except where they cannot be separated from the machine and itsequipment.It is assumed that the turbine of the set or the pump-turbine operating as turbine has beencommissioned beforehand in accordance with IEC Publication 545: Guide for Commissioning,Operation and Maintenance of Hydraulic Turbines, where applicable. Where this is not possible,an individual commissioning procedure shall be established at an early stage of plan design.2. Terms and definitions2.1 CommissioningTesting of new equipment to check its conformity with contractual specifications, as well asoperation of the equipment until formally accepted by the purchaser.SIST IEC 60805:1999



805 © IEC 1985— 9 —2.2 OperationUtilization of the equipment to convert energy, or state of readiness for such utilization.2.3 MaintenanceActivity on the equipment directed to its conservation in a state of optimum operating condi-tion.2.4 Pre-start testsTests between completion of erection of the equipment and initial run.2.5 Initial runFirst movement of rotating parts after erection.2.6 Test runOperating to obtain one set of data for a specific test.2.7 Test operationUtilization of the equipment for testing purposes.2.8 Test operation periodTest period following initial run and followed by test service. It includes runs for checking plantequipment, as well as pumping and energy supply interruption tests.2.9 Test serviceOperation of the equipment for an agreed period, during which the manufacturer is generallyresponsible for the way in which it is operated.2.10 Commercial serviceOperation of the equipment, under the purchaser's responsibility, to convert energy, or state ofreadiness for such operation.2.11 Commercial service periodPeriod starting after acceptance, including service periods as well as periods when the equipmentmay be out of operation for maintenance, inspection, repairs, etc.2.12 Guarantee periodTime, extending through an agreed part of the commercial service period, during which themanufacturer has commercial obligations to correct defects of his equipment in order to bring itinto conformity with the contract. For this purpose, tests in accordance with appropriate parts ofSub-clause 11.1.4 shall be performed.2.13 InspectionCheck on the condition of equipment.SIST IEC 60805:1999



805 ©IEC 1985— 11 —2.14 Maintenance inspectionInspection at regular intervals, often combined with incidental repairs.2.15 RepairRestoration after wear or damage.2.16 ModificationChange intended to improve performance.2.17 OverhaulComplete inspection with necessary repairs and/or modifications intended to bring the unit orpart of it to, or nearly to, the optimum functioning condition.3. Data on operating conditions3.1 A fundamental requirement for proper operation and maintenance is a satisfactory knowledge of themachine and its accessories by the purchaser's personnel.The manufacturer(s) shall deliver to the purchaser's responsible representative all necessarydocuments, instructions and information. They shall include:—general drawings and data;—material lists, descriptions and test certificates for main parts;—function diagrams;—instructions for operation, inspection and maintenance of the supply, and tables of measure-ments as given in Appendix A;—assembly and dismantling instructions and/or exploded views of assemblies as necessary;—safety instructions;—list of original spare parts, as stated in the contract.These documents, even in a preliminary form, shall be given to the purchaser's representative assoon as required and in any case, before the initial run.3.2 The documents submitted by the manufacturer(s) shall include the following data, some of whichmay be amended according to experience gathered during commissioning.1)Diagram of the storage pump or pump-turbine operating as pump, relating head, discharge,input and suction head and indicating operation limits: in applicable cases, at different wicket-gate and runner blade openings.2)Statement of similitude of model and prototype according to I EC Publication 497: Interna-tional Code for Model Acceptance Tests of Storage Pumps, if appropriate.3)Detailed sequential diagrams, illustrating ordinary changes from one mode of operation toanother (pumping, generation, synchronous condenser operation, stand-by, stop for repair) aswell as changes due to various electrical or mechanical faults.4)Wicket-gate and impeller blade openings during the starting period and as a function of head andtailwater level, if applicable.SIST IEC 60805:1999



805 © IEC 1985— 13 —5)If applicable, description of the de-watering and refilling system of the pump impeller, data forpressure air tanks, compressors, valves, automatic control devices, maximum and minimumwater level in the draft tube, etc.6)If applicable, description of the coupling type between pump shaft and motor-generator shaftand of the synchronizing arrangement.7)Control and servo systems.Description of type and control system of the main valve, the distributor and the movableblade impeller (as applicable) as well as closing and opening laws for the main valve, thewicket-gates and impeller blades.Characteristics of the fluids for the control system, filter specifications and information onpurification frequency and method.8)Speed and pressure variations at various operating heads under transient conditions. Maxi-mum steady-state reversed runaway speed, maximum momentary reversed runaway speed.9)Minimum continuous operating speed for initial run (depending on design of thrust bearing andother bearings).10)Description of lubrication systems, stating amount and characteristics of lubricants and fre-quency of replacement.11)If applicable, data on jacking system for rotating parts and time after shut-down for a re-startwithout operating the jacking system.12)Speed limit below which operation of the thrust-bearing oil injection system is necessary.13)If applicable, maximum and minimum speeds for brake operation.14)Intermediate oil levels and pressures at which pumps and compressors should be started oralarms given; also data for automatic signalling and emergency devices.15)Maximum and minimum temperatures in bearings, seals and in the pressure oil or fluid circuits(alarm and shut-down values).16)Maximum and minimum pressures, flows and levels in oil or fluid pressure systems andbearings, according to control system (alarm and shut-down values).17)Maximum and minimum pressures and flows in coolers (alarm and shut-down values).18)Maximum water levels in drainage pits (alarm and shut-down values).19)Data for electric motors, pumps, and other accessories.20)Safety instructions for test and operating staff.21)Other data of importance which are considered necessary by either purchaser or manufacturerfor safe operation of the equipment or for instruction of personnel.Some of this information may be required already for other modes of operation of the hydraulicmachine.3.3 The manufacturer(s) shall give appropriate instruction to the purchaser's staff and provide theinformation stated in Sub-clause 3.2 on operating the equipment during the final stage of erection,initial run, test runs and test service particularly for all items not previously used by the pur-chaser.During the final stage of erection, the start, the test runs and test service, the purchaser shall makehis staff available to the manufacturer for instruction and training in order that they may be capableof operating properly the machinery and associated equipment.3.4 During the guarantee period, the equipment shall not be operated outside the limits stated in thecontract without agreement with the manufacturer.SIST IEC 60805:1999



805 © IEC 1985— 15 —SECTION TWO — COMMISSIONING4.General4.1 Commissioning occurs after first erection of a new unit. Similar procedures may be utilized with orwithout formal acceptance also after repair or overhaul.4.2 Operating requirements given in Clause 3 shall be complied with during commissioning. If operatingcircumstances of the power station prevent commissioning within these requirements, the pur-chaser and manufacturer should agree on the commissioning procedure.4.3 It will be advantageous for correct interpretation and determination of the causes of any unexpectedphenomena, to make simultaneous recordings of significant parameters (such as pressure in spiraland draft tube, openings of control mechanisms, speed, displacement and vibration of shaft, wicket-gates and covers, and possibly power, forces on bearings, pressures in servo-motors, etc.) onrecording instruments. These should be in operation during the initial run and onwards. Eachrecording shall have a time and date registration.4.4 It is the purchaser's responsibility to obtain from all the authorities concerned, agreements onspecified conditions (flow, grid, etc.) for test operation.5.Test co-ordinatorUnless otherwise specified, a test co-ordinator shall be chosen by the purchaser, the main con-tractors, and the manufacturers. He shall be in charge of drawing up the programme of commis-sioning. His programme shall be checked and approved by the purchaser, the main manufacturersand contractors before and during the tests if changes appear to be necessary. He shall conduct thetests and check the results.The test co-ordinator is responsible for the execution of all tests up to the beginning of the testservice period. Operating and commissioning personnel shall report to the co-ordinator on alloperations performed on the equipment.6.Pre-start testsThe rigidity and resonance frequency of the supporting structures for reversible machines shallpreferably be checked before erection of the unit, and later checked with the machine in place, ifpossible.These test checks shall be carried out before the hydraulic machine is filled for the first time withwater; they may be part of the turbine commissioning programme for pump-turbines. According toSub-clause 6.1 of I EC Publication 545, at least the following items shall be checked and the resultsrecorded by the test co-ordinator.6.1 Pre-tests check1)Inspection of all waterways of the hydraulic machine and removal of all foreign or loose objects(these might cause damage). If required, the measuring sections and pressure taps for futureacceptance tests shall be checked and confirmed as satisfactory and dimensions recorded asappropriate.2)Calibration of scales for control devices (such as wicket-gates and impeller blades), measure-ment of clearances, see Appendix A.SIST IEC 60805:1999



805 © IEC 1985— 17 —3)Operation of pressure oil units, hydraulic shut-offvalves, pressure relief valves, control systems,automatic and manually-operated starting and stopping devices, and signalling devices.4)Oil levels and pressures in the control system; condition of all oil filters.5)All protective devices, such as oil level, pressure and temperature alarms and relays, withadjustment if needed.6)Times of opening and closing of control elements, and their function.7)Times of opening and closing of valves or gate (if possible), and their function.8)Bearing and seal clearances. In the case of pressure-lubricated bearings, the oil injection systemshould be checked.9)Operation of leakage and drainage pumps and separately-driven lubrication pumps.10)Oil, grease, and water supply to all bearings and seals requiring lubrication and cooling.11)Operation of the braking system of the unit.12)Proper fastening of all manhole covers.The following will also be checked, if applicable:13)Runner blades set at starting position.14)Operation of the de-watering and refilling system and its control devices.15)Operation of the disengageable coupling mechanism.After the above checks, the valves or gates shall be closed, the control devices on the machineshall be closed (and locked, if applicable), the brakes applied and the electrical board energized.When satisfied, the test co-ordinator shall give instructions for the waterways to be filled.6.2 Filling waterwaysThe suction side valve or gate shall be opened first. A check for leaks shall be made.The waterway shall be filled slowly, and preferably manually either through a by-pass or a smallopening of the gate or stop-logs. When the waterway has been sufficiently filled, the gate shall beopened or the stop-logs removed.The casing of a machine with a distributor shall be pressurized through a by-pass and checkedbefore the main valve is opened. The operation of automatic protection devices which actuate thisvalve shall then be checked, and the functioning of the valve itself tested.The casings of hydraulic machines without a distributor, can be partially pressurized only withthe tailwater pressure.Automatic protection devices which actuate the intake or outlet gate or valve shall be checked,especially, the emergency gate. Also automatic locking devices shall be checked. If the protectivedevices and the emergency gate work satisfactorily and no unusual leakage or deformation has beenobserved; further tests, such as a de-watering and refilling test, shall be carried out and the controldevices adjusted.SIST IEC 60805:1999



805 © IEC 1985— 19 —7. Initial runIn the case of pump-turbines with movable wicket-gates the initial run of the set will normally takeplace while commissioning the turbine according to the I EC Publication 545. In the case of pump-turbines without movable wicket-gates the initial run of the set will normally take place after openingthe main valve and/or a by-pass. Reversible machines with movable wicket-gates will usually bestarted in the pumping direction with the impeller in air. It will then be necessary to provide coolingfor the impeller seals or labyrinths and for the shaft seal. The provisions for this cooling shall betested before rotating the empty machine. This test condition is normally achieved as part of thetransition (during start-up) from releasing air to best gate opening for pumping.Hydraulic machines without movable wicket-gates will usually be started as pumps with theimpeller in water: in the case of reversible machines or of storage pumps without coupling, theelectrical machine shall be brought to rated speed by the equipment provided, synchronized andconnected to the grid.In the case of ternary sets with a disengageable coupling to the storage pump, two cases shall beforeseen:a)coupling engageable only with unit at standstill: in this case the coupling will be engaged and thenthe electrical machine will be brought to rated speed by the turbine, synchronized and connectedto the grid;b)coupling engageable on speed: in this case the coupling will be disengaged and the storage pumpshall be brought to speed by for example, a torque converter with increasing rate of filling of theconverter, so that at the end normal speed will be reached. Then the coupling to the electricalmachine, started meanwhile by the turbine, shall be engaged.If any abnormal noise, scraping or other phenomena are observed, or if an authorized repre-sentative of any manufacturer so requests, the unit shall be stopped immediately.8. Test runs8.1 No-discharge tests8.1.1 The pump shall operate with its main valve or wicket-gates closed and therefore under no-dischargeconditions for a short period only, to avoid a dangerous temperature rise of the water inside thepump. This period depends on the design of the machine, its head and its input and shall be welldefined beforehand.The unit shall be operating at rated speed with the motor-generator connected to the grid. Thetemperatures of all bearings, oil and of the cooling water shall be continuously recorded. If anytemperature becomes excessive or increases too rapidly, the unit shall be shut down and the reasonfor this phenomenon investigated and corrected.For these tests,—the hydraulic machine that has been started with its impeller(s) in water shall have the main valveand by-pass closed;—the hydraulic machine that has been started with its impeller(s) in air shall be refilled. The fillingprocedure has to be done carefully so that pressure and input increase smoothly and so that noundue pressure or power peaks occur.The time needed for reaching no-discharge operation shall be recorded.The unit shall be stopped before the agreed permissible no-discharge operation periodelapses.SIST IEC 60805:1999



805 © IEC 1985— 21 —These no-discharge tests shall be repeated in order to allow the necessary adjustments of differentstarting devices, such as coupling mechanisms, de-watering and refilling valves, etc.8.1.2 During the no-discharge run, the following measurements and/or observations shall be made (ifpossible) :1)Input and pressure (including suction head).2)Vibration and deformation of covers and bearing supports.3)Vibration in the hydraulic machine, especially of shaft and wicket-gates and of the motor-generator. Noise.4)Water supply pressure in water-lubricated bearings and packings, and cooling of shaft sealingdevices.5)Operating conditions of stand-by, safety, and signal devices, in particular those which could notbe properly tested before water was admitted and the unit rotated.6)Operating conditions for the oil pressure and other pumps.7)Conditions of oil in bearings and pressure oil systems. If water is observed in oil, leaks shouldbe located and eliminated. If oil foam or smell is observed, the cause shall be located and elimi-nated.8)Oil pressure in pressure and lubrication systems; pressure, circulation, and inlet and outlettemperature of cooling water.9)Levels of oil in the lubricating system of bearings and in oil pressure systems.10)Temperatures of bearings, packings, and lubrication oil.8.2 Pumping and energy supply interruption tests8.2.1 GeneralWhen no-discharge operation is satisfactory, the flow control device(s) shall be opened slowly topumping position. Measurements and observations shall be continued and especially input, pressureand vibration shall be carefully studied. On pump-turbines with movable wicket-gates and impellerblades, the relation between them shall be checked and improved, for example by index tests (seeSub-clause 11.1.4).An energy supply interruption may occur at any moment during these tests. Necessary precau-tions must be taken to ensure that safe values of penstock and pump stresses will not be exceeded.The tests should be arranged to include an agreed degree of reverse rotation, subject to the provisionof Sub-clause 11.1.5.8.2.2 PurposesThe purposes are:a) for tests during steady-state operation, including synchronous condenser operation:1)to check with an agreed accuracy, the input, pressure and discharge of the pump, comparedwith the documents listed in Sub-clause 3.2;2)to check the unit for instability, vibration, or cavitation in the range of guaranteed loads.b) for tests during transients; including changing mode of operation:1) to check the functioning of control devices, as well as that of the entire unit;SIST IEC 60805:1999



805 © IEC 1985— 23 —2)to determine the momentary pressure and speed variations of the unit and to find theirmaximum values;3)to adjust the operating law and times of the control equipment (main valve, wicket-gates orimpeller blades) for the hydraulic machine in order to minimize pressure variations;4)to permit testing of the operation of other hydraulic and electrical equipment, includingprotection and other automatic devices.8.2.3 ProcedureDischarge will be started from no-discharge conditions:—for hydraulic machines without distributor, by opening the main valve according to a suitableopening law;—for hydraulic machines with distributor, by opening the main valve and the distributor in asuitable sequence and according to a suitable opening law.If the machine has adjustable impeller blades, these shall be set at the proper angle.Discharge tests shall be carried out at various water levels in the reservoirs and therefore atvarious pump heads.At least two energy supply interruption tests shall be made, at high and low pump head.For a pump-turbine with movable wicket-gates a low gate opening pump power failure test and abest gate opening failure test are necessary. Comparison with predicted results can indicate if otherconditions should be tested.If specified, energy supply interruption tests shall also be made under control of the emergencydevice and—if applicable—with inoperative wicket-gates.The tests and adjustments shall be repeated until all sequences follow automatically in a satis-factory manner, that can be relied on during normal operation.8.2.4 Observations, measurements and recordings:During these tests, the test co-ordinator shall ensure, where possible, that the following observa-tions and measurements are made and recorded:a) during steady-state operation (including synchronous condenser operation):1)head and tailwater levels (or pressures);2)motor-generator input;3)where applicable, wicket-gate and/or impeller blade angles;4)pressure at pump inlet and outlet sections;5)axial thrust;6)pressures and levels in oil pressure systems;7)operating times of oil pressure pumps;8)where applicable, pressure differentials in servo-motors required for wicket-gate closure andpressure setting for emergency valve tripping;9)temperatures in bearings and seals;10)noise levels, if values are specified in contract;11)abnormal noises, if any;SIST IEC 60805:1999



805 O IEC 1985— 25 —12)vibration and other displacements of rotating and fixed parts (e.g. vibrations of shaft bearingsupports and vibrations of the wicket-gates if any);13)steady-state air consumption;b) during transients (including changing mode of operation):1)head and suction water levels (or pressure);2)motor-generator input;3)opening and closing law and time of the main valve, where applicable strokes, opening andclosing times of wicket-gate and of the impeller blades;4)pressure at pump inlet and outlet sections;5)level variations at intake and surge tanks;6)time/speed relationship of the unit;7)maximum axial thrust;8)opening strokes and opening/closing times for pressure relief valve (if any);9)pressure drop in the oil-pressure system in the process of governing and variation of level inthe pressure reservoir;10)noise levels, if values are specified in contract;11)abnormal noises, if any;12)vibration and other displacements of rotating and fixed parts (e.g. vibrations of shaftbearing support and vibrations of the wicket-gate vanes, if any);13)action of emergency valve or gate under flow, if specified;14)sequence and time for changing mode of operation;15)air consumption during a de-watering operation.If possible, an estimate of leakage through the pump-turbine with closed wicket-gates should bemade preferably just after the energy supply interruption tests.It is recommended that after the test operation period (Sub-clause 2.8) an inspection of the unit becarried out, for example to check for any possible rubbing contact which may have occurred. Ifduring the tests (see Sub-clause 8.2) unusual phenomena or vibration have been observed, or if areversed speed test has been performed (see Sub-clause 11.1.5) this inspection shall be carried outprior to the start of the test service period (see Sub-clause 2.9).SECTION THREE — OPERATION9. GeneralThe operation period includes the following parts:—test service;—commercial service, divided into:—guarantee period,-post-guarantee period.The sub-clauses on test service and the guarantee period may be applied to new units after firsterection and commissioning, or to any unit after repair or overhaul carried out by a manufacturer ora contractor.Sub-clause 11.2 on commercial service after the guarantee period may be applied as long as theunit is in operation.SIST IEC 60805:1999



805 © IEC 1985— 27 —10. Test service period10.1 If nothing unusual has been observed during the pumping and energy supply interruption tests, thetest service period shall follow. As the manufacturer is generally responsible for his equipmentduring the test service and also for the way in which it is operated, it is recommended that thepurchaser's personnel operate the equipment under the manufacturer's guidance during the testservice.After the first erection or a modification, the test service should be long enough to satisfy thepurchaser that all components are in satisfactory operating condition. After a repair or an overhaul,the test service period can be short or even omitted.10.2 The duration of the final part of the test service period, during which the unit must operate withoutany interruption
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