1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 modified at Melbourne, 1988)IntroductionIf a buried telecommunication cable without an insulating layer around the metal sheath is located in thevicinity of a high voltage earthin
2、g system, part of the earth potential rise (EPR) in the event of an earth fault in the highvoltage system can transfer to the telecommunication system through resistive coupling.According to CCITT and CIGRE1)documents 1-3, EPR from high voltage power installations is recognizedas a source of dangero
3、us disturbance to telecommunication systems and a hazard to service personnel.It is possible to calculate EPR near power installations following the methods given in the Directives 1 (seeVolumes II and III), and this is especially recommended for dealing with switchyard earthing systems.The object o
4、f the present Recommendation is to give practical guidelines in determining safe distances betweenburied telecommunication cables and earthing systems of power facilities in the absence of local measurements orcalculated values of EPR.1 ScopeEarth fault in a power system causes earth currents which
5、raise the earth potential where the fault current leavesand enters the earth. The magnitude and extension of the EPR depends on the fault current level, the earthingresistance, the soil resistivity and the layout of the earthing arrangement. The duration of an earth fault depends on thetype of power
6、 network.This Recommendation gives information about:a) locations where EPR may occur;b) duration of EPR in different types of power networks;c) “safe distance” between telecommunication cables and power installations;d) measures to be taken if the safe distance is not achieved.2 General considerati
7、onsThe minimum separation in soil to be recommended between an earthing system of a power installation andtelecommunication cables depends on a number of factors: type of power network; fault current level; power earthing system; soil resistivity; local conditions.3 Type of power networkPower networ
8、ks are classified according to how the neutral point is connected to earth. The earthing systemaffects both the level and duration of the fault current, and hence the EPR._1)CIGRE International Conference on Large High-Tension Electric Systems.2 Volume IX - Rec. K.83.1 Networks with the neutral poin
9、t earthed directly or through a low impedanceThe level of an earth-fault current is high. A relay system will clear the fault in a short time.3.2 Networks with the neutral point earthed through an arc suppression coilThe level of an earth-fault current is small, usually not exceeding 100 amperes for
10、 each coil. The duration of anearth fault is relatively short.Such networks may be equipped with delayed tripping to clear permanent earth faults.3.3 Networks with the neutral point isolated from earthThe level of an earth-fault current is normally low, but the fault duration might be very long. Net
11、works of largeextent may give rise to large capacitive fault currents.If such networks are equipped with devices for automatic fault clearing, the fault duration is short to medium.4 Locations where earth potential rise may occur4.1 Power stations and sub-stationsPower stations and sub-stations are
12、most likely to experience EPR. The size of the station, the number andconstruction of power lines attached to the station, and the earthing arrangement are factors influencing the level andstation, and the earthing arrangement are factors influencing the level and zone of EPR. As given in reference
13、4 thelayout and structure of the earthing arrangement depends on regulations, size, age, purpose and location. If the powerlines entering the station are provided with earth wires, they will be connected to the earthing system in the station.4.2 Power line towersPower line towers with footing electr
14、odes are subjected to EPR due to earth-fault current in the power system,and currents from lightning strikes. If the power line is equipped with earth wires, these will normally be connected tothe tower electrodes. The probability of high EPR decreases when a power line is equipped with earth wires.
15、5 Magnitude of earth potential riseThe magnitude of the EPR depends on the power system voltage, the power line construction, the fault currentlevel and the earthing resistance.6 Zone of earth potential riseEPR is measured as the earth potential referred to a distant neutral earth. The zone of EPR,
16、near an earthingsystem, varies from some tens to some thousands of metres, depending on soil resistivity, the layout of the earthelectrode, and other local conditions. Further information is found in reference 5. The zones of EPR in urban areasare small compared to what can be expected in rural area
17、s. Only EPR zones having a potential higher than values givenin reference 1 are considered as dangerous. Measurements and calculation of the EPR zones are made by the powerdistribution authorities.7 Duration of earth potential riseThe duration of an earth fault and hence the EPR, depends on the type
18、 of power network.7.1 Networks with the neutral point earthed directly or through a low impedanceThe duration of an earth fault is generally less than 0.2-0.5 s.Volume IX - Rec. K.8 37.2 Networks with the neutral point earthed through an arc suppression coilThe duration of an earth fault is normally
19、 less than 0.8 s, but may in some cases last for several seconds. Suchnetworks may be equipped with delayed (a few seconds) tripping to clear permanent earth faults.7.3 Networks with an isolated neutral pointThe duration of an earth fault can be very long, and may last until another earth fault occu
20、rs.If such networks are equipped with automatic fault-clearing devices, the fault duration may be as short as in 7.1.8 Minimum separation in soil between buried telecommunication cables and power earthing systemsThe EPR near a high voltage earthing system can be estimated from calculations based on
21、idealized earthelectrodes and a homogeneous soil resistivity in the EPR zone. In practice it is not possible to make an exactcalculation of the potential transferred from a high voltage earthing system to an adjacent telecommunication cable.However, by feeding a current into the high voltage earthin
22、g system from a sufficiently great distance, the voltagebetween the cable sheath and an auxiliary electrode in the area of neutral potential can be measured. The result must becorrected proportionately to the actual earth-fault current. (On armoured cables the correction factor is not linear, butdep
23、ends on the magnetic characteristic of the ferromagnetic cable screen.) In the absence of other experiments, localmeasurements or calculated values of EPR, the values in Table 1/K.8 for the minimum separation in soil between“ordinary” telecommunication cable with a metal sheath in direct contact wit
24、h the soil and a high voltage powerearthing system should be observed.TABLE 1/K.8Separation in soil (in metres) between telecommunication cables and high voltage earthing systemsbeyond which no calculation nor measurement is necessaryPower network system withEarth resistivityisolated neutral or arcs
25、uppression coildirectly earthed neutralLocationLess than2 5 Urban50 ohm m5 10 Rural50-500 ohm m5 10 Urban10 20 Rural500-5000 ohm m10 50 Urban20 100 RuralGreater than10 50 Urban5000 ohm m20 100-200 a)Rurala)200 metres in areas with extremely severe soil conditions, i.e. greater than 10 000 ohm m.Note
26、 1 The values in the table normally refer to lines and installations which have a nominal voltage equal to or greater than132 kV.Note 2 The hazards due to lightning strokes on electric plants are not covered and may require taking into considerating themethods of 9 for high keraunic level areas.Note
27、 3 In the case of tower earthing, much shorter distances can be used if the power lines include earth wires.Note 4 Hazards for people working on telecommunication lines inside the zone of EPR is not taken into consideration bythese values; such hazards require additional measures or precautions.4 Vo
28、lume IX - Rec. K.89 Measures to be taken to avoid hazards from EPRThe primary method to avoid dangerous influence from EPR is to increase the distance betweentelecommunication cables and power earthing systems. If local conditions do not permit sufficient separation to avoiddangerous EPR, the teleco
29、mmunication cables should be provided with insulation, for example by placing the cables ininsulating plastic tubes.When the magnitude of EPR is extremely high, or the zone of EPR is of very great extension, optical fibrecables or radio-relay systems may be used instead of metallic cables.References
30、1 CCITT manual Directives concerning the protection of telecommunication lines against harmful effects fromelectrified power and electrified railway lines, Vols. II and III, ITU, Geneva, 1988.2 CCITT Study Group V Contribution No. 61/1979.3 CIGRE No. 36-04/1970 Ground potential rise and telecommunication lines.4 ELECTRA No. 71/1980 Station grounding Safety and interference aspects.5 ELECTRA No. 60/1978 Zone of influence of ground potential rise.
