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Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El. Enginering Guides EE General guides Power Substations Schneider Electric CTs Siemens Basics of EE ABB Drives Guides Industry Automation Relay control/protection Alternative EnergyElectrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resources Video Lectures Electrical Engineering PLC Programming Training Electric Testing and Maintenance (VIDEO) Network Theorems and Laws -Home / Technical Articles / Differences Between Earthed and Unearthed Cables Differences Between Earthed and Unearthed CablesPosted Jan 21 2013 by jiguparmar in Cables, High Voltage, Low Voltage, Medium Voltage, Transmission and Distribution with 14 Comments

Differences Between Earthed and Unearthed CablesIntroductionIn HT electrical distribution, the system can be earthed or unearthed.The selection of unearthed or earthed cable depends on distribution system. If such system is earthed, then we have to use cable which is manufactured for earthed system. (which the specifies the manufacturer). If the system is unearthed then we need to use cable which is manufactured for unearthed system.The unearthed system requires high insulation level compared to earthed system.For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.

Earthed SystemEarlier the generators and transformers were of small capacities and hence the fault current was less. The star point was solidly grounded. This is called earthed system.In three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.Where in unearthed system, (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to armor should be equal to insulation level of conductor to conductor.In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth.Example: 1.9/3.3 KV, 3.8/6.6 KV system

Unearthed SystemToday generators of 500MVA capacities are used and therefore the fault level has increased. In case of an earth fault, heavy current flows into the fault and this lead to damage of generators and transformers. To reduce the fault current, the star point is connected to earth through a resistance. If an earth fault occurs on one phase, the voltage of the faulty phase with respect to earth appears across the resistance.Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times.If the insulation of these phases is not designed for these increased voltages, they may develop earth fault. This is called unearthed system.In an unearth system, the phases are not grounded to earth .As a result of which there are chances of getting shock by personnel who are operating it.Example: 6.6/6.6 KV, 3.3/3.3 KV system.Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is 3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture.So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used.NomenclatureIn simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same.The size of cable will depend on current rating and voltage level. Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage. Earthed system has insulation grade of KV / 1.75 x KV. For Earthed System (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV. Unearthed system has insulation grade of KV / KV. For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV. 3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less.Thumb RuleAs a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.Because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kV+6.6kV = 11kVFor transmission of HT, earthed cable will be more economical due to low cost where as unearthed cables are not economical but insulation will be good.Generally 6.6 kV and 11kV systems are earthed through a neutral grounding resistor and the shield and armor are also earthed, especially in industrial power distribution applications. Such a case is similar to an unearthed application but with earthed shield (sometimes called solid bonding).In such cases, unearthed cables may be used so that the core insulation will have enough strength but current rating is de-rated to the value of earthed cables.But it is always better to mention the type of system earthing in the cable specification when ordering the cables so that the cable manufacturer will take care of insulation strength and de rating.Recommended EE articles // 6 Transformer Types You Can See In Commercial InstallationsMarch 16, 2015 Degradation of Insulation in Switchgear (Whats Really Happening)March 4, 2015 Test On 110kV Power Cable After Installation (2)December 26, 2014 Test On 110kV Power Cable After Installation (1)December 22, 2014Share with engineers //Article Tags //cable, distribution, earth, earth fault, earthed cables, insulation, unearthed cables, Filed Under Category //Cables High Voltage Low Voltage Medium Voltage Transmission and DistributionAbout Author //

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.RSS Feed for Comments14 Comments1. General Guidelines for Online Partial Discharge Testing of Power Cables | EEPMar 30, 2015[] Cable earth shield coming out of the cable (whether single core or three core) must not be touching any earthed metal except at terminating point to Substation Earth. Earth shield at cable termination should be insulated using shrink tubes in order to get access to individual cable earths. [](reply)2. DK SharmaMar 24, 2015We use 11 KV cable to connect our Distribution transformer which has its primary or HV winding in Delta connection and Secondary or LV winding in Star connection. Since Delta connection has no earth, we always need 11 KV Unearthed (UE) cable.So we should always ask for 11 KV UE cable.(reply)3. Manuel BolotinhaMar 15, 2015This article about earthed and unearthed cables is very basic and with poor technical content.(reply)4. AMEER AHMADMar 15, 2015I am not agree with As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.Reason: For 6.6/11kV Earthed cable, the system nominal voltage is 11kV. If we use 6.6/6.6kV Unearthed cable for 6.6/11kV Earthed system with 11kV nominal voltage, the phase to earth voltage remain 6.6kV for Earthed system(Agreed), but phase to phase voltage remain 11kV for Earthed system and this 6.6/6.6kV Unearthed cable can not meet the 11kV nominal system phase to phase voltage insulation.(reply)5. vikas j trivediNov 02, 2014How to calculate feeder htvr through metlab or etap(reply)6. JAYESH R MANKADJul 25, 2014It is good and interesting web. Informative topics are covered(reply)7. noureddineJan 26, 2014Mr. Peteryou must check set time for overload in your breakermay be the setting is wrong.thanks.(reply)8. guestJan 03, 2014Hows 6.6kV+6.6kV = 11kV? As far as I know 6.6+6.6=13.2(reply) ParagMay 08, 2014It means it is suitable for 11KV insulation requirement (since 13.2KV>11KV). Standard cable insulation ratings are as 1.1KV, 3.3KV, 11KV, 33KV)(reply)9. BalachandarDec 10, 2013I can not agree this statement ie., 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.but we can use 6.6/11kv cable for 6.6/6.6kv system.(reply)10. Peter AbbeyJan 21, 2013I have a question and it goes thus:A motor Filter Fans RPM on motor data plate is 3545 RPM, Relay overload set is 2.9A. During solo run, motor runs at 1.6A (starting current) at 3615 RPM. The LV motor tripped at 4.9A with enclosure door closed during load run. What are the possible causes of the motor trip? What can be done to avoid such trippings? (What are your recommendations?)(the motor specification are: Voltage = 460 V, Power = 1.5kW (2HP), No. of Phase = 3,Frequency = 60Hz,No. of poles = 2)Your prompt response will highly be apprexciated.Kind Regards,Peter.(reply) RagnarokMar 04, 2013Hi Peter, the starting current of 3 ph induction motor has nothing to do with the load. (Load affect the run up time and thermal withstand)The starting current for induction motors can as high as 7-8x the rated full load current. By your description of 1.5kW 460 3ph motors, I would think 4.9A sounds like a reasonable starting current. (not taking PF/eff into account, the rated full load current is at least 1.89A)(reply) TeguhMar 08, 2013Dear Mr Peter,if the motor trip at load run, means we have to check the load first. is it suitable for your motor or not, you can ask the mechanical engineer for that. well, for the motor itself, you can do normal check such as its impedance, megger, bearing etc.(reply) AKReddyMay 13, 2013I too experienced similar problem, in my case motor fan belt coupled, we loosen the belt and it worked for us(reply)RSS Feed for CommentsLeave a CommentClick here to cancel reply.Top of FormTell us what you're thinking... we care about your opinion!and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!Name *Email *Website

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Bottom of FormCONNECT //Subscribe to Weekly Download Updates //Free MS Excel calculation spreadsheets, electrical software and engineering guides.SubscribeLatest Technical Articles 3 Lighting Essentials You Can't DenyApril 3, 2015 Good Voltage Regulation and Justified Power Factor CorrectionApril 1, 2015 General Guidelines for Online Partial Discharge Testing of Power CablesMarch 30, 2015

Electrical Engineering Daily DoseDaily dose of knowledge and news fromElectrical Engineering WorldScience at GlanceOur mission is to be the leading provider of scientific information in the field of power and engineering in general. We publish, we share and we spread the knowledge.You're welcome to read, write and contribute to EEP in any way!Energy EfficencyWe see the results of power usage all around us. Every day, the generation and usage of energy produces more pollution than any other single industry.It's our time to use energy on smart and more efficent way. EEP is on your side!Electrical SoftwareElectrical design without appropriate software and tools is impossible to imagine. This section covers professional engineering software for design, MV and LV network and photometric calculations, voltage drop and short circuit calculations etc. 2015 EEP - Electrical Engineering Portal. All Rights Reserved | Privacy Policy | 42 queries in 0.215 seconds.Powered by CsanyiGroup SHARE TOPGetPDF Home Technical ArticlesRead and Learn Electrical Engineering!1. Technical Articles - EE Blog2. Article categories (36)3. Subscribe to technical articles4. Become Contributor (write for EEP)Energy & Power5. Energy and Power6. T&D7. Power Transformers8. Power SubstationsHV/MV/LV9. High Voltage10. Medium Voltage11. Low Voltage12. Testing & Commission.Industry13. Motor Applications14. Variable Speed Drives15. Automation in Industry16. Energy EfficencyPower Grid17. SCADA Systems18. Smart Grid19. Metering20. Wind Power Electrical Software & GuidesExcel Spreadsheets & Electrical Tools1. MS Excel Calculation Spreadsheets2. Electrical Software3. HVAC Spreadsheets4. Online Electrical CalculatorsEE Books and Guides5. Electrical Engineering in general (HV/MV/LV)6. Power Substations7. Relaying - Control and protection8. Industry Automation (PLC, SCADA, VSD etc.)Manufacturers9. Schneider Electric CTs10. ABB Drives Guides11. ABB Protection Guides12. Siemens basics of EEOther...13. Alternative Energy14. HVAC Guides15. Design Documentation (SLD, drawings etc.) EE LecturesNetwork Theorems and Laws1. Ohms Laws2. Kirchhoffs Laws3. Thvenins Theorem4. Other theorems (Nortons Theorems, etc.)Electrical Courses / Video Lectures5. Electrical Engineering in General6. PLC Programming VIDEO Courses7. Testing and CommissioningReasearch Papers8. Researches and Projects (RaP) ConnectGet in touch!1. Contact Editorial TeamAdvertisement2. AdvertisementGeneral3. Sitemap4. RSS Feed5. LoginSocial networks6. Google+ Page7. LinkedIn Page8. Twitter9. Facebook page Top of Form

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Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El. Enginering Guides EE General guides Power Substations Schneider Electric CTs Siemens Basics of EE ABB Drives Guides Industry Automation Relay control/protection Alternative EnergyElectrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resources Video Lectures Electrical Engineering PLC Programming Training Electric Testing and Maintenance (VIDEO) Network Theorems and Laws -Home / Technical Articles / Differences Between Earthed and Unearthed Cables Differences Between Earthed and Unearthed CablesPosted Jan 21 2013 by jiguparmar in Cables, High Voltage, Low Voltage, Medium Voltage, Transmission and Distribution with 14 Comments

Differences Between Earthed and Unearthed CablesIntroductionIn HT electrical distribution, the system can be earthed or unearthed.The selection of unearthed or earthed cable depends on distribution system. If such system is earthed, then we have to use cable which is manufactured for earthed system. (which the specifies the manufacturer). If the system is unearthed then we need to use cable which is manufactured for unearthed system.The unearthed system requires high insulation level compared to earthed system.For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.

Earthed SystemEarlier the generators and transformers were of small capacities and hence the fault current was less. The star point was solidly grounded. This is called earthed system.In three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.Where in unearthed system, (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to armor should be equal to insulation level of conductor to conductor.In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth.Example: 1.9/3.3 KV, 3.8/6.6 KV system

Unearthed SystemToday generators of 500MVA capacities are used and therefore the fault level has increased. In case of an earth fault, heavy current flows into the fault and this lead to damage of generators and transformers. To reduce the fault current, the star point is connected to earth through a resistance. If an earth fault occurs on one phase, the voltage of the faulty phase with respect to earth appears across the resistance.Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times.If the insulation of these phases is not designed for these increased voltages, they may develop earth fault. This is called unearthed system.In an unearth system, the phases are not grounded to earth .As a result of which there are chances of getting shock by personnel who are operating it.Example: 6.6/6.6 KV, 3.3/3.3 KV system.Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is 3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture.So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used.NomenclatureIn simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same.The size of cable will depend on current rating and voltage level. Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage. Earthed system has insulation grade of KV / 1.75 x KV. For Earthed System (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV. Unearthed system has insulation grade of KV / KV. For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV. 3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less.Thumb RuleAs a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.Because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kV+6.6kV = 11kVFor transmission of HT, earthed cable will be more economical due to low cost where as unearthed cables are not economical but insulation will be good.Generally 6.6 kV and 11kV systems are earthed through a neutral grounding resistor and the shield and armor are also earthed, especially in industrial power distribution applications. Such a case is similar to an unearthed application but with earthed shield (sometimes called solid bonding).In such cases, unearthed cables may be used so that the core insulation will have enough strength but current rating is de-rated to the value of earthed cables.But it is always better to mention the type of system earthing in the cable specification when ordering the cables so that the cable manufacturer will take care of insulation strength and de rating.Recommended EE articles // 6 Transformer Types You Can See In Commercial InstallationsMarch 16, 2015 Degradation of Insulation in Switchgear (Whats Really Happening)March 4, 2015 Test On 110kV Power Cable After Installation (2)December 26, 2014 Test On 110kV Power Cable After Installation (1)December 22, 2014Share with engineers //Article Tags //cable, distribution, earth, earth fault, earthed cables, insulation, unearthed cables, Filed Under Category //Cables High Voltage Low Voltage Medium Voltage Transmission and DistributionAbout Author //

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.RSS Feed for Comments14 Comments1. General Guidelines for Online Partial Discharge Testing of Power Cables | EEPMar 30, 2015[] Cable earth shield coming out of the cable (whether single core or three core) must not be touching any earthed metal except at terminating point to Substation Earth. Earth shield at cable termination should be insulated using shrink tubes in order to get access to individual cable earths. [](reply)2. DK SharmaMar 24, 2015We use 11 KV cable to connect our Distribution transformer which has its primary or HV winding in Delta connection and Secondary or LV winding in Star connection. Since Delta connection has no earth, we always need 11 KV Unearthed (UE) cable.So we should always ask for 11 KV UE cable.(reply)3. Manuel BolotinhaMar 15, 2015This article about earthed and unearthed cables is very basic and with poor technical content.(reply)4. AMEER AHMADMar 15, 2015I am not agree with As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.Reason: For 6.6/11kV Earthed cable, the system nominal voltage is 11kV. If we use 6.6/6.6kV Unearthed cable for 6.6/11kV Earthed system with 11kV nominal voltage, the phase to earth voltage remain 6.6kV for Earthed system(Agreed), but phase to phase voltage remain 11kV for Earthed system and this 6.6/6.6kV Unearthed cable can not meet the 11kV nominal system phase to phase voltage insulation.(reply)5. vikas j trivediNov 02, 2014How to calculate feeder htvr through metlab or etap(reply)6. JAYESH R MANKADJul 25, 2014It is good and interesting web. Informative topics are covered(reply)7. noureddineJan 26, 2014Mr. Peteryou must check set time for overload in your breakermay be the setting is wrong.thanks.(reply)8. guestJan 03, 2014Hows 6.6kV+6.6kV = 11kV? As far as I know 6.6+6.6=13.2(reply) ParagMay 08, 2014It means it is suitable for 11KV insulation requirement (since 13.2KV>11KV). Standard cable insulation ratings are as 1.1KV, 3.3KV, 11KV, 33KV)(reply)9. BalachandarDec 10, 2013I can not agree this statement ie., 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.but we can use 6.6/11kv cable for 6.6/6.6kv system.(reply)10. Peter AbbeyJan 21, 2013I have a question and it goes thus:A motor Filter Fans RPM on motor data plate is 3545 RPM, Relay overload set is 2.9A. During solo run, motor runs at 1.6A (starting current) at 3615 RPM. The LV motor tripped at 4.9A with enclosure door closed during load run. What are the possible causes of the motor trip? What can be done to avoid such trippings? (What are your recommendations?)(the motor specification are: Voltage = 460 V, Power = 1.5kW (2HP), No. of Phase = 3,Frequency = 60Hz,No. of poles = 2)Your prompt response will highly be apprexciated.Kind Regards,Peter.(reply) RagnarokMar 04, 2013Hi Peter, the starting current of 3 ph induction motor has nothing to do with the load. (Load affect the run up time and thermal withstand)The starting current for induction motors can as high as 7-8x the rated full load current. By your description of 1.5kW 460 3ph motors, I would think 4.9A sounds like a reasonable starting current. (not taking PF/eff into account, the rated full load current is at least 1.89A)(reply) TeguhMar 08, 2013Dear Mr Peter,if the motor trip at load run, means we have to check the load first. is it suitable for your motor or not, you can ask the mechanical engineer for that. well, for the motor itself, you can do normal check such as its impedance, megger, bearing etc.(reply) AKReddyMay 13, 2013I too experienced similar problem, in my case motor fan belt coupled, we loosen the belt and it worked for us(reply)RSS Feed for CommentsLeave a CommentClick here to cancel reply.Top of FormTell us what you're thinking... we care about your opinion!and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!Name *Email *Website

We believe you're an engineer, but nevertheless ... *six = 2Sign me up for the newsletter

Bottom of FormCONNECT //Subscribe to Weekly Download Updates //Free MS Excel calculation spreadsheets, electrical software and engineering guides.SubscribeLatest Technical Articles 3 Lighting Essentials You Can't DenyApril 3, 2015 Good Voltage Regulation and Justified Power Factor CorrectionApril 1, 2015 General Guidelines for Online Partial Discharge Testing of Power CablesMarch 30, 2015

Electrical Engineering Daily DoseDaily dose of knowledge and news fromElectrical Engineering WorldScience at GlanceOur mission is to be the leading provider of scientific information in the field of power and engineering in general. We publish, we share and we spread the knowledge.You're welcome to read, write and contribute to EEP in any way!Energy EfficencyWe see the results of power usage all around us. Every day, the generation and usage of energy produces more pollution than any other single industry.It's our time to use energy on smart and more efficent way. EEP is on your side!Electrical SoftwareElectrical design without appropriate software and tools is impossible to imagine. This section covers professional engineering software for design, MV and LV network and photometric calculations, voltage drop and short circuit calculations etc. 2015 EEP - Electrical Engineering Portal. All Rights Reserved | Privacy Policy | 42 queries in 0.215 seconds.Powered by CsanyiGroup SHARE TOPGetPDF Top of Form

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NotesLoading, please wait.Cables for MV Power Distribution - Earthed versus Unearthed Systems By Jeson Pitt on August 12th, 2013

Power cables can basically be classified into earthed and unearthed cables, where earthed and unearthed refer to the application for which the cable is used. Earthed system refers to a three phase system whose star point is grounded directly and the voltage between the healthy phases and the ground will be - 11kV/1.732 or 6.6/1.732. In the case of unearthed cable, ground voltage is equal to phase to phase voltage.Earthed CablesUnearthed Cables

Medium Voltage (MV) voltage power distribution system cables can be both earthed and unearthed. If the system is earthed, then we use earthed rated cable for manufacturing; and if the system is unearthed, we use an unearthed rated cable for manufacturing. Compared with the earthed cable as per the manufacturer's specifications, the unearthed cable needs higher insulation levels.The greatest difference arises in the voltage grade (Uo/U), which is:Earthed SystemUnearthed System

1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV3.3/3.3 kV and 11/11 kV

In an exception to the abover, the cables of 6.35/11kV for an earthed system can also be used in the place of 6.6/6.6 kV for an unearthed system. This is because each core of the cable has the insulation level to withstand 6.6kV due to which between core to core insulation level will be 6.6kV+6.6kV = 11kV.Contents [hide]The Difference in OriginInsulation StrengthCable RequirementsPreferable Cable for MV TransmissionThe Difference in OriginThe first generators and transformers had small capacities in which the fault current was less and the star point was solidly grounded due to which they were called earthed system. Generators that are now available have 500MVA capacity and higher fault levels. So, if there is an earth fault, a heavy current flows into the fault, which leads to the damage of the generators and transformers. In such a scenario, to reduce the fault current, the star point is connected to the earth through a resistance. In case of an earth fault in one phase, the voltage of the faulty phase with respect to the earth appears across the resistance. Due to this, the voltage of the remaining two healthy phases with respect to the earth rises by 1.7 times. If the insulation system is not designed to sustain these increased voltages, they are likely to develop earth faults.In case of earthed cable, three phase cables are earthed to a ground and each of the phase system is grounded to the earth. While the unearthed system (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage; in such situations the insulation level of the conductor to the armor should be equal to the insulation level of conductor to conductor. In the three phase earthed systems, phase to earth voltage is 1.732 times less than phase to phase voltage. Thus, the voltage stress on the cable to armor is 1.732 times less than the voltage stress between conductor to conductor.Insulation StrengthUnearthed cable requires more insulation strength than earthed cable. If a fault occurs in the phase to ground voltage is 3 time the normal phase to ground voltage. So, if an earthed rated cable is used in an unearthed system, it may result in an insulation puncture. Hence, it is essential to use unearthed rated cable in such situations, especially in the case of 6.6kV systems where resistance type earthing is used. Cable RequirementsCarrying forward the above point, 11kV earthed cable can be used in place of 6.6kV unearthed system since the cable manufacturing process is the same. The size of the cable will depend on the current rating and voltage level. So, Voltage grade (Uo/U) where Uo is phase to earth voltage and U is phase to phase voltageEarthed system has an insulation grade of kV/1.75x kV For earthed system (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV Unearthed system has insulation grade kV/kV 3 phase 3 wires system generally comes with unearthed grade cable and 3 phase 4 wire systems can be used as earthed grade cables Preferable Cable for MV TransmissionFor MV transmission, earthed cable will be more economical, but unearthed cable offers more insulation. This is because, if an earth fault occurs in the underground system, the voltage between the healthy phases and the ground will be equal to phase to phase voltage - 11kV or 6.6kV and higher insulation level is required. The voltage of the healthy phases rises by nearly 1.7 times resulting in an earth fault since the insulation of these phases is not designed for increases voltage. It is advisable to opt for an unearthed cable so that the core insulation has enough strength. Earthing, Cables, High Voltage, Distribution, Medium Voltage

More interesting Notes:Possibly related posts:Laplace TransformPower Transformers - An IntroductionPhotovoltaic (PV) Panel - Performance ModellingPower FactorIntroduction to Current Transformers

Jeson PittJeson works as a Sales Representative for D&F Liquidators, a leading supplier of electrical products. He is passionate about everything electrical.About the author

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Local_EngCommentator

Join Date: May 2011Posts: 80CT Sizing of Transformer Ground Cable05/11/2011 11:44 PM

Dear all,Is it normal to have half sizing of the nominal current of transformer to detect Earth Fault/Restricted Earth Fault?I have 5MVA 11/.38 kV transformer, with 5000/5 CT on the secondary side.I'm planning to install 2500/5 or 2000/5 CT for the ground cable to detect Earth Fault/REF, as I know the setting value is always low.Thank you for all of your help

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Bottom of FormInterested in this topic? By joining CR4 you can "subscribe" tothis discussion and receive notification when new comments are added. Join CR4, The Engineer's Place for News and Discussion!ramvinodGuru

Join Date: Jun 2007Location: chennai,IndiaPosts: 536Good Answers: 14#1Re: CT Sizing of Transformer Ground Cable 05/12/2011 4:52 AM

For REF protection all the four CTs to be of same specification and turns ratio.Earthing of transformer neutral is to be after CT.__________________Ramesh,Freelance Electrical/automation Consultant

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Join Date: May 2011Posts: 80#2In reply to #1Re: CT Sizing of Transformer Ground Cable 05/12/2011 5:26 AM

As can be seen in the drawing, there is one CT, on the right side, for grounding cable (green cable).How do we calculate the size of the CT? What class should be used?

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hien.nguyenquocPower-User

Join Date: Sep 2009Location: BH, VietnamPosts: 438Good Answers: 1#6In reply to #2Re: CT Sizing of Transformer Ground Cable 05/12/2011 12:25 PM

Local_Eng:What I see, is the only one smaller green wire connected to the neutral bar. It is Ground Return Sensing Method, isn't it? You just need to size the CT to operate at the current that the grounding wire can carry.__________________Anything that can go wrong, will go wrong

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electricalexpert65Guru

Join Date: Oct 2009Posts: 2015Good Answers: 164#16In reply to #2Re: CT Sizing of Transformer Ground Cable 05/15/2011 10:35 AM

I assume that you are talking about non-restricted earthfault protection. If so, the CT size depends upon the required protection level that you want. At the same tim if you keep it so small, then during a high fault, the CT might saturate. It has to be a balancing act. The class can be 5P10.

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TonySGuru

Join Date: Aug 2010Location: Under the Major OakPosts: 3629Good Answers: 143#3Re: CT Sizing of Transformer Ground Cable 05/12/2011 5:36 AM

That's earth fault for the system, not restricted earth fault.To be honest the earth (ground) looks far to small, what size are the conductors?__________________Pessimists are rarely disappointed.

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Join Date: May 2011Posts: 80#4Re: CT Sizing of Transformer Ground Cable 05/12/2011 6:17 AM

Yes.. It's an earth fault for the system.The cable size is 185mm2. How do we calculate the size of the CT then?

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TonySGuru


Your going to have to look at the entire transformer set up if you want restricted earth fault. The neutral earth connection will have to be moved for a start.Your existing 5000/5A CT's are too small for the maximum transformer rated current of 7600A. What are they used for at the moment, or is it just the E/F C/T you refer to?__________________Pessimists are rarely disappointed.

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jack of all tradesGuru

Join Date: Sep 2006Location: Auckland, New ZealandPosts: 6799Good Answers: 265#7Re: CT Sizing of Transformer Ground Cable 05/12/2011 3:14 PM

What country is this in (this is an international forum)?Are you using a neutral earthing (or grounding) resistor to limit the earth fault current (may explain the small size of the earth wire)?Do you know what your (approximate) fault current is and what transformer relay protection settings (especially the time delays relating to earth fault disconnection) are?Thought I better ask for clarity.__________________jack of all trades

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Akihito ShigenoPower-User

Join Date: Oct 2008Location: Jakarta IndonesiaPosts: 123Good Answers: 7#8Re: CT Sizing of Transformer Ground Cable 05/12/2011 9:57 PM

Dear all,A REF protection requires all CTs with the same CT ratios. Therefore, total five CTs (for 3 phase lines, 1 neutral and 1 neutral earth conductor) are all 5000/5A.If the protection relay is micro-processor type, the CT ratio for the neutral earth conductor can be selected with a different CT ratio from other 4 CTs. Please refer to your relay manual.Regards__________________Akihito Shigeno @ JGC-Indonesia

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rhkramerGuru


What does REF mean?

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Akihito ShigenoPower-User

Join Date: Oct 2008Location: Jakarta IndonesiaPosts: 123Good Answers: 7#12In reply to #11Re: CT Sizing of Transformer Ground Cable 05/13/2011 8:48 AM

Dear all,REF: Restricted earth fault is differential protection for transformer Wye winding.Each of all conductors brought out from the winding must have a CT of the same ratios. The total of the CT secondary currents is zero, if no earth fault on the winding. If non-zero, an earth fault is inside the winding.Regards__________________Akihito Shigeno @ JGC-Indonesia

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rhkramerGuru


Thanks!

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Join Date: May 2011Posts: 80#9Re: CT Sizing of Transformer Ground Cable 05/12/2011 11:59 PM

@ TonyS: I'm sorry for the limited information. Actually the 5MVA transformer installation is in progress. The photo is the existing transformer with EF issue, and we have to follow it.For the new transformer of 5MVA, the ip LN fault current is 211kA with 8000/5 CT.Is it ok if I use 800/5 CT to detect the EF?

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TonySGuru


REF requires 4 matched C/T's but you've obviously got that sorted.As to the general E/F C/T I think you need to talk to others on the board, my main concern is saturation of the C/T under fault conditions. I've never fitted a neutral/earth C/T that was underrated.__________________Pessimists are rarely disappointed.

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hkianCommentator

Join Date: Jan 2010Posts: 73Good Answers: 1#14Re: CT Sizing of Transformer Ground Cable 05/13/2011 12:23 PM

it depends.. know the acceptable unbalance current as well as the maximum fault current that could pass through the current transformer.. you may change the ratio of your CT if it will lessen the burden that the CT will handle..

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rudy_fernando5rActive Contributor

Join Date: May 2009Location: PhilippinesPosts: 23#15Re: CT Sizing of Transformer Ground Cable 05/13/2011 1:07 PM

Re earth fault protection (REF) 3 nos. phase CTs are required at the line side of the LV breaker (above the breaker), 1 no. neutral CT is also required to be located at the neutral of the transformer secondary, ground should be installed below this neutral CT. The 4 nos. CTs shall be identical or same spec & turns ratio. The CTs shall be sized more than the FLA of the transformer at the secondary.Under normal operation current will remain stabilized , full load current should be flowing through the 3 phase CTs and neutral CT, all connected in parallel including REF relay. In the event of LV line to earth fault (between phase CT to transformer secondary winding) , current will only flow through neutral CT and the REF relay. This relay could be set to operate at 10 to 20% of FLA.During stability test, stabilizing resistor in series with the REF relay should be adjusted to avoid nuisance tripping.thanks and regards,rudy_fernando5r

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BKRParticipant

Join Date: Apr 2011Posts: 3#17Re: CT Sizing of Transformer Ground Cable 05/15/2011 11:52 AM

Dear friend,There are two things, one is EARTH FAULT i e it is a line protection,second REF it is for transformer winding earth fault protection.It is not necessary to provide C.T. in neutral conductor for line earth fault detection but earth fault current can be sense by providing an earth fault relay in between common point of R_Y_B O/C element and C.T.star point.For REF protection we needs all C.T. of same ratio and same protection class generally it is PS class having ratio near to but more than full load of the protected object.RAKHOLIA B.K.CR4 Admin - email address removedFrom the Site FAQ: Do not post phone numbers or email addresses. The CR4 Admin will delete all phone numbers posted in threads or comments, and we strongly urge you not to put up email addresses. You can share this information via the CR4 internal messaging system.

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Interested in this topic? By joining CR4 you can "subscribe" tothis discussion and receive notification when new comments are added. Join CR4, The Engineer's Place for News and Discussion!Copy to ClipboardUsers who posted comments:Akihito Shigeno (2); BKR (1); electricalexpert65 (1); hien.nguyenquoc (1); hkian (1); jack of all trades (1); Local_Eng (3); ramvinod (1); rhkramer (2); rudy_fernando5r (1); TonyS (3)

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About UsINDCOIL transformers incorporate most modern engineering concepts and high quality materials, processed under expert supervision and rigid quality control.

INDCOIL covers a full range of Current Transformers both for metering and protection manufactured conforming to International Standards.

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Products: LT Instrument Transformers Product Details Mainly used for differential protection in order to match the ratios of main feeder CTs, which can protect sensitive instruments and devices during a fault. These CTs will saturate and limit their secondary currents within specified values.

Technical SpecificationsSummation Current Transformers: These are used to measure sum of the currents in more than one line from the same phase of the system. Normally used for monitoring total current/power drawn from a system. Input to a Summation CT should be from the individual line CTs. It is necessary for the Line CTs to have same secondary currents. The out puts from Line CTs 500/5 A, 200/5A, 150/5A ...(not 150/1A...) can be connected to a summation CT Primary windings.

Interposing Current Transformers: Mainly used for differntial protection in order to match the ratios of main feeder CTs. These can also be used as saturation CTs, which can protect sensitive Instruments and devices during a fault. These CTs will saturate and limit their secondary currents within specified values.

Cast Resin Types: Recommended for higher mechanical strength and safety. Ideal for use in corosive atmosphere. All the above mentioned types of CTs can be supplies in Cast Resin Construction also.turate and limit their secondary currents within specified values.

Neutral Current Transformers: Used for measuring unbalance current in the neutral of a three phase system. Since the neutral current should be zero in a balance three phase system, the primary of these CTs are related for a fraction of the line currents in that system.

EnquiriesThe following details if furnished with enquiries can help us to quote the right type of Transformers for your specific requirement TypePurposeRatioBurden

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