{"schema_version":"1.0","package_type":"agent_readable_article","generated_at":"2026-05-17T02:42:40+00:00","article":{"id":8280,"slug":"current-transformer-secondary-burden-calculation","title":"Υπολογισμός δευτερεύοντος φορτίου τρέχοντος μετασχηματιστή","url":"https://voltgrids.com/el/blog/current-transformer-secondary-burden-calculation/","language":"el","published_at":"2026-04-09T06:26:48+00:00","modified_at":"2026-05-10T02:34:21+00:00","author":{"id":1,"name":"Bepto"},"summary":"Ο υπολογισμός του δευτερεύοντος φορτίου του μετασχηματιστή ρεύματος είναι απαραίτητος για τη διασφάλιση της αξιοπιστίας του συστήματος ηλεκτρικής ενέργειας. Αυτός ο οδηγός μηχανικής παρέχει μια βήμα προς βήμα μεθοδολογία για τον υπολογισμό της συνολικής σύνθετης αντίστασης -συμπεριλαμβανομένων των VA ρελέ, της αντίστασης καλωδίων και των απωλειών σύνδεσης- για την αποφυγή κορεσμού του CT και την...","word_count":2468,"taxonomies":{"categories":[{"id":159,"name":"Μετασχηματιστής ρεύματος (CT)","slug":"current-transformerct","url":"https://voltgrids.com/el/blog/category/instrument-transformer/current-transformerct/"},{"id":146,"name":"Μετασχηματιστής οργάνων","slug":"instrument-transformer","url":"https://voltgrids.com/el/blog/category/instrument-transformer/"}],"tags":[{"id":190,"name":"Μέση τάση","slug":"medium-voltage","url":"https://voltgrids.com/el/blog/tag/medium-voltage/"},{"id":188,"name":"Διανομή ισχύος","slug":"power-distribution","url":"https://voltgrids.com/el/blog/tag/power-distribution/"},{"id":248,"name":"Προστασία","slug":"protection","url":"https://voltgrids.com/el/blog/tag/protection/"},{"id":191,"name":"Αξιοπιστία","slug":"reliability","url":"https://voltgrids.com/el/blog/tag/reliability/"},{"id":247,"name":"Τεχνικές προδιαγραφές","slug":"technical-specification","url":"https://voltgrids.com/el/blog/tag/technical-specification/"}]},"media_links":[{"type":"video","provider":"YouTube","url":"https://youtu.be/qWZAHtxO5oU","embed_url":"https://www.youtube.com/embed/qWZAHtxO5oU","video_id":"qWZAHtxO5oU"},{"type":"audio","provider":"SoundCloud","url":"https://soundcloud.com/bepto-247719800/current-transformer-secondary/s-9PGbjfVSzb2?si=99109b79ef9841d492d68fd7321726e5\u0026utm_source=clipboard\u0026utm_medium=text\u0026utm_campaign=social_sharing","embed_url":"https://w.soundcloud.com/player/?url=https://soundcloud.com/bepto-247719800/current-transformer-secondary/s-9PGbjfVSzb2?si=99109b79ef9841d492d68fd7321726e5\u0026utm_source=clipboard\u0026utm_medium=text\u0026utm_campaign=social_sharing\u0026auto_play=false\u0026buying=false\u0026sharing=false\u0026download=false\u0026show_artwork=true\u0026show_playcount=false\u0026show_user=true\u0026single_active=true"}],"sections":[{"heading":"Εισαγωγή","level":2,"content":"In medium voltage protection systems, even a perfectly specified Current Transformer can fail to deliver reliable fault signals if the secondary burden is miscalculated. **Secondary burden — the total impedance connected to the CT secondary terminals — directly determines whether your CT maintains accuracy during fault conditions, or saturates and sends corrupted signals to your protection relays.** For electrical engineers designing MV protection schemes and procurement managers sourcing CTs for industrial substations or power grid feeders, an incorrect burden calculation is one of the most common yet most consequential specification errors in the field. This guide provides a structured, engineering-grade methodology for calculating CT secondary burden, covering every resistance component in the secondary loop, and translating that calculation into a correct CT specification per IEC 61869-2."},{"heading":"Πίνακας περιεχομένων","level":2,"content":"- [What Is CT Secondary Burden and What Does It Include?](#what-is-ct-secondary-burden-and-what-does-it-include)\n- [How Do You Calculate Total Secondary Burden Step by Step?](#how-do-you-calculate-total-secondary-burden-step-by-step)\n- [How Does Secondary Burden Affect CT Selection for MV Protection?](#how-does-secondary-burden-affect-ct-selection-for-mv-protection)\n- [What Are the Most Common Burden Calculation Errors in Protection Circuits?](#what-are-the-most-common-burden-calculation-errors-in-protection-circuits)"},{"heading":"What Is CT Secondary Burden and What Does It Include?","level":2,"content":"![Detailed technical visualization of current transformer (CT) secondary burden components, presented in a laboratory context. A cutaway of a CT shows internal winding resistance (Rct), connected by secondary cables (Rcable) to industrial terminal blocks (Rterminal), leading to a modern numerical protection relay (Relay Burden, Srelay). The total impedance path, combining all these elements, is visually emphasized with a unified glowing blue and orange current flow and labels like \u0027CT SECONDARY BURDEN (Total Impedance - expressed in VA or Ω)\u0027, referencing the IEC 61869-2 standard.](https://voltgrids.com/wp-content/uploads/2026/04/CT-Secondary-Burden-Components-and-Total-Impedance-Visualization-1024x687.jpg)\n\nCT Secondary Burden Components and Total Impedance Visualization\n\nCT secondary burden is the **total impedance (expressed in VA or Ω) presented to the CT secondary winding** by all connected devices and conductors in the secondary loop. It is not simply the relay coil impedance — it is the sum of every resistive and reactive element the secondary current must drive through.\n\nΑνά **IEC 61869-2**, the [rated burden (Sₙ) of a protection CT is defined at rated secondary current](https://webstore.iec.ch/publication/28612)[1](#fn-1) (typically 1A or 5A) and rated power factor (usually cos φ = 0.8). The CT must maintain its accuracy class up to this burden value. Exceed it, and the effective ALF drops — potentially below your system fault level requirement."},{"heading":"Components of CT Secondary Burden","level":3,"content":"The total secondary burden comprises four distinct elements:\n\n- **Relay Burden (S_relay):** The VA consumption of all connected protection relays — overcurrent, earth fault, differential, distance. [Modern numerical protection relays typically consume 0.1–0.5VA per phase](https://www.gegridsolutions.com/multilin/catalog/850.htm)[2](#fn-2); electromechanical relays can consume 3–10VA\n- **Cable Burden (R_cable):** Resistance of the secondary wiring between CT terminals and relay panel — often the largest single burden component in field installations\n- **Terminal Block and Connection Resistance (R_terminal):** Small but non-negligible in long secondary chains; typically 0.01–0.05Ω per terminal block pair\n- **CT Secondary Winding Resistance (R_ct):** Internal winding resistance of the CT itself — not part of the external burden but critical for ALF calculation; [measured at 75°C per IEC standard](https://webstore.iec.ch/publication/28612)[3](#fn-3)"},{"heading":"Key Technical Specifications to Confirm","level":3,"content":"- **Rated Secondary Current:** 1A or 5A — this choice dramatically affects cable burden (5A secondary produces 25× more cable voltage drop than 1A for same resistance)\n- **Σύστημα μόνωσης:** Epoxy resin cast, rated 12kV / 24kV / 36kV per IEC 61869\n- **Κατηγορία ακρίβειας:** 5P or 10P for protection circuits\n- **Rated Burden Range:** Standard values — 2.5VA, 5VA, 10VA, 15VA, 30VA\n- **Operating Temperature:** [Class E (120°C) or Class F (155°C)](https://webstore.iec.ch/publication/583)[4](#fn-4) — affects Rct correction factor"},{"heading":"How Do You Calculate Total Secondary Burden Step by Step?","level":2,"content":"![Detailed technical illustration of a current transformer (CT) secondary burden calculation worksheet. The infographic shows a sequence of four graphical steps across a blueprint background: determining Relay Burden (Srelay) and converting to Rrelay, calculating Cable Resistance (Rcable_75) with temperature correction for one-way length and copper properties, adding Terminal resistance (Rterminal) for multiple pairs, and summing total burden resistance. It concludes with a summation of example values (0.02 + 0.511 + 0.18 = 0.549Ω) converted to 13.7VA at 5A, pointing to the final specification: \u0027Specify CT rated burden ≥ 15VA\u0027. A comparison highlights the massive impact of 5A secondary on cable burden.](https://voltgrids.com/wp-content/uploads/2026/04/CT-Secondary-Burden-Step-by-Step-Calculation-Worksheet-1024x687.jpg)\n\nCT Secondary Burden Step-by-Step Calculation Worksheet\n\nA rigorous secondary burden calculation follows a four-step process. Each step must be completed before CT specification is finalized — skipping any step introduces risk of under-specification."},{"heading":"Step 1: Determine Relay Burden","level":3,"content":"Obtain VA consumption from relay manufacturer datasheets for each connected device:\n\nSrelay=∑i=1nSrelay,iS_{relay} = \\sum_{i=1}^{n} S_{relay,i}\n\nConvert VA to resistance at rated secondary current:\n\nRrelay=SrelayI2n2R_{relay} = \\frac{S_{relay}}{I_{2n}^2}\n\n**Παράδειγμα:** Numerical overcurrent relay = 0.3VA, earth fault relay = 0.2VA, total = 0.5VA\nAt I₂ₙ = 5A: Rrelay=0.525=0.02,ΩR_{relay} = \\frac{0.5}{25} = 0.02 , \\Omega\nAt I₂ₙ = 1A: Rrelay=0.51=0.5,ΩR_{relay} = \\frac{0.5}{1} = 0.5 , \\Omega"},{"heading":"Step 2: Calculate Cable Resistance","level":3,"content":"This is the most critical calculation step, especially for installations where CTs are located far from relay panels:\n\nRcable=2×L×ρAR_{cable} = \\frac{2 \\times L \\times \\rho}{A}\n\nΠού:\n\n- LL = μήκος μονόδρομου καλωδίου (μέτρα)\n- ρ\\rho = [resistivity of copper = **0.0175 Ω·mm²/m**](https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity)[5](#fn-5) (at 20°C)\n- AA = επιφάνεια διατομής καλωδίου (mm²)\n- Factor **2** accounts for both outgoing and return conductors\n\n**Temperature correction to 75°C:**\n\nRcable,75=Rcable,20×[1+0.00393×(75−20)]R_{cable,75} = R_{cable,20} \\times [1 + 0.00393 \\times (75 – 20)]\n\nRcable,75=Rcable,20×1.216R_{cable,75} = R_{cable,20} \\times 1.216\n\n**Παράδειγμα:** 30m cable run, 2.5mm² copper:\nRcable,20=2×30×0.01752.5=0.42,ΩR_{cable,20} = \\frac{2 \\times 30 \\times 0.0175}{2.5} = 0.42 , \\Omega\nRcable,75=0.42×1.216=0.511,ΩR_{cable,75} = 0.42 \\times 1.216 = 0.511 , \\Omega"},{"heading":"Step 3: Add Terminal and Connection Resistance","level":3,"content":"For a typical secondary circuit with 6 terminal block pairs:\n\nRterminal=6×0.03=0.18,ΩR_{terminal} = 6 \\times 0.03 = 0.18 , \\Omega"},{"heading":"Step 4: Sum Total External Burden","level":3,"content":"Rburden,total=Rrelay+Rcable,75+RterminalR_{burden,total} = R_{relay} + R_{cable,75} + R_{terminal}\n\nRburden,total=0.02+0.511+0.018=0.549,ΩR_{burden,total} = 0.02 + 0.511 + 0.018 = 0.549 , \\Omega\n\nConvert to VA at rated secondary current:\n\nSburden,total=Rburden,total×I2n2=0.549×25=13.7,VAS_{burden,total} = R_{burden,total} \\times I_{2n}^2 = 0.549 \\times 25 = 13.7 , VA\n\n→ **Specify CT rated burden ≥ 15VA** (next standard value above 13.7VA)"},{"heading":"Burden Comparison: 1A vs 5A Secondary","level":3,"content":"| Παράμετρος | 1A Secondary | 5A Secondary |\n| Cable Resistance Impact | Low (I² effect minimal) | High (25× more VA loss) |\n| Relay Burden (VA→Ω) | Higher Ω per VA | Lower Ω per VA |\n| Recommended Cable Run | Up to 100m practical | Keep under 30m ideally |\n| Standard Burden Rating | 2.5VA–15VA typical | 10VA–30VA typical |\n| Μέγεθος πυρήνα | Smaller | Larger |\n| Εφαρμογή | Remote installations, long cable runs | Local panel installations |\n\n**The key takeaway:** For CT installations more than 20 meters from the relay panel, **1A secondary is strongly preferred** — cable burden at 5A secondary can consume the entire rated VA budget before the relay even receives a signal.\n\n**Client Case — Power Grid EPC Contractor, 33kV Substation:**\nAn EPC contractor in South Asia specified 5A secondary CTs for a 33kV outdoor substation where CT marshalling boxes were located 45 meters from the main relay panel. Initial burden calculation (relay only) showed 8VA — well within the 15VA rated burden. However, Bepto’s application engineer recalculated including cable resistance: 45m × 2.5mm² copper at 75°C added **1.23Ω = 30.7VA** to the burden. Total burden exceeded 38VA — more than double the CT rating. The specification was revised to 1A secondary CTs with 15VA burden rating, resolving the issue before manufacturing. **This single calculation prevented a complete protection system failure on a live grid feeder.**"},{"heading":"How Does Secondary Burden Affect CT Selection for MV Protection?","level":2,"content":"![Detailed technical infographic visualizing the impact of burden selection on current transformer (CT) accuracy and reliability. It shows a split comparison: the left side illustrates a calculated burden of 13.7 VA resulting in a saturated fault signal, while the right side shows a specified rated burden of 15 VA resulting in an accurate, linear fault signal reproducing the fault current multiplier. Labels highlight the calculation example and final specification: \u0027SPECIFIED RATED BURDEN: 15 VA (Class 5P20)\u0027.](https://voltgrids.com/wp-content/uploads/2026/04/Burden-Selection-Impact-on-CT-ALF-and-Protection-Accuracy-1024x687.jpg)\n\nBurden Selection Impact on CT ALF and Protection Accuracy\n\nOnce total secondary burden is calculated, it directly drives three CT specification parameters: rated burden class, accuracy class selection, and the verification of actual ALF against system fault level requirements."},{"heading":"Step 1: Select Rated Burden Class","level":3,"content":"Always select the **next standard burden value above your calculated total burden:**\n\n- Calculated burden = 13.7VA → Specify **15VA**\n- Calculated burden = 22VA → Specify **30VA**\n- Never specify a CT with rated burden equal to calculated burden — this leaves zero margin"},{"heading":"Step 2: Verify Actual ALF Against Fault Level","level":3,"content":"With rated burden selected, verify the actual ALF using:\n\nALFactual=ALFrated×Rct+Rburden,ratedRct+Rburden,actualALF_{actual} = ALF_{rated} \\times \\frac{R_{ct} + R_{burden,rated}}{R_{ct} + R_{burden,actual}}\n\nEnsure: ALFactual≥Isc,maxI1n×1.1ALF_{actual} \\geq \\frac{I_{sc,max}}{I_{1n}} \\times 1.1"},{"heading":"Step 3: Application-Specific Burden Recommendations","level":3,"content":"- **Βιομηχανική διανομή MV (6-12kV):** 5A secondary, 15VA, Class 5P20 — short cable runs in compact MCC panels\n- **Υποσταθμός δικτύου ηλεκτρικής ενέργειας (33-36kV):** 1A secondary, 15VA, Class 5P30 — long cable runs to remote relay rooms\n- **Συλλογή MV ηλιακού πάρκου (33kV):** 1A secondary, 10VA, Class 10P10 — lower fault levels, cost-optimized\n- **Κύρια μονάδα αστικού δακτυλίου (12kV):** 1A secondary, 5VA, Class 5P20 — compact epoxy-cast CT, space-constrained\n- **Θαλάσσια / υπεράκτια πλατφόρμα:** 1A secondary, 10VA, Class 5P20, IP67 epoxy encapsulation — corrosive environment"},{"heading":"Reliability Impact of Correct Burden Specification","level":3,"content":"- ✅ CT operates within linear region during fault → relay receives accurate fault current signal\n- ✅ Protection relay trips within correct time-current characteristic\n- ✅ Differential protection maintains stability on through-faults\n- ✅ System reliability and uptime preserved across full fault level range\n- ❌ Over-burdened CT saturates → relay under-reads fault current → delayed or failed trip\n- ❌ Under-specified burden rating → effective ALF reduced → protection blind spot at high fault multiples"},{"heading":"What Are the Most Common Burden Calculation Errors in Protection Circuits?","level":2,"content":"![A comprehensive technical infographic detailing four primary mistakes in CT burden calculation—temperature effects, return conductors, terminal blocks, and length changes—and visually mapping them to their operational impacts: reduced effective ALF, relay under-reading, and system failures like motor damage.](https://voltgrids.com/wp-content/uploads/2026/04/Analysis-of-CT-Overburdening-Causes-and-Consequences-1024x687.jpg)\n\nAnalysis of CT Overburdening Causes and Consequences"},{"heading":"Installation and Verification Checklist","level":3,"content":"1. **Μετρήστε το πραγματικό μήκος του καλωδίου** — use as-built drawings, not design estimates; field routing adds 15–25% to calculated length\n2. **Obtain relay burden from current datasheet** — not from memory or previous project specs; relay models vary significantly\n3. **Apply temperature correction to Rct and cable resistance** — always calculate at 75°C, not ambient\n4. **Account for all terminal blocks** — especially in marshalling kiosks with multiple intermediate terminal strips\n5. **Verify with burden meter during commissioning** — measure actual secondary loop impedance before energization\n6. **Check for parallel relay connections** — multiple relays on same CT secondary reduce total burden but require individual verification"},{"heading":"Common Errors That Cause Protection Failures","level":3,"content":"- **Using relay nameplate VA without temperature correction** — electromechanical relay coil resistance increases significantly at operating temperature\n- **Ignoring return conductor resistance** — the factor of 2 in the cable formula is frequently omitted, halving the calculated cable burden\n- **Assuming numerical relay burden equals electromechanical relay burden** — numerical relays consume 10–50× less VA; over-specifying burden wastes cost, but under-specifying for legacy relay replacements causes errors\n- **Failing to recalculate burden after relay panel relocation** — cable length changes during construction are common and must trigger burden recalculation\n- **Specifying CT burden based on relay room distance only** — forgetting intermediate junction boxes, marshalling kiosks, and test terminal blocks\n\n**Client Case — Procurement Manager, Industrial Petrochemical Plant:**\nA procurement manager at a petrochemical facility in the Middle East ordered replacement CTs based on the original 1995 project specification — 5A secondary, 15VA, Class 5P20. The relay panel had been relocated during a 2018 plant expansion, extending cable runs from 12m to 38m. Nobody recalculated the burden. After CT replacement, the overcurrent protection on a 11kV motor feeder failed to trip during a phase-to-phase fault, causing motor winding damage. Post-incident analysis revealed the actual burden was 28.4VA — nearly double the 15VA CT rating. Bepto now provides **free burden calculation review as part of CT replacement consultation**, ensuring specification accuracy before any order is placed."},{"heading":"Συμπέρασμα","level":2,"content":"CT secondary burden calculation is not a formality — it is a foundational engineering step that determines whether your entire MV protection scheme functions correctly under fault conditions. By systematically accounting for relay burden, cable resistance at operating temperature, terminal block resistance, and verifying the result against CT rated burden and ALF requirements, engineers ensure that Current Transformers deliver accurate, reliable signals when the power system needs protection most. For medium voltage power distribution, substations, and industrial installations, correct burden specification is the bedrock of protection reliability."},{"heading":"FAQs About CT Secondary Burden Calculation","level":2},{"heading":"**Ερ: Ποιο είναι το τυπικό εύρος ονομαστικού φορτίου για μετασχηματιστές ρεύματος κατηγορίας προστασίας σε συστήματα μέσης τάσης;**","level":3,"content":"**A:** Οι τυπικές ονομαστικές τιμές φορτίου σύμφωνα με το πρότυπο IEC 61869-2 είναι 2,5VA, 5VA, 10VA, 15VA και 30VA. Οι περισσότερες εφαρμογές προστασίας MV χρησιμοποιούν 10VA έως 30VA ανάλογα με τον τύπο του ρελέ και το μήκος του καλωδίου."},{"heading":"**Ε: Γιατί προτιμάται το δευτερεύον 1Α έναντι του δευτερεύοντος 5Α για μεγάλες διαδρομές καλωδίων σε κυκλώματα CT υποσταθμών;**","level":3,"content":"**A:** Cable burden scales with I²R. At 5A secondary, a 0.5Ω cable resistance consumes 12.5VA; at 1A, the same cable consumes only 0.5VA — a 25× reduction, preserving CT accuracy margin."},{"heading":"**Q: How does CT secondary burden affect the [Περιοριστικός παράγοντας ακρίβειας (ALF)](https://voltgrids.com/el/blog/ct-accuracy-limiting-factor-calculation-guide/) in protection circuits?**","level":3,"content":"**A:** Higher actual burden reduces effective ALF. If actual burden exceeds rated burden, the CT saturates at a lower fault current multiple, potentially leaving protection relays blind to high-magnitude fault events."},{"heading":"**Q: What cable cross-section is recommended for CT secondary wiring in MV protection panels?**","level":3,"content":"**A:** Minimum 2.5mm² copper for runs up to 30m with 5A secondary. For runs exceeding 30m or 1A secondary systems, 1.5mm² is acceptable. Always verify with burden calculation — never select cable size by rule of thumb alone."},{"heading":"**Q: How do you verify CT secondary burden correctly during commissioning of a protection system?**","level":3,"content":"**A:** Use a calibrated burden meter to measure actual secondary loop impedance with all relays connected. Compare against calculated value and CT rated burden. Perform secondary injection test to confirm relay operation at expected current multiples.\n\n1. “IEC 61869-2:2012 Μετασχηματιστές οργάνων - Μέρος 2: Πρόσθετες απαιτήσεις για μετασχηματιστές ρεύματος”, `https://webstore.iec.ch/publication/28612`. Official international standard defining testing and rating specifications for protection current transformers. Evidence role: general_support; Source type: standard. Supports: rated burden (Sₙ) of a protection CT is defined at rated secondary current. [↩](#fnref-1_ref)\n2. “850 Feeder Protection System”, `https://www.gegridsolutions.com/multilin/catalog/850.htm`. Technical specifications for modern numerical relays showing typical power consumption values. Evidence role: statistic; Source type: industry. Supports: Modern numerical protection relays typically consume 0.1–0.5VA per phase. [↩](#fnref-2_ref)\n3. “IEC 61869-2:2012 Μετασχηματιστές οργάνων - Μέρος 2”, `https://webstore.iec.ch/publication/28612`. IEC standards mandate resistance measurement at 75°C for thermal class alignment. Evidence role: standard; Source type: standard. Supports: measured at 75°C per IEC standard. [↩](#fnref-3_ref)\n4. “IEC 60085:2007 Ηλεκτρική μόνωση - Θερμική αξιολόγηση και προσδιορισμός”, `https://webstore.iec.ch/publication/583`. Defines standard thermal classes including Class E (120°C) and Class F (155°C) for electrical insulation materials. Evidence role: standard; Source type: standard. Supports: Class E (120°C) or Class F (155°C). [↩](#fnref-4_ref)\n5. “Ηλεκτρική αντίσταση και αγωγιμότητα”, `https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity`. Material properties database showing the standard electrical resistivity of copper at room temperature. Evidence role: statistic; Source type: research. Supports: resistivity of copper = 0.0175 Ω·mm²/m. [↩](#fnref-5_ref)"}],"source_links":[{"url":"https://voltgrids.com/el/product-category/instrument-transformer/current-transformerct/","text":"Μετασχηματιστής ρεύματος (CT)","host":"voltgrids.com","is_internal":true},{"url":"#what-is-ct-secondary-burden-and-what-does-it-include","text":"What Is CT Secondary Burden and What Does It Include?","is_internal":false},{"url":"#how-do-you-calculate-total-secondary-burden-step-by-step","text":"How Do You Calculate Total Secondary Burden Step by Step?","is_internal":false},{"url":"#how-does-secondary-burden-affect-ct-selection-for-mv-protection","text":"How Does Secondary Burden Affect CT Selection for MV Protection?","is_internal":false},{"url":"#what-are-the-most-common-burden-calculation-errors-in-protection-circuits","text":"What Are the Most Common Burden Calculation Errors in Protection Circuits?","is_internal":false},{"url":"https://webstore.iec.ch/publication/28612","text":"rated burden (Sₙ) of a protection CT is defined at rated secondary current","host":"webstore.iec.ch","is_internal":false},{"url":"#fn-1","text":"1","is_internal":false},{"url":"https://www.gegridsolutions.com/multilin/catalog/850.htm","text":"Modern numerical protection relays typically consume 0.1–0.5VA per phase","host":"www.gegridsolutions.com","is_internal":false},{"url":"#fn-2","text":"2","is_internal":false},{"url":"#fn-3","text":"3","is_internal":false},{"url":"https://webstore.iec.ch/publication/583","text":"Class E (120°C) or Class F (155°C)","host":"webstore.iec.ch","is_internal":false},{"url":"#fn-4","text":"4","is_internal":false},{"url":"https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity","text":"resistivity of copper = 0.0175 Ω·mm²/m","host":"en.wikipedia.org","is_internal":false},{"url":"#fn-5","text":"5","is_internal":false},{"url":"https://voltgrids.com/el/blog/ct-accuracy-limiting-factor-calculation-guide/","text":"Περιοριστικός παράγοντας ακρίβειας (ALF)","host":"voltgrids.com","is_internal":true},{"url":"#fnref-1_ref","text":"↩","is_internal":false},{"url":"#fnref-2_ref","text":"↩","is_internal":false},{"url":"#fnref-3_ref","text":"↩","is_internal":false},{"url":"#fnref-4_ref","text":"↩","is_internal":false},{"url":"#fnref-5_ref","text":"↩","is_internal":false}],"content_markdown":"![LA-10 LAJ-10 Μετασχηματιστής ρεύματος 10kV Εσωτερική εποξειδική ρητίνη - 5-1200A 0.2S 0.5 10P Κατηγορία 12 42 75kV Μόνωση 265mm Creepage GB1208 IEC60044-1](https://voltgrids.com/wp-content/uploads/2026/01/LA-10-LAJ-10-Current-Transformer-10kV-Indoor-Epoxy-Resin-5-1200A-0.2S-0.5-10P-Class-12-42-75kV-Insulation-265mm-Creepage-GB1208-IEC60044-1.jpg)\n\n[Μετασχηματιστής ρεύματος (CT)](https://voltgrids.com/el/product-category/instrument-transformer/current-transformerct/)\n\n## Εισαγωγή\n\nIn medium voltage protection systems, even a perfectly specified Current Transformer can fail to deliver reliable fault signals if the secondary burden is miscalculated. **Secondary burden — the total impedance connected to the CT secondary terminals — directly determines whether your CT maintains accuracy during fault conditions, or saturates and sends corrupted signals to your protection relays.** For electrical engineers designing MV protection schemes and procurement managers sourcing CTs for industrial substations or power grid feeders, an incorrect burden calculation is one of the most common yet most consequential specification errors in the field. This guide provides a structured, engineering-grade methodology for calculating CT secondary burden, covering every resistance component in the secondary loop, and translating that calculation into a correct CT specification per IEC 61869-2.\n\n## Πίνακας περιεχομένων\n\n- [What Is CT Secondary Burden and What Does It Include?](#what-is-ct-secondary-burden-and-what-does-it-include)\n- [How Do You Calculate Total Secondary Burden Step by Step?](#how-do-you-calculate-total-secondary-burden-step-by-step)\n- [How Does Secondary Burden Affect CT Selection for MV Protection?](#how-does-secondary-burden-affect-ct-selection-for-mv-protection)\n- [What Are the Most Common Burden Calculation Errors in Protection Circuits?](#what-are-the-most-common-burden-calculation-errors-in-protection-circuits)\n\n## What Is CT Secondary Burden and What Does It Include?\n\n![Detailed technical visualization of current transformer (CT) secondary burden components, presented in a laboratory context. A cutaway of a CT shows internal winding resistance (Rct), connected by secondary cables (Rcable) to industrial terminal blocks (Rterminal), leading to a modern numerical protection relay (Relay Burden, Srelay). The total impedance path, combining all these elements, is visually emphasized with a unified glowing blue and orange current flow and labels like \u0027CT SECONDARY BURDEN (Total Impedance - expressed in VA or Ω)\u0027, referencing the IEC 61869-2 standard.](https://voltgrids.com/wp-content/uploads/2026/04/CT-Secondary-Burden-Components-and-Total-Impedance-Visualization-1024x687.jpg)\n\nCT Secondary Burden Components and Total Impedance Visualization\n\nCT secondary burden is the **total impedance (expressed in VA or Ω) presented to the CT secondary winding** by all connected devices and conductors in the secondary loop. It is not simply the relay coil impedance — it is the sum of every resistive and reactive element the secondary current must drive through.\n\nΑνά **IEC 61869-2**, the [rated burden (Sₙ) of a protection CT is defined at rated secondary current](https://webstore.iec.ch/publication/28612)[1](#fn-1) (typically 1A or 5A) and rated power factor (usually cos φ = 0.8). The CT must maintain its accuracy class up to this burden value. Exceed it, and the effective ALF drops — potentially below your system fault level requirement.\n\n### Components of CT Secondary Burden\n\nThe total secondary burden comprises four distinct elements:\n\n- **Relay Burden (S_relay):** The VA consumption of all connected protection relays — overcurrent, earth fault, differential, distance. [Modern numerical protection relays typically consume 0.1–0.5VA per phase](https://www.gegridsolutions.com/multilin/catalog/850.htm)[2](#fn-2); electromechanical relays can consume 3–10VA\n- **Cable Burden (R_cable):** Resistance of the secondary wiring between CT terminals and relay panel — often the largest single burden component in field installations\n- **Terminal Block and Connection Resistance (R_terminal):** Small but non-negligible in long secondary chains; typically 0.01–0.05Ω per terminal block pair\n- **CT Secondary Winding Resistance (R_ct):** Internal winding resistance of the CT itself — not part of the external burden but critical for ALF calculation; [measured at 75°C per IEC standard](https://webstore.iec.ch/publication/28612)[3](#fn-3)\n\n### Key Technical Specifications to Confirm\n\n- **Rated Secondary Current:** 1A or 5A — this choice dramatically affects cable burden (5A secondary produces 25× more cable voltage drop than 1A for same resistance)\n- **Σύστημα μόνωσης:** Epoxy resin cast, rated 12kV / 24kV / 36kV per IEC 61869\n- **Κατηγορία ακρίβειας:** 5P or 10P for protection circuits\n- **Rated Burden Range:** Standard values — 2.5VA, 5VA, 10VA, 15VA, 30VA\n- **Operating Temperature:** [Class E (120°C) or Class F (155°C)](https://webstore.iec.ch/publication/583)[4](#fn-4) — affects Rct correction factor\n\n## How Do You Calculate Total Secondary Burden Step by Step?\n\n![Detailed technical illustration of a current transformer (CT) secondary burden calculation worksheet. The infographic shows a sequence of four graphical steps across a blueprint background: determining Relay Burden (Srelay) and converting to Rrelay, calculating Cable Resistance (Rcable_75) with temperature correction for one-way length and copper properties, adding Terminal resistance (Rterminal) for multiple pairs, and summing total burden resistance. It concludes with a summation of example values (0.02 + 0.511 + 0.18 = 0.549Ω) converted to 13.7VA at 5A, pointing to the final specification: \u0027Specify CT rated burden ≥ 15VA\u0027. A comparison highlights the massive impact of 5A secondary on cable burden.](https://voltgrids.com/wp-content/uploads/2026/04/CT-Secondary-Burden-Step-by-Step-Calculation-Worksheet-1024x687.jpg)\n\nCT Secondary Burden Step-by-Step Calculation Worksheet\n\nA rigorous secondary burden calculation follows a four-step process. Each step must be completed before CT specification is finalized — skipping any step introduces risk of under-specification.\n\n### Step 1: Determine Relay Burden\n\nObtain VA consumption from relay manufacturer datasheets for each connected device:\n\nSrelay=∑i=1nSrelay,iS_{relay} = \\sum_{i=1}^{n} S_{relay,i}\n\nConvert VA to resistance at rated secondary current:\n\nRrelay=SrelayI2n2R_{relay} = \\frac{S_{relay}}{I_{2n}^2}\n\n**Παράδειγμα:** Numerical overcurrent relay = 0.3VA, earth fault relay = 0.2VA, total = 0.5VA\nAt I₂ₙ = 5A: Rrelay=0.525=0.02,ΩR_{relay} = \\frac{0.5}{25} = 0.02 , \\Omega\nAt I₂ₙ = 1A: Rrelay=0.51=0.5,ΩR_{relay} = \\frac{0.5}{1} = 0.5 , \\Omega\n\n### Step 2: Calculate Cable Resistance\n\nThis is the most critical calculation step, especially for installations where CTs are located far from relay panels:\n\nRcable=2×L×ρAR_{cable} = \\frac{2 \\times L \\times \\rho}{A}\n\nΠού:\n\n- LL = μήκος μονόδρομου καλωδίου (μέτρα)\n- ρ\\rho = [resistivity of copper = **0.0175 Ω·mm²/m**](https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity)[5](#fn-5) (at 20°C)\n- AA = επιφάνεια διατομής καλωδίου (mm²)\n- Factor **2** accounts for both outgoing and return conductors\n\n**Temperature correction to 75°C:**\n\nRcable,75=Rcable,20×[1+0.00393×(75−20)]R_{cable,75} = R_{cable,20} \\times [1 + 0.00393 \\times (75 – 20)]\n\nRcable,75=Rcable,20×1.216R_{cable,75} = R_{cable,20} \\times 1.216\n\n**Παράδειγμα:** 30m cable run, 2.5mm² copper:\nRcable,20=2×30×0.01752.5=0.42,ΩR_{cable,20} = \\frac{2 \\times 30 \\times 0.0175}{2.5} = 0.42 , \\Omega\nRcable,75=0.42×1.216=0.511,ΩR_{cable,75} = 0.42 \\times 1.216 = 0.511 , \\Omega\n\n### Step 3: Add Terminal and Connection Resistance\n\nFor a typical secondary circuit with 6 terminal block pairs:\n\nRterminal=6×0.03=0.18,ΩR_{terminal} = 6 \\times 0.03 = 0.18 , \\Omega\n\n### Step 4: Sum Total External Burden\n\nRburden,total=Rrelay+Rcable,75+RterminalR_{burden,total} = R_{relay} + R_{cable,75} + R_{terminal}\n\nRburden,total=0.02+0.511+0.018=0.549,ΩR_{burden,total} = 0.02 + 0.511 + 0.018 = 0.549 , \\Omega\n\nConvert to VA at rated secondary current:\n\nSburden,total=Rburden,total×I2n2=0.549×25=13.7,VAS_{burden,total} = R_{burden,total} \\times I_{2n}^2 = 0.549 \\times 25 = 13.7 , VA\n\n→ **Specify CT rated burden ≥ 15VA** (next standard value above 13.7VA)\n\n### Burden Comparison: 1A vs 5A Secondary\n\n| Παράμετρος | 1A Secondary | 5A Secondary |\n| Cable Resistance Impact | Low (I² effect minimal) | High (25× more VA loss) |\n| Relay Burden (VA→Ω) | Higher Ω per VA | Lower Ω per VA |\n| Recommended Cable Run | Up to 100m practical | Keep under 30m ideally |\n| Standard Burden Rating | 2.5VA–15VA typical | 10VA–30VA typical |\n| Μέγεθος πυρήνα | Smaller | Larger |\n| Εφαρμογή | Remote installations, long cable runs | Local panel installations |\n\n**The key takeaway:** For CT installations more than 20 meters from the relay panel, **1A secondary is strongly preferred** — cable burden at 5A secondary can consume the entire rated VA budget before the relay even receives a signal.\n\n**Client Case — Power Grid EPC Contractor, 33kV Substation:**\nAn EPC contractor in South Asia specified 5A secondary CTs for a 33kV outdoor substation where CT marshalling boxes were located 45 meters from the main relay panel. Initial burden calculation (relay only) showed 8VA — well within the 15VA rated burden. However, Bepto’s application engineer recalculated including cable resistance: 45m × 2.5mm² copper at 75°C added **1.23Ω = 30.7VA** to the burden. Total burden exceeded 38VA — more than double the CT rating. The specification was revised to 1A secondary CTs with 15VA burden rating, resolving the issue before manufacturing. **This single calculation prevented a complete protection system failure on a live grid feeder.**\n\n## How Does Secondary Burden Affect CT Selection for MV Protection?\n\n![Detailed technical infographic visualizing the impact of burden selection on current transformer (CT) accuracy and reliability. It shows a split comparison: the left side illustrates a calculated burden of 13.7 VA resulting in a saturated fault signal, while the right side shows a specified rated burden of 15 VA resulting in an accurate, linear fault signal reproducing the fault current multiplier. Labels highlight the calculation example and final specification: \u0027SPECIFIED RATED BURDEN: 15 VA (Class 5P20)\u0027.](https://voltgrids.com/wp-content/uploads/2026/04/Burden-Selection-Impact-on-CT-ALF-and-Protection-Accuracy-1024x687.jpg)\n\nBurden Selection Impact on CT ALF and Protection Accuracy\n\nOnce total secondary burden is calculated, it directly drives three CT specification parameters: rated burden class, accuracy class selection, and the verification of actual ALF against system fault level requirements.\n\n### Step 1: Select Rated Burden Class\n\nAlways select the **next standard burden value above your calculated total burden:**\n\n- Calculated burden = 13.7VA → Specify **15VA**\n- Calculated burden = 22VA → Specify **30VA**\n- Never specify a CT with rated burden equal to calculated burden — this leaves zero margin\n\n### Step 2: Verify Actual ALF Against Fault Level\n\nWith rated burden selected, verify the actual ALF using:\n\nALFactual=ALFrated×Rct+Rburden,ratedRct+Rburden,actualALF_{actual} = ALF_{rated} \\times \\frac{R_{ct} + R_{burden,rated}}{R_{ct} + R_{burden,actual}}\n\nEnsure: ALFactual≥Isc,maxI1n×1.1ALF_{actual} \\geq \\frac{I_{sc,max}}{I_{1n}} \\times 1.1\n\n### Step 3: Application-Specific Burden Recommendations\n\n- **Βιομηχανική διανομή MV (6-12kV):** 5A secondary, 15VA, Class 5P20 — short cable runs in compact MCC panels\n- **Υποσταθμός δικτύου ηλεκτρικής ενέργειας (33-36kV):** 1A secondary, 15VA, Class 5P30 — long cable runs to remote relay rooms\n- **Συλλογή MV ηλιακού πάρκου (33kV):** 1A secondary, 10VA, Class 10P10 — lower fault levels, cost-optimized\n- **Κύρια μονάδα αστικού δακτυλίου (12kV):** 1A secondary, 5VA, Class 5P20 — compact epoxy-cast CT, space-constrained\n- **Θαλάσσια / υπεράκτια πλατφόρμα:** 1A secondary, 10VA, Class 5P20, IP67 epoxy encapsulation — corrosive environment\n\n### Reliability Impact of Correct Burden Specification\n\n- ✅ CT operates within linear region during fault → relay receives accurate fault current signal\n- ✅ Protection relay trips within correct time-current characteristic\n- ✅ Differential protection maintains stability on through-faults\n- ✅ System reliability and uptime preserved across full fault level range\n- ❌ Over-burdened CT saturates → relay under-reads fault current → delayed or failed trip\n- ❌ Under-specified burden rating → effective ALF reduced → protection blind spot at high fault multiples\n\n## What Are the Most Common Burden Calculation Errors in Protection Circuits?\n\n![A comprehensive technical infographic detailing four primary mistakes in CT burden calculation—temperature effects, return conductors, terminal blocks, and length changes—and visually mapping them to their operational impacts: reduced effective ALF, relay under-reading, and system failures like motor damage.](https://voltgrids.com/wp-content/uploads/2026/04/Analysis-of-CT-Overburdening-Causes-and-Consequences-1024x687.jpg)\n\nAnalysis of CT Overburdening Causes and Consequences\n\n### Installation and Verification Checklist\n\n1. **Μετρήστε το πραγματικό μήκος του καλωδίου** — use as-built drawings, not design estimates; field routing adds 15–25% to calculated length\n2. **Obtain relay burden from current datasheet** — not from memory or previous project specs; relay models vary significantly\n3. **Apply temperature correction to Rct and cable resistance** — always calculate at 75°C, not ambient\n4. **Account for all terminal blocks** — especially in marshalling kiosks with multiple intermediate terminal strips\n5. **Verify with burden meter during commissioning** — measure actual secondary loop impedance before energization\n6. **Check for parallel relay connections** — multiple relays on same CT secondary reduce total burden but require individual verification\n\n### Common Errors That Cause Protection Failures\n\n- **Using relay nameplate VA without temperature correction** — electromechanical relay coil resistance increases significantly at operating temperature\n- **Ignoring return conductor resistance** — the factor of 2 in the cable formula is frequently omitted, halving the calculated cable burden\n- **Assuming numerical relay burden equals electromechanical relay burden** — numerical relays consume 10–50× less VA; over-specifying burden wastes cost, but under-specifying for legacy relay replacements causes errors\n- **Failing to recalculate burden after relay panel relocation** — cable length changes during construction are common and must trigger burden recalculation\n- **Specifying CT burden based on relay room distance only** — forgetting intermediate junction boxes, marshalling kiosks, and test terminal blocks\n\n**Client Case — Procurement Manager, Industrial Petrochemical Plant:**\nA procurement manager at a petrochemical facility in the Middle East ordered replacement CTs based on the original 1995 project specification — 5A secondary, 15VA, Class 5P20. The relay panel had been relocated during a 2018 plant expansion, extending cable runs from 12m to 38m. Nobody recalculated the burden. After CT replacement, the overcurrent protection on a 11kV motor feeder failed to trip during a phase-to-phase fault, causing motor winding damage. Post-incident analysis revealed the actual burden was 28.4VA — nearly double the 15VA CT rating. Bepto now provides **free burden calculation review as part of CT replacement consultation**, ensuring specification accuracy before any order is placed.\n\n## Συμπέρασμα\n\nCT secondary burden calculation is not a formality — it is a foundational engineering step that determines whether your entire MV protection scheme functions correctly under fault conditions. By systematically accounting for relay burden, cable resistance at operating temperature, terminal block resistance, and verifying the result against CT rated burden and ALF requirements, engineers ensure that Current Transformers deliver accurate, reliable signals when the power system needs protection most. For medium voltage power distribution, substations, and industrial installations, correct burden specification is the bedrock of protection reliability.\n\n## FAQs About CT Secondary Burden Calculation\n\n### **Ερ: Ποιο είναι το τυπικό εύρος ονομαστικού φορτίου για μετασχηματιστές ρεύματος κατηγορίας προστασίας σε συστήματα μέσης τάσης;**\n\n**A:** Οι τυπικές ονομαστικές τιμές φορτίου σύμφωνα με το πρότυπο IEC 61869-2 είναι 2,5VA, 5VA, 10VA, 15VA και 30VA. Οι περισσότερες εφαρμογές προστασίας MV χρησιμοποιούν 10VA έως 30VA ανάλογα με τον τύπο του ρελέ και το μήκος του καλωδίου.\n\n### **Ε: Γιατί προτιμάται το δευτερεύον 1Α έναντι του δευτερεύοντος 5Α για μεγάλες διαδρομές καλωδίων σε κυκλώματα CT υποσταθμών;**\n\n**A:** Cable burden scales with I²R. At 5A secondary, a 0.5Ω cable resistance consumes 12.5VA; at 1A, the same cable consumes only 0.5VA — a 25× reduction, preserving CT accuracy margin.\n\n### **Q: How does CT secondary burden affect the [Περιοριστικός παράγοντας ακρίβειας (ALF)](https://voltgrids.com/el/blog/ct-accuracy-limiting-factor-calculation-guide/) in protection circuits?**\n\n**A:** Higher actual burden reduces effective ALF. If actual burden exceeds rated burden, the CT saturates at a lower fault current multiple, potentially leaving protection relays blind to high-magnitude fault events.\n\n### **Q: What cable cross-section is recommended for CT secondary wiring in MV protection panels?**\n\n**A:** Minimum 2.5mm² copper for runs up to 30m with 5A secondary. For runs exceeding 30m or 1A secondary systems, 1.5mm² is acceptable. Always verify with burden calculation — never select cable size by rule of thumb alone.\n\n### **Q: How do you verify CT secondary burden correctly during commissioning of a protection system?**\n\n**A:** Use a calibrated burden meter to measure actual secondary loop impedance with all relays connected. Compare against calculated value and CT rated burden. Perform secondary injection test to confirm relay operation at expected current multiples.\n\n1. “IEC 61869-2:2012 Μετασχηματιστές οργάνων - Μέρος 2: Πρόσθετες απαιτήσεις για μετασχηματιστές ρεύματος”, `https://webstore.iec.ch/publication/28612`. Official international standard defining testing and rating specifications for protection current transformers. Evidence role: general_support; Source type: standard. Supports: rated burden (Sₙ) of a protection CT is defined at rated secondary current. [↩](#fnref-1_ref)\n2. “850 Feeder Protection System”, `https://www.gegridsolutions.com/multilin/catalog/850.htm`. Technical specifications for modern numerical relays showing typical power consumption values. Evidence role: statistic; Source type: industry. Supports: Modern numerical protection relays typically consume 0.1–0.5VA per phase. [↩](#fnref-2_ref)\n3. “IEC 61869-2:2012 Μετασχηματιστές οργάνων - Μέρος 2”, `https://webstore.iec.ch/publication/28612`. IEC standards mandate resistance measurement at 75°C for thermal class alignment. Evidence role: standard; Source type: standard. Supports: measured at 75°C per IEC standard. [↩](#fnref-3_ref)\n4. “IEC 60085:2007 Ηλεκτρική μόνωση - Θερμική αξιολόγηση και προσδιορισμός”, `https://webstore.iec.ch/publication/583`. Defines standard thermal classes including Class E (120°C) and Class F (155°C) for electrical insulation materials. Evidence role: standard; Source type: standard. Supports: Class E (120°C) or Class F (155°C). [↩](#fnref-4_ref)\n5. “Ηλεκτρική αντίσταση και αγωγιμότητα”, `https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity`. Material properties database showing the standard electrical resistivity of copper at room temperature. Evidence role: statistic; Source type: research. Supports: resistivity of copper = 0.0175 Ω·mm²/m. 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