--- title: "Applications of CTs/PTs; CT burden" subject: "Switchgear and Protection" chapter: 2 key: "semester_6/switchgear_and_protection:ct-pt-applications-burden" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-pt-applications-burden site: BCT Engineering --- # Applications of CTs/PTs; CT burden Switchgear and Protection · Chapter 2: Current and Potential Transformers - Asked in: 2072 Magh, 4 marks - Video: [Current Transformers (CT) | Eaton PSEC](https://www.youtube.com/watch?v=32Vw40nUSwA) — Eaton - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-pt-applications-burden - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2 ## Question What are the applications of CTs and PTs in power system? What is the meaning of burden on CT? ## Short answer CTs used for metering (ammeters, energy meters), protection (overcurrent, differential, distance relays), SCADA, and core-balance earth-fault detection. PTs used for metering (voltmeters, energy meters), protection (distance, directional, voltage/frequency relays), synchronising, open-delta residual voltage, and CVTs for carrier coupling. CT burden is the total secondary impedance (relays, meters, leads) expressed in VA at rated secondary current (usually 0.8 PF lagging); exceeding rated burden increases errors and causes saturation. ## Answer #### Applications of CTs and PTs in power system **Current transformers (CTs)** - Metering: ammeters, wattmeters, energy meters (kWh, kVArh) and tariff metering on HV and LV feeders. - Protection: overcurrent, earth fault, differential (transformer, generator, busbar), distance and directional relays. - Supplying current to power-factor meters, maximum demand indicators and SCADA transducers. - Core-balance CTs for sensitive earth-fault detection on cables and motors. **Potential (voltage) transformers (PTs/VTs)** - Metering: voltmeters, wattmeters, energy meters, frequency and power-factor meters. - Protection: distance relays, directional relays, under/over-voltage, under/over-frequency, reverse power relays. - Synchronising of generators and synchro-check. - Open-delta (broken-delta) connection for residual voltage in earth-fault detection. - CVTs at EHV also couple power-line carrier signals. Both provide isolation from high voltage and standardise signals to 1 A/5 A and 110 V. #### Burden on CT The **burden** of a CT is the load connected to its secondary: the total impedance of relay coils, meters and connecting leads. It is expressed in **volt-amperes (VA) at rated secondary current** and specified power factor (usually 0.8 lagging), or in ohms. ```text Burden (VA) = I_s² × Z_burden e.g. 5 A CT, Z = 0.6 Ω → 5² × 0.6 = 15 VA ``` - Standard ratings: 2.5, 5, 10, 15, 30 VA. - If the connected burden exceeds the rated burden, the CT needs more e.m.f., flux and exciting current, so ratio and phase errors increase and the CT may saturate during faults. - For 1 A CTs the lead burden is 25 times smaller than for 5 A CTs for the same lead resistance, so 1 A CTs are preferred for long leads. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the burden of a CT? A. The load on the secondary: total impedance of relays, meters and leads, expressed in VA at rated secondary current B. The primary line current C. The core weight D. The primary voltage drop Answer: A. Burden is the secondary load in VA (or ohms) at rated current and stated power factor. (Key point: CT burden meaning) 2. A 5 A CT has a burden impedance of 0.6 ohm. What is the burden? A. 3 VA B. 15 VA C. 30 VA D. 0.12 VA Answer: B. VA = I^2 x Z = 25 x 0.6 = 15 VA. (Key point: Burden calculation) 3. What happens if the connected burden exceeds the rated burden? A. Errors decrease B. Primary current falls C. More emf, flux and exciting current are needed, so ratio and phase errors rise and the CT may saturate in faults D. Nothing changes Answer: C. A higher burden demands more secondary emf and flux, increasing errors and saturation risk. (Key point: Excess burden) 4. Why are 1 A CTs preferred for long leads? A. They have higher VA ratings B. They never saturate C. Leads become shorter D. Lead burden is 25 times smaller than for 5 A CTs for the same lead resistance Answer: D. Burden is I^2 R, so 1 A gives 1/25 of the lead burden of 5 A. (Key point: 1 A vs 5 A) ## Related in Switchgear and Protection - [IDMT grading: R1, R2 with 1000/5 CTs](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-r1-r2-1000-5) — Switchgear and Protection, chapter 7, asked 2× - [CT working principle and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-definition-principle-applications) — Switchgear and Protection, chapter 2 - [CT ratio and connection for 11/132 kV](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-11-132kv) — Switchgear and Protection, chapter 7 - [CT ratio for 33/6.6 kV star/delta (300:5)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adifferential-ct-33-6-6kv-300) — Switchgear and Protection, chapter 7 - [Knee point voltage and CT selection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-selection) — Switchgear and Protection, chapter 2 - [Need of instrument transformers; CT errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ainstrument-transformers-need-ct-errors) — Switchgear and Protection, chapter 2 ## Related in other subjects - [Define embedded system and its applications](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Adefine-embedded-system-applications) — Embedded System, chapter 1, asked 4× - [Energy and power signal; bandwidth of a system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aenergy-power-bandwidth-system) — Communication Systems, chapter 1 - [Frequency response of a CT LTI system](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Act-frequency-response) — Signals and Systems, chapter 5, asked 3× - [Causality condition for CT LTI system](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Acausal-ct-lti-condition) — Signals and Systems, chapter 5