--- title: "Distinguishing CT and PT" subject: "Switchgear and Protection" chapter: 2 key: "semester_6/switchgear_and_protection:distinguish-ct-pt" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/distinguish-ct-pt site: BCT Engineering --- # Distinguishing CT and PT Switchgear and Protection · Chapter 2: Current and Potential Transformers - Asked in: 2073 Bhadra, 4 marks - Video: [Current Transformer vs Voltage Transformer : What’s the Difference ? TheElectricalGuy](https://www.youtube.com/watch?v=p4fXV8FwmLY) — Gaurav J - TheElectricalGuy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/distinguish-ct-pt - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2 ## Question What are the main facts to distinguish current transformer and potential transformer? ## Short answer CT primary is series-connected with line, reducing current to 1A/5A; PT primary is parallel-connected, reducing voltage to 110V. CT has few primary turns, many secondary turns, low secondary impedance (near short-circuit); PT has many primary turns, few secondary turns, high secondary impedance (near open-circuit). CT secondary open-circuit dangerous (high voltage); PT secondary short-circuit dangerous (high current). CT burden affects secondary voltage; PT burden affects secondary current. ## Answer A **current transformer** reduces a large line current to a standard value (1 A or 5 A), while a **potential transformer** reduces a high voltage to a standard value (110 V). Their main distinguishing facts are: | Point | Current transformer (CT) | Potential transformer (PT) | |---|---|---| | Connection | Primary in **series** with line | Primary in **parallel** (line to line or line to earth) | | Primary quantity | Line current; not decided by CT burden | System voltage; fixed | | Primary winding | Few turns (often a single bar) | Many turns | | Secondary winding | Many turns | Few turns | | Standard secondary | 1 A or 5 A | 110 V or 110/√3 V | | Core flux | Varies with line current; low normally | Almost constant (voltage fixed) | | Secondary load | Very low impedance (ammeter, relay current coil) – nearly short circuit | High impedance (voltmeter, relay voltage coil) – nearly open circuit | | Secondary open circuit | Dangerous: very high voltage, core overheats | Normal (no-load) condition | | Secondary short circuit | Normal condition | Dangerous: very high current, winding burns | | Burden effect | Burden changes secondary voltage, primary current unchanged | Burden changes secondary current | | Errors due to | Exciting current | Winding voltage drops and exciting current | | Construction at EHV | Bar/ring/bushing type | Electromagnetic or capacitor (CVT) type | Both provide isolation, standardise relay and meter inputs, and have ratio and phase angle errors. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are CT and PT primaries connected? A. CT in series with the line, PT in parallel across it B. CT in parallel, PT in series C. Both in series D. Both in parallel Answer: A. A CT carries line current in series; a PT is connected across the line voltage. (Key point: Connection) 2. Which winding arrangement is correct? A. CT: many primary turns; PT: few primary turns B. CT: few primary turns, many secondary turns; PT: many primary turns, few secondary turns C. Both have equal turns D. CT: many turns on both Answer: B. A CT steps current down (few primary, many secondary turns); a PT steps voltage down. (Key point: Windings) 3. Which is the dangerous secondary condition for each? A. CT: short circuit; PT: open circuit B. Both: open circuit C. CT: open circuit; PT: short circuit D. Both: short circuit Answer: C. An open CT secondary produces very high voltage; a shorted PT secondary draws very high current. (Key point: Dangerous condition) 4. How does core flux behave? A. CT flux is almost constant; PT flux varies with load current B. Both vary strongly C. Both are zero D. CT flux varies with line current and is low normally; PT flux is almost constant since voltage is fixed Answer: D. The PT works at fixed voltage, hence nearly constant flux; the CT flux follows line current. (Key point: Core flux) ## Related in Switchgear and Protection - [CT secondary open and PT secondary short](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-open-pt-short) — Switchgear and Protection, chapter 2 - [CT with ammeter numerical](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-ammeter-numerical) — Switchgear and Protection, chapter 2 - [Accuracy classes of CT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-accuracy-class) — Switchgear and Protection, chapter 2 - [IDMT settings for 20 MVA transformer and feeder](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-feeder-idmt-20mva) — Switchgear and Protection, chapter 7, asked 2× - [Percentage differential relay: 125 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-125mva) — Switchgear and Protection, chapter 7 - [Percentage differential relay: 150 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-150mva) — Switchgear and Protection, chapter 7 ## Related in other subjects - [Current transformer vs conventional; CT open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Acurrent-transformer-secondary-open) — Electrical Machine, chapter 2 - [CT secondary open; auto transformer economy](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Act-open-autotransformer-economical) — Electrical Machines, chapter 2 - [Main winding current and pf at 5% slip](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-im-main-current) — Electric Machines II, chapter 3 - [Explain emf, potential difference and current](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aemf-potential-difference-current) — Basic Electrical Engineering, chapter 1