--- title: "CT secondary open and PT secondary short" subject: "Switchgear and Protection" chapter: 2 key: "semester_6/switchgear_and_protection:ct-open-pt-short" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-open-pt-short site: BCT Engineering --- # CT secondary open and PT secondary short Switchgear and Protection · Chapter 2: Current and Potential Transformers - Asked in: 2075 Bhadra, 4 marks - Video: [🔰Current Transformer Shorting Link Explanation | Working | CT block Terminal | CT Shorting Terminal](https://www.youtube.com/watch?v=1A5wsxKIQo8) — The Engineering Waley - 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-open-pt-short - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2 ## Question Explain why the secondary of CT should not be left open and secondary of PT should be short circuited. ## Short answer CT secondary must not be open because primary current becomes magnetizing current, driving core to saturation, producing dangerously high secondary voltage (kV), core overheating, and residual magnetism. PT secondary must not be shorted because fixed primary voltage causes very large short-circuit current limited only by leakage impedance, overheating and burning windings. Safe condition: CT secondary shorted, PT secondary open. ## Answer #### Why the CT secondary must not be left open - A CT primary is in series with the line, so its current I_p is fixed by the load, not by the CT secondary. - Normally, secondary ampere-turns N_s·I_s nearly cancel primary ampere-turns; only a small exciting current magnetises the core and the flux density is low. - If the secondary is opened, there is no opposing m.m.f. The **entire primary current becomes magnetising current**, so the flux rises to saturation. - Consequences: 1. **Very high secondary voltage:** the large secondary turns and rapid flux change (flat-topped flux, steep reversals near current zero) induce peak voltages of several kilovolts. This is dangerous to persons and can break down the secondary insulation. 2. **Core overheating:** high flux causes large eddy-current and hysteresis loss, which can burn the insulation. 3. **Residual magnetism:** the core may keep remanent flux, increasing future ratio and phase errors. - Therefore the CT secondary must always be connected to a burden or **short-circuited** (using a shorting link or test block) before removing a meter or relay. #### Why the PT secondary must not be short-circuited (The correct practice is that the PT secondary must **not** be short-circuited.) - A PT primary is connected across the line and works like a normal power transformer with a fixed primary voltage; the secondary feeds high-impedance loads and runs almost on open circuit. - If the secondary is short-circuited, the only limit to current is the small leakage impedance of the windings, so a **very large short-circuit current** flows. - This overheats and burns the windings within a short time and can damage the PT and cause a fault on the HV side. - Hence the PT secondary must be left open (no-load condition is safe) and is protected by **fuses or MCBs**; the PT primary is also protected by HV fuses. | | CT | PT | |---|---|---| | Safe condition | Secondary shorted | Secondary open | | Dangerous condition | Secondary open | Secondary shorted | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does an open CT secondary produce very high voltage? A. Entire primary current becomes magnetising current, driving saturation and rapid flux changes in many secondary turns B. The primary voltage rises C. Burden becomes zero D. Resistance drops Answer: A. With no opposing m.m.f., flux saturates and steep flux reversals induce kV peaks. (Key point: Open CT cause) 2. Which is a consequence of an open CT secondary? A. Lower core loss B. Core overheating and residual magnetism increasing later errors C. Reduced flux D. Safer operation Answer: B. High flux causes large eddy/hysteresis loss and remanent flux that increases future errors. (Key point: Open CT consequences) 3. Why must the PT secondary not be short-circuited? A. Voltage doubles B. The core saturates C. Only leakage impedance limits the current, so very large current overheats and burns the windings D. The primary opens Answer: C. The primary voltage is fixed, so a shorted secondary draws huge current. (Key point: PT short circuit) 4. What is the proper practice before removing a CT burden and for PT protection? A. Open the CT secondary; short the PT B. Leave both open C. Short both D. Short the CT secondary with a link or test block; protect the PT secondary with fuses/MCBs Answer: D. CT is shorted before removing a meter; PT runs near open circuit and is fused. (Key point: Practice) ## Related in Switchgear and Protection - [Distinguishing CT and PT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adistinguish-ct-pt) — 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 ## Related in other subjects - [CT secondary should not be kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Act-secondary-open-true-false) — Electrical Machines, 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 - [True/false: CT secondary kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-false-ct-secondary-open) — Electrical Machine, chapter 2 - [Open-circuited double stub for 60−j80 Ω](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Adouble-stub-open-60-j80) — RF and Microwave Engineering, chapter 2