--- title: "Induction type directional overcurrent relay" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:induction-directional-overcurrent" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/induction-directional-overcurrent site: BCT Engineering --- # Induction type directional overcurrent relay Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2075 Bhadra, 8 marks - Video: [Induction Type Directional overcurrent relay](https://www.youtube.com/watch?v=AR_hxoebKCU) — Ultimate Tech education - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/induction-directional-overcurrent - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question Describe the construction and principle of operation of induction type directional over-current relay. Explain IDMT characteristics and how they are obtained in an induction type relay. ## Short answer An induction type directional overcurrent relay combines a directional element (wattmetric disc with voltage coil from PT and current coil from CT, torque ∝ V·I·cos(θ‑τ)) and an overcurrent element. Directional contacts close only for forward current, allowing the overcurrent disc to operate. IDMT characteristic gives inverse time at low multiples and definite minimum time at high multiples, achieved by saturating the electromagnet core at about 10× pick‑up. ## Answer An **induction type directional overcurrent relay** operates only when the current exceeds its setting **and** flows in a chosen direction (e.g. towards a fault in the protected line). It has two elements in one case: a directional (power) element and a non-directional induction overcurrent element. ```text CT PT │ │ ┌────┴─────────────┐ ┌──────┴──────┐ │ Directional elem │ │ voltage coil│ │ (wattmetric disc)│◄──┤ │ │ current coil │ └─────────────┘ │ contacts 1-2 ───┼──┐ └──────────────────┘ │ (in series with ┌──────────────────┐ │ lower coil of O/C) │ O/C element │◄─┘ │ upper coil (CT) │ │ lower coil │ │ disc + spring │──► trip contacts └──────────────────┘ ``` #### Construction 1. **Directional element:** works like an induction wattmeter. The upper magnet has a voltage coil (fed from the PT); the lower magnet has a current coil (fed from the CT). An aluminium disc or cup lies between them. Torque T ∝ V·I·cos(θ − τ), where τ is the maximum torque angle. 2. **Overcurrent element:** a wattmeter-type induction disc relay. The upper magnet carries the main current coil (with plug setting tappings) and a secondary winding; the secondary winding feeds the lower magnet coil **through the contacts of the directional element**. 3. A control spring, a brake (permanent) magnet for damping, and a disc spindle carrying the moving contact; the time is set by moving the backstop (TMS). #### Principle of operation - **Current in the non-tripping (reverse) direction:** the directional torque acts in the reverse direction, so its contacts stay open. The lower coil of the O/C element is then open-circuited and the O/C disc has no torque, so there is no trip even if the current is high. - **Fault with power flow in the tripping direction:** the directional disc closes its contacts within a few cycles; the lower coil gets current and the O/C disc develops torque. If the current exceeds pick-up, the disc rotates and closes the trip contacts after a time decided by the current magnitude and the TMS. #### IDMT characteristic and how it is obtained **IDMT (Inverse Definite Minimum Time):** the operating time is inversely proportional to current at low multiples of setting, and tends to a definite minimum time at high multiples (PSM about 10–20). ```text time│\ │ \ │ \__ │ \____ │ ‾‾‾‾‾‾‾‾ definite minimum └─────────────────── PSM 2 5 10 20 ``` How it is obtained in the induction relay: - Driving torque ∝ I², so at low currents the disc speed rises with current and the time falls (inverse part). - The **electromagnet core is designed to saturate** at higher currents (about 10× setting). Beyond saturation the flux and torque hardly rise, so the time stays nearly constant: the definite minimum part. - The brake magnet gives a damping torque ∝ speed, so the disc runs at a steady speed set by the driving torque. - Standard IDMT (3-s, normal inverse) curve: t = 0.14 × TMS / (PSM^0.02 − 1), which gives about 3 s at PSM 10 with TMS = 1. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the two elements of an induction type directional overcurrent relay? A. A directional (wattmetric) element and a non-directional induction overcurrent element B. Two identical overcurrent discs connected in parallel C. A voltage element and a frequency element D. A thermal element and a differential element Answer: A. The relay combines a directional power element with an induction disc overcurrent element in one case. (Key point: Two-element construction) 2. How does the directional element control the overcurrent element? A. Its contacts close the circuit feeding the lower coil of the overcurrent element B. It changes the spring tension of the disc C. It short-circuits the brake magnet D. It changes the CT ratio Answer: A. The secondary winding feeds the lower coil only through the directional contacts, so the disc gets torque only for tripping-direction power flow. (Key point: Role of directional contacts) 3. With normal power flow and a very high current, why does the relay not trip? A. The directional torque is reverse, its contacts stay open, so the overcurrent disc has no torque B. The brake magnet stops the disc C. The plug setting automatically rises D. The PT voltage is zero Answer: A. The lower coil circuit stays open, so no torque develops even if current is high. (Key point: Normal power flow behaviour) 4. How is the definite minimum time part of the IDMT characteristic obtained? A. The electromagnet core saturates at about 10 times the setting, so torque stops rising B. The brake magnet is removed at high current C. The spring stiffness increases D. The PT limits the current Answer: A. Torque is proportional to I^2 at low current (inverse part); beyond saturation flux and torque hardly rise, so the time becomes constant. (Key point: Obtaining IDMT) ## Related in Switchgear and Protection - [IDMT characteristics and induction disc relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-induction-disc-relay) — Switchgear and Protection, chapter 7, asked 2× - [Directional static overcurrent relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adirectional-static-overcurrent) — Switchgear and Protection, chapter 8, asked 2× - [Voltage-monitored overcurrent relay timing](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avoltage-monitored-overcurrent) — Switchgear and Protection, chapter 7 - [Design IDMT grading for relays A and B](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aradial-feeder-idmt-design-ab) — Switchgear and Protection, chapter 7 - [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× - [IDMT relays across 132/11 kV transformer (3500 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-132-11kv-3500a) — Switchgear and Protection, chapter 7 ## Related in other subjects - [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5 - [Construction and operation of synchronous generator](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asynchronous-generator-construction-principle) — Electric Machines II, chapter 1 - [JFET construction, curve and drain current equation](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ajfet-construction-equation) — Electronics Devices and Circuit, chapter 3 - [Reluctance motor construction and principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Areluctance-motor) — Electric Machines II, chapter 3, asked 3×