--- title: "SP, DP and TP miniature circuit breakers" subject: "Industrial Power Distribution and Illumination" chapter: 3 key: "semester_6/industrial_power_distribution_and_illumination:sp-dp-tp-mcb" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/sp-dp-tp-mcb site: BCT Engineering --- # SP, DP and TP miniature circuit breakers Industrial Power Distribution and Illumination · Chapter 3: Power Carrying Devices - Asked in: 2068 Magh (old course), 8 marks - Video: [What Are MCB SP, DP, TP, and TPN? Where Are They Used?](https://www.youtube.com/watch?v=a9PJ8FwNYO8) — Gyan Center - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/sp-dp-tp-mcb - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-3 ## Question Write a detailed note on the construction, operation and use of SP, DP and TP miniature circuit breakers. ## Short answer An MCB has a bimetallic strip for overload inverse-time tripping and a solenoid coil for instantaneous short-circuit tripping, contacts parting inside an arc chute for fast quenching. SP MCB switches phase only for single-phase final circuits; DP switches phase and neutral, used for main switch or geyser; TP switches all three phases with common trip bar for 3-phase motors and sub-mains to prevent single phasing. ## Answer A miniature circuit breaker (MCB) is a small, automatically operated switch that protects LT circuits against overload and short circuit, and can be reset after tripping. SP, DP and TP refer to the number of poles switched together. #### Construction ```text ON/OFF toggle | [latch & trip mechanism] | | bimetal strip solenoid coil (overload) + plunger (short circuit) | | fixed & moving contacts -> arc chute (arc splitter plates) moulded insulating case (DIN rail mount) ``` - **Moulded case** of insulating material, mounted on a 35 mm DIN rail. - **Operating toggle** with a trip-free mechanism: it trips even if the knob is held ON. - **Contacts**: fixed and moving contacts of silver alloy. - **Thermal element**: bimetallic strip carrying load current. - **Magnetic element**: coil with a plunger (armature). - **Arc chute**: stack of steel splitter plates that divides and cools the arc. #### Operation 1. **Overload**: current above rating heats the bimetal strip; it bends with time (inverse-time characteristic) and releases the latch, opening the contacts. 2. **Short circuit**: high current creates a strong magnetic field in the coil; the plunger strikes the latch and opens the contacts in a few milliseconds. 3. The arc formed between the contacts is driven into the arc chute, split into small arcs, cooled and extinguished. 4. Tripping curves: **B** (trips at 3–5 In, lighting/resistive), **C** (5–10 In, general and small motors), **D** (10–20 In, high inrush loads like transformers and motors). 5. Typical ratings: 0.5–125 A, breaking capacity 6 kA or 10 kA. #### SP, DP and TP MCBs | Type | Poles | Use | |---|---|---| | SP (single pole) | Switches phase only | Single-phase final circuits: lighting, fans, sockets in an SDB | | SP+N | Phase protected, neutral switched | Single-phase circuits where neutral isolation is needed | | DP (double pole) | Phase and neutral switched together | Main switch of single-phase DB, geysers, ACs, outdoor circuits | | TP (triple pole) | Three phases, common trip bar | Three-phase motors, 3-phase sub-mains and SDB incomer | | TPN / 4P | Three phases + neutral | Incomer of 3-phase 4-wire DBs | In DP and TP MCBs, the poles are mechanically linked by a common trip bar, so a fault on any one pole trips all poles. This avoids single phasing of 3-phase motors and ensures full isolation. **Advantages**: resettable (no fuse renewal), quick and reliable tripping, indicates faulty circuit, can be used as a switch, tamper-proof, compact. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a trip-free mechanism mean? A. It trips without any fault B. The MCB trips even if the toggle is held in the ON position C. It cannot be reset D. It trips only manually Answer: B. This prevents defeating protection by holding the knob. (Key point: Trip-free mechanism) 2. Why are the poles of DP and TP MCBs linked by a common trip bar? A. So each pole can be reset separately B. To make the MCB thermal only C. A fault on any one pole trips all poles, avoiding single phasing and ensuring isolation D. To increase breaking capacity Answer: C. Linked poles protect three-phase motors from running on two phases. (Key point: Linked poles) 3. Which curve fits high inrush loads like transformers and motors? A. B curve (3-5 In) B. C curve (1-2 In) C. B curve (10-20 In) D. D curve (10-20 In) Answer: D. B suits lighting, C general and small motors, D high inrush. (Key point: Trip curves) 4. Where is a TP or 4P MCB typically used? A. Three-phase motors and sub-mains; TPN/4P as incomer of 3-phase 4-wire DBs B. Single-phase lighting circuits C. Switching only the neutral of a socket D. Single-phase fans in an SDB Answer: A. SP is for single-phase final circuits; TP/TPN for three-phase. (Key point: Choosing pole type) ## Related in other subjects - [Arc extinction in vacuum circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-arc-extinction) — Switchgear and Protection, chapter 6 - [Classification of circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-classification) — Switchgear and Protection, chapter 6 - [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 - [Wien bridge oscillator circuit and operation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awien-bridge-draw-explain) — Basic Electronics Engineering, chapter 4, asked 4×