--- title: "Substation protection against direct strokes" subject: "Switchgear and Protection" chapter: 5 key: "semester_6/switchgear_and_protection:substation-direct-stroke-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/substation-direct-stroke-protection site: BCT Engineering --- # Substation protection against direct strokes Switchgear and Protection · Chapter 5: System Earthing - Asked in: 2078 Chaitra, 6 marks - Video: [High Voltage Engineering: Lect-31: Protection of Sub-stations from Direct Strokes](https://www.youtube.com/watch?v=ZhFmE0xVzxU) — Electrical Engineering Lectures-T. G. Arora - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/substation-direct-stroke-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-5 ## Question Describe the protection of stations and sub-stations against direct lightning strokes. ## Short answer Substations are protected from direct strokes by earthing screens (wire mesh over equipment), overhead shield wires extended from lines, and lightning masts, all bonded to a low-resistance earth mat. These intercept strokes and conduct current to earth. Protective zones are verified by angle or rolling-sphere methods. Arresters at line entrances complement this by clamping travelling waves. ## Answer Stations and substations contain costly equipment (transformers, breakers, busbars), so a **direct lightning stroke** must be intercepted before it reaches live parts and conducted to earth through a path of low impedance. The common methods are earthing screens, overhead shield wires and lightning masts, all bonded to the station earth mat. #### 1. Earthing screen ```text ==+=====+=====+=====+== earthed wire mesh | | | | above equipment [T/F] [CB] [Bus] [CT] | | | | ======== earth mat ======== ``` - A network (mesh) of copper or galvanised steel wires is erected over the whole substation on the steel structures. - It is connected at many points to the **low-resistance earth mat** of the substation. - A direct stroke hits the screen instead of the equipment and the current flows to earth. - **Advantages:** simple and cheap; covers the whole yard. - **Limitations:** it does not protect against travelling waves entering through incoming lines; if the earth resistance is high, the potential of the screen rises and can cause back flashover to equipment. #### 2. Overhead ground wires extended over the station - The shield wires of incoming lines are carried over the substation and earthed on the gantries. - Equipment lies within the **protective zone** (shielding angle of about 30°–45° under the wire). #### 3. Lightning masts / spikes - Tall earthed steel masts or spikes on gantries (Franklin rods) placed so that all equipment is within the cone or **rolling-sphere** protection zone. - Each mast has its own earth connection bonded to the earth mat. #### Design points 1. Earth mat resistance should be low (typically below 1 Ω for large substations). 2. Adequate clearance between shield wires/masts and live parts to avoid back flashover. 3. Down conductors short and straight to reduce inductance. 4. Protective zone checked by the protective-angle or rolling-sphere method. Protection against direct strokes is always used with **lightning arresters** at line entrances and transformer terminals, which protect against surges travelling in along the lines. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does an earthing screen protect a substation? A. A mesh of earthed wires over the yard takes the direct stroke and passes it to the earth mat B. It absorbs travelling waves C. It reduces voltage by resistance D. It isolates the busbar Answer: A. The stroke hits the screen instead of equipment and flows to earth. (Key point: Earthing screen principle) 2. What can go wrong if the earth resistance is high? A. Nothing, the screen still works B. The screen potential rises and can cause back flashover to equipment C. The CB opens D. Arresters fail to seal Answer: B. A high-resistance earth lets the screen potential rise during a stroke. (Key point: Screen limitation) 3. How is the protective zone of masts or spikes checked? A. By the rolling-sphere or protective-angle method B. By the resistance of the CB C. By the insulator creepage D. By power factor Answer: A. Equipment must lie within the cone or rolling-sphere zone of the masts. (Key point: Mast protection method) 4. Why are lightning arresters used along with direct stroke protection? A. Direct stroke protection is illegal alone B. Arresters protect against surges travelling in along the lines C. Arresters replace the earth mat D. Arresters stop switching Answer: B. Screens, wires and masts do not stop travelling waves entering at line entrances. (Key point: Arresters still needed) ## Related in Switchgear and Protection - [Lightning protection schemes](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Alightning-protection-schemes) — Switchgear and Protection, chapter 5 - [Causes and protection against overvoltage](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aovervoltage-protection-methods) — Switchgear and Protection, chapter 5, asked 2× - [Earth fault protection: 20 MVA alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-20mva-5-ohm) — Switchgear and Protection, chapter 7 ## Related in other subjects - [Direct lightning stroke overvoltage and tower footing](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Adirect-stroke-overvoltage-footing) — High Voltage Engineering, chapter 3 - [Direct methods of sub-surface investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Adirect-subsurface-methods) — Engineering Geology II, chapter 4, asked 3× - [Protection against ESD](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aesd-protect) — Instrumentation II, chapter 6, asked 4× - [Importance of distribution substation with SLD](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Adistribution-substation-importance-sld) — Industrial Power Distribution and Illumination, chapter 4