--- title: "Leakage current and catheterization protection" subject: "Biomedical Instrumentation" chapter: 8 key: "semester_7/biomedical_instrumentation:leakage-current-catheterization-grounding" source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/leakage-current-catheterization-grounding site: BCT Engineering --- # Leakage current and catheterization protection Biomedical Instrumentation · Chapter 8: Electrical Safety of Medical Equipment - Asked in: 2075 Chaitra, 8 marks - Video: [Electrical Safety Of Medical Equipment's | Biomedical Engineers TV |](https://www.youtube.com/watch?v=ko-zuQ3-hHY) — Biomedical Engineers TV - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/leakage-current-catheterization-grounding - Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-8 ## Question What is leakage currents? How a Heart patient who is under catheterization procedure is protected from the leakage current? Assume the values of leakage current and patient resistance with suitable ground wire for proper grounding. ## Short answer Leakage current is the unwanted small current flowing through insulation and stray capacitance to the chassis or ground. For a catheterized heart patient, protect by low-resistance grounding, equipotential bonding, and isolation so leakage bypasses the patient. With 100 µA leakage, 0.1 Ω ground wire, and 1 kΩ patient, patient current is 10 nA, safe. ## Answer **Leakage current** is the unwanted small current that flows from the live parts of an electrical device through its insulation and stray capacitance to the metal chassis (case), and then to the ground, or through a person touching it. Even with no fault, 50/60 Hz mains produces tens of µA through stray capacitance between the power wires and the chassis. Types: enclosure (chassis) leakage, patient leakage, and patient auxiliary leakage. **Limits:** for general patient equipment, 100 µA (chassis); for devices with direct cardiac connection (catheter, pacing wire), the limit is **10 µA**, because microshock of about 100 µA through the heart can cause ventricular fibrillation. #### Protection of a catheterised heart patient 1. **Grounding of the chassis through a low-resistance ground wire** (three-wire system) so the leakage current flows to the ground and not through the patient. 2. **Equipotential grounding:** all metal surfaces near the patient (bed, instruments, pipes) are connected by heavy wires to a common ground point, so there is no potential difference between them. 3. **Isolation transformer / isolated power** (with a line isolation monitor), so the first fault does not cause shock. 4. **Isolated (floating) patient connections:** isolation amplifiers (transformer or optical) with very small coupling capacitance. 5. **Double insulation** and battery-operated equipment; GFCI/earth leakage breaker; regular testing of leakage. 6. Catheter ends insulated, staff handling with dry gloves, and only one device connected to the catheter at a time. #### Example calculation (assumed values) ```text Assume: chassis leakage current Ileak = 100 uA ground wire resistance Rg = 0.1 ohm patient (catheter-to-ground body path) Rp = 1 kohm ``` With the ground wire intact, the chassis voltage is Vc = Ileak × Rg = 100 µA × 0.1 Ω = **10 µV**. The current through the patient is Ip = Vc/Rp = 10 µV/1 kΩ = **10 nA**, far below the 10 µA limit; so almost all the leakage current goes to the ground wire. If the ground wire breaks (Rg → ∞), the whole 100 µA can flow through the patient: Ip ≈ 100 µA, which is 10 times the safe limit and may produce fibrillation. Hence a good ground wire (Rg < 0.1 Ω, thick copper), equipotential grounding and periodic checks give the necessary protection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is leakage current? A. The deliberate current delivered to the patient B. Unwanted small current flowing through insulation and stray capacitance from live parts to the chassis, ground or a person C. The current drawn by the transformer D. The current in the ground wire only when it breaks Answer: B. Even in a healthy device, stray capacitance causes tens of µA to the chassis. (Key point: Leakage current) 2. What is the leakage limit for equipment with direct cardiac connection, and why? A. 100 µA, because macroshock needs 100 µA B. 10 µA, because microshock of about 100 µA through the heart can cause VF C. 1 mA, because skin is dry D. 10 mA, because the heart tolerates it Answer: B. Direct cardiac connections bypass skin resistance, so the limit is 10 µA. (Key point: Limit for cardiac devices) 3. What is the purpose of equipotential grounding? A. To isolate the power supply B. To connect all metal surfaces near the patient to a common ground so there is no potential difference between them C. To increase the leakage current D. To replace the ground wire Answer: B. With no potential difference between nearby conductors, no current flows through the patient. (Key point: Equipotential grounding) 4. With 100 µA leakage, Rg = 0.1 Ω and Rp = 1 kΩ, what is the patient current with intact ground wire? A. 100 µA B. 10 µA C. 10 nA D. 10 mA Answer: C. Vc = 100 µA × 0.1 Ω = 10 µV; Ip = 10 µV/1 kΩ = 10 nA, far below 10 µA. 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