---
title: "Protection against ESD"
subject: "Instrumentation II"
chapter: 6
key: "semester_5/instrumentation_ii:esd-protect"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/esd-protect
site: BCT Engineering
---

# Protection against ESD

Instrumentation II · Chapter 6: Grounding And Shielding

- Asked in: 2073 Chaitra, 2 marks; 2068 Chaitra, 2 marks; 2075 Chaitra; 2068 Baisakh (old course), 4 marks (asked 4 times)
- Video: [ESD Essentials: How to Select ESD Protection](https://www.youtube.com/watch?v=eiK79hCFff0) — Texas Instruments
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/esd-protect
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6

## Question

How can a circuit be protected against electrostatic discharge (ESD)?

## Short answer

Protection involves prevention (wrist straps, humidity control), diversion via grounded metal enclosures and guard rings, and clamping at I/O pins with TVS diodes, series resistors, and capacitors to shunt the discharge energy to chassis ground.

## Answer

**Electrostatic discharge (ESD)** is a sudden flow of charge between two bodies at different potentials, caused by contact or by an electric field. It can reach several kV with nanosecond rise time and damage ICs (gate-oxide breakdown, junction burn-out). Protection is applied at three levels: prevention at the source, shielding/diversion, and circuit protection.

#### Prevent charge build-up
- Use anti-static mats, wrist straps and grounded work tables during handling and assembly.
- Keep humidity at 40-60 percent; use anti-static packaging (conductive bags, foam).
- Avoid synthetic clothing and carpets near sensitive equipment; use ionisers.

#### Divert the discharge
- Put the circuit in a **grounded metal enclosure** so the discharge flows to ground, not through the electronics.
- Keep gaps and seams small; connect connector shells to chassis ground, not to signal ground.
- Use a **guard ring/ground plane** around I/O connectors and keep traces short.

#### Protect the circuit
- Add **TVS diodes (transient voltage suppressors)** or zener diodes at every external input/output to clamp the voltage.
- Use series resistors with a shunt capacitor (RC filter) or a ferrite bead to slow the pulse.
- Use varistors (MOV) or gas discharge tubes for larger surges.
- Use opto-isolators or transformers where galvanic isolation is needed.

```text
 Connector pin --[R]--+--> to IC input
                      |
                  TVS / diode
                      |
                    GND (chassis)
```

The TVS conducts only when the voltage exceeds its breakdown value, sending the energy to ground.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is electrostatic discharge?
   A. A slow leakage current
   B. A sudden flow of charge between bodies at different potentials, with kV and ns rise time
   C. A steady dc supply
   D. A magnetic effect
   Answer: B. It can break down gate oxides and burn junctions. (Key point: ESD)

2. Which is a source-level prevention?
   A. Larger supply capacitors
   B. Using synthetic carpets
   C. Wrist straps, anti-static mats and 40-60% humidity
   D. Dry air below 10% humidity
   Answer: C. These stop charge build-up. (Key point: Prevention)

3. How does a grounded metal enclosure protect a circuit?
   A. It raises the voltage
   B. It absorbs magnetic field only
   C. It stores charge
   D. It diverts the discharge to ground instead of through the electronics
   Answer: D. Connector shells should go to chassis ground. (Key point: Diverting)

4. How does a TVS diode at an input protect the IC?
   A. Conducts only above its breakdown voltage, sending energy to ground
   B. Conducts at all voltages
   C. Blocks the signal permanently
   D. Increases input impedance only
   Answer: A. It clamps the voltage. (Key point: TVS)

## Related in other subjects

- [Substation protection against direct strokes](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asubstation-direct-stroke-protection) — Switchgear and Protection, chapter 5
- [Merz-Price: protected winding (13.8 kV)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amerz-price-13-8kv-protected) — Switchgear and Protection, chapter 7, asked 2×
- [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×
- [Snubber circuit and dv/dt protection](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asnubber-dvdt-protection) — Power Electronics, chapter 1, asked 2×