---
title: "Touching energized microwave oven body"
subject: "Industrial Power Distribution and Illumination"
chapter: 2
key: "semester_6/industrial_power_distribution_and_illumination:microwave-oven-earthed-unearthed"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/microwave-oven-earthed-unearthed
site: BCT Engineering
---

# Touching energized microwave oven body

Industrial Power Distribution and Illumination · Chapter 2: Earthing

- Asked in: 2072 Asoj, 4 marks
- 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/microwave-oven-earthed-unearthed
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-2

## Question

A person comes in contact with the metallic body of an energized microwave oven. Discuss the consequences during unearthed and earthed conditions.

## Short answer

Without earthing, the oven body becomes live at 230 V; a person touching it conducts about 230 mA, causing ventricular fibrillation and death, while the MCB does not trip. With earthing, the fault current flows through the earth wire, tripping the MCB instantly and limiting touch voltage to a safe level, especially if an RCCB is fitted.

## Answer

Assume a 230 V supply whose neutral is earthed at the transformer, and that the live wire inside the oven has touched its metal body because of insulation failure.

```text
 L ─[MCB]──┐
           │ fault
       ┌───┴───┐
       │ oven  │──── earth wire (if present)
       │ body  │        │
       └───┬───┘        │
         person         │
           │            │
 ══════════╧════════════╧══ earth
 N earthed at transformer
```

#### Unearthed condition (no earth wire)
- The oven body rises to about **230 V** with respect to earth.
- The person touching it becomes the only path to earth. With body plus contact resistance of about 1000 Ω, current ≈ 230/1000 = **230 mA**.
- Currents above about 30 mA through the body can cause muscle spasm, inability to let go, and ventricular fibrillation, leading to death.
- The current is too small to blow the fuse or trip the MCB, so the body stays live and the danger continues for anyone who touches it.

#### Earthed condition (body connected to a good earth)
- As soon as the fault occurs, a **large fault current** flows through the low-resistance earth wire back to the transformer neutral (for example, with total loop resistance of about 2 Ω, the current is over 100 A).
- This current **trips the MCB or blows the fuse** within a fraction of a second, so the oven is disconnected, often before anyone touches it.
- If the person touches it during this short time, the body is in parallel with the very low earth path, so only a small current passes through the person. With an **RCCB (30 mA)** the supply is cut in about 30 ms.

**Conclusion:** the unearthed oven is a serious shock hazard; the earthed oven makes the fault safe by clearing it quickly. This is why every appliance with a metal body must use a 3-pin plug with the earth pin connected.

## Check yourself

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

1. A live wire touches the body of an unearthed 230 V oven. What is the body's voltage with respect to earth?
   A. About 0 V
   B. About 230 V
   C. About 23 V
   D. About 400 kV
   Answer: B. With no earth path, the body rises to about the supply voltage. (Key point: Unearthed oven body voltage)

2. Why does the MCB not trip in the unearthed case?
   A. The fault current is large but delayed
   B. The MCB is faulty
   C. The current through the person (about 230 mA) is too small to trip it, so the body stays live
   D. The earth wire absorbs the current
   Answer: C. The only path is through the person, giving a current far below the trip threshold, so the danger continues. (Key point: Why protection does not trip)

3. What happens in the earthed oven when the fault occurs?
   A. A large fault current flows through the earth wire and trips the MCB or fuse quickly
   B. Nothing happens
   C. The person gets all the current
   D. The neutral is disconnected
   Answer: A. The low-resistance loop gives over 100 A in the example, so the device disconnects the oven within a fraction of a second. (Key point: Earthed oven outcome)

## Related in Industrial Power Distribution and Illumination

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