---
title: "Characteristics of a safety ground"
subject: "Instrumentation II"
chapter: 6
key: "semester_5/instrumentation_ii:safety-ground"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/safety-ground
site: BCT Engineering
---

# Characteristics of a safety ground

Instrumentation II · Chapter 6: Grounding And Shielding

- Asked in: 2071 Chaitra, 2 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/safety-ground
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6

## Question

What are the characteristics of a safety ground?

## Short answer

A safety ground is a low-impedance (<1 Ω) path connecting exposed metal parts to earth, carrying no current normally but enough fault current to trip protection. It must be reliable, continuous, with adequate ampacity, and isolated from signal returns.

## Answer

A **safety ground** is a path that carries fault current to earth to protect people and equipment from shock. Its characteristics:

1. **Very low impedance** (resistance, typically below 1 ohm) so that a fault current is large enough to trip the fuse/breaker quickly.
2. **Connected to earth** and to exposed metal parts (chassis, enclosure) of the equipment, normally via the green/yellow earth wire.
3. **Carries no current in normal operation**; it conducts only during a fault.
4. **Adequate current capacity** (thick conductor) to carry the maximum fault current without melting.
5. **Continuous and reliable**: no switch or fuse in the path, with secure mechanical connections.
6. Not shared with signal returns, to avoid injecting noise into the circuits.

## Check yourself

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

1. Why should a safety ground have very low impedance?
   A. So it carries signal return current
   B. So it limits fault current to a tiny value
   C. So it isolates the chassis from earth
   D. So fault current is large enough to trip the fuse/breaker quickly
   Answer: D. Low impedance allows a high fault current that operates protection promptly. (Key point: Impedance)

2. How much current does a safety ground carry in normal operation?
   A. The full load current
   B. Signal return current
   C. None; it conducts only during a fault
   D. Half of the load current
   Answer: C. It is a protective path, used only in fault conditions. (Key point: Normal operation)

3. Which is a proper characteristic of a safety ground path?
   A. Fused so it opens on fault
   B. Continuous, with no switch or fuse in the path and adequate conductor size
   C. Shared with signal returns
   D. Thin conductor to limit current
   Answer: B. The path must stay intact and carry the maximum fault current without melting. (Key point: Construction)

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