---
title: "Knee point voltage; measuring vs protective CT"
subject: "Switchgear and Protection"
chapter: 2
key: "semester_6/switchgear_and_protection:knee-point-measuring-protective-ct"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/knee-point-measuring-protective-ct
site: BCT Engineering
---

# Knee point voltage; measuring vs protective CT

Switchgear and Protection · Chapter 2: Current and Potential Transformers

- Asked in: 2070 Bhadra, 4 marks
- Video: [Why Measurement Current Transformer (CT) can't be used as Protection CT ?](https://www.youtube.com/watch?v=gLcZ370UyLk) — Er. Vivek Choubey
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/knee-point-measuring-protective-ct
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2

## Question

What is 'knee point voltage'? Mention the difference between measuring and protective CT.

## Short answer

Knee point voltage is where a 10% rise in secondary emf causes 50% rise in exciting current, indicating saturation. Meeasuring CTs have low knee point, saturate early to protect meters; protective CTs have high knee point to avoid saturation during faults, ensuring accurate relay current.

## Answer

#### Knee point voltage
The **knee point voltage** of a CT is the point on its excitation (magnetisation) curve where a 10 % increase in secondary e.m.f. causes a 50 % increase in exciting current (IEC/IS definition). Beyond this point the core saturates, the exciting current rises rapidly and the CT can no longer reproduce the primary current faithfully.

```text
 Vs (sec. emf)
  │            ___________ saturation
  │        _--
  │   Vk ●   knee point
  │     /
  │    /  linear region
  │   /
  └──┴────────────────── Ie (exciting current)
```
A protective CT needs a high knee point voltage so that it does not saturate during heavy fault current; it is chosen so that Vk ≥ K × If(sec) × (R_CT + R_lead + R_relay).

#### Measuring CT vs protective CT
| Point | Measuring CT | Protective CT |
|---|---|---|
| Purpose | Feeds meters (ammeter, energy meter) | Feeds relays |
| Working range | Up to about 120 % of rated current | Up to 10–30 times rated current |
| Accuracy needed | High at normal current (class 0.1, 0.2, 0.5, 1) | Moderate, maintained up to fault current (5P, 10P, PS) |
| Knee point | Low – saturates early | High – no saturation during faults |
| Core | Small, high-permeability (nickel-iron) | Large cross-section, CRGO steel |
| Saturation | Desirable: protects meters during faults | Undesirable: relay must see the fault |
| Instrument security factor | Low (ISF ≤ 5) | Accuracy limit factor 10, 15, 20, 30 |

## Check yourself

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

1. How is knee point voltage defined?
   A. 10% rise in secondary e.m.f. causes 50% rise in exciting current
   B. 50% rise in voltage causes 10% rise in current
   C. 10% rise in voltage causes 10% rise in current
   D. Where exciting current is zero
   Answer: A. This marks the start of core saturation. (Key point: Knee point definition)

2. What happens beyond the knee point?
   A. CT becomes more accurate
   B. Secondary voltage drops to zero
   C. Core saturates and the CT can no longer reproduce primary current faithfully
   D. Ratio becomes exact
   Answer: C. Exciting current rises rapidly. (Key point: Beyond knee)

3. Why does a measuring CT have a low knee point?
   A. To feed relays accurately at 30 times rated current
   B. It saturates early at fault, protecting meters
   C. To reduce its accuracy at normal current
   D. Because it has a large core
   Answer: B. Saturation at fault is desirable for meters. (Key point: Measuring CT knee)

4. What is the working range of a protective CT?
   A. Up to 120% rated current
   B. Up to 5% rated current
   C. Only no-load
   D. 10-30 times rated current
   Answer: D. Measuring CTs work up to about 120%. (Key point: Protective CT range)

## Related in Switchgear and Protection

- [Knee point voltage and CT selection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-selection) — Switchgear and Protection, chapter 2
- [Knee point voltage and CT errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-errors) — Switchgear and Protection, chapter 2
- [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1

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