---
title: "Merz-Price CT ratio for 66000/6600 V"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:merz-price-66kv-ct-ratio"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/merz-price-66kv-ct-ratio
site: BCT Engineering
---

# Merz-Price CT ratio for 66000/6600 V

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2077 Chaitra, 4 marks
- Video: [CURRENT TRANSFORMER  TURNS RATIO CALCULATION  || MERZ PRICE DIFFERENTIAL PROTECTION](https://www.youtube.com/watch?v=KNFVWeYsUE4) — ELECTRICAL NEST
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/merz-price-66kv-ct-ratio
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

A 3 phase 66000/6600 V star/delta transformer is protected by Merz-Price system. What will be the ratio of current transformer on the high voltage side if the current transformers on the low voltage side have a ratio of 400/5 A?

## Short answer

For the 66/6.6 kV star/delta transformer, LV CTs 400/5 in star give 5 A pilot current. HV line current is 400 x 6.6/66 = 40 A; HV CTs in delta need secondary 5/√3 A. HV CT ratio is 40 : 5/√3 ≈ 69.3 : 5.

## Answer

In the Merz-Price (circulating current) scheme for a star/delta transformer, the HV (star) side CTs are connected in **delta** and the LV (delta) side CTs in **star**, so that the pilot currents from both sides are equal and in phase.

**Data:** 66 000/6600 V, star/delta; LV CTs 400/5.

```text
LV line current = 400 A → LV CT secondary (pilot) = 5 A

HV line current = 400 × 6600/66 000 = 40 A

HV CTs in delta: pilot current = √3 × CT secondary
CT secondary = 5/√3 = 2.887 A

HV CT ratio = 40 : 5/√3 = 40 : 2.887
            = 69.28 : 5   (≈ 13.86 : 1)
```

**Answer:** HV CT ratio = **40 : 5/√3 ≈ 69.3 : 5**, with HV CTs connected in delta and LV CTs in star.

## Check yourself

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

1. In the Merz-Price scheme for a 66/6.6 kV star/delta transformer, the LV (delta) side CTs are connected in:
   A. Delta
   B. Star
   C. Open delta
   D. Zigzag
   Answer: B. The delta winding side gets star-connected CTs, and the star side gets delta-connected CTs. (Key point: Merz-Price CT connections)

2. LV CTs are 400/5. For 400 A LV line current, what is the HV line current?
   A. 4000 A
   B. 400 A
   C. 69.3 A
   D. 40 A
   Answer: D. I_HV = 400 × 6600/66000 = 40 A. (Key point: HV line current)

3. What is the HV CT ratio?
   A. 40 : 5
   B. 40 : 5/sqrt(3), about 69.3 : 5
   C. 40 : 5 × sqrt(3)
   D. 13.86 : 5
   Answer: B. Delta-connected CTs give a 2.887 A secondary for 5 A pilot current, so 40 : 2.887 = 69.3 : 5. (Key point: HV CT ratio)

## Related in Switchgear and Protection

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