---
title: "Relay 1 time and relay 2 TMS (3000 A)"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:idmt-300-1-3000a"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-300-1-3000a
site: BCT Engineering
---

# Relay 1 time and relay 2 TMS (3000 A)

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2075 Baisakh, 6 marks
- Video: [PSM and TMS Numericals](https://www.youtube.com/watch?v=1Za43V_-9tM) — Basic Electrical
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-300-1-3000a
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

Calculate the time of operation of relay 1 and time setting multiplier for relay 2 for the conditions given below: (i) Time grading margin between the relays is 0.5 sec (ii) The time setting multiplier of relay 1 is 0.4 (iii) CT ratio of both CTs is 300/1 (iv) The fault occurs at point F and fault current is 3000 A (v) Time current characteristic of both relays is given in table.
[Figure: CB – CT with Relay 2 (plug setting = 150%) – CB – CT with Relay 1 (plug setting = 125%) – fault point F.]

| PSM | 2 | 3.6 | 5 | 6.67 | 8 | 10 | 15 | 20 |
|---|---|---|---|---|---|---|---|---|
| Time in sec | 10 | 6 | 3.9 | 3.5 | 3.15 | 2.8 | 2.2 | 2.1 |

## Short answer

For relay 1: pickup 1.25×300=375 A, PSM=3000/375=8, t at TMS=1=3.15 s, operating time=0.4×3.15=1.26 s. For relay 2: pickup 1.5×300=450 A, PSM=6.67, t at TMS=1=3.5 s, required time=1.26+0.5=1.76 s, TMS=1.76/3.5=0.503.

## Answer

Relay 1 (near fault F) is the primary relay; relay 2 backs it up with a 0.5 s margin.

**Data:** CT 300/1 for both, plug setting R1 = 125%, R2 = 150%, TMS of R1 = 0.4, If = 3000 A.

#### Relay 1

```text
Pick-up = 1.25 × 300 = 375 A
PSM = 3000/375 = 8 → t(TMS=1) = 3.15 s
t1 = 0.4 × 3.15 = 1.26 s
```

#### Relay 2

```text
Pick-up = 1.5 × 300 = 450 A
PSM = 3000/450 = 6.67 → t(TMS=1) = 3.5 s
Required t2 = 1.26 + 0.5 = 1.76 s
TMS2 = 1.76 / 3.5 = 0.503
```

| Relay | Pick-up | PSM | t at TMS=1 | TMS | Time |
|---|---|---|---|---|---|
| 1 | 375 A | 8 | 3.15 s | 0.4 | 1.26 s |
| 2 | 450 A | 6.67 | 3.5 s | 0.503 | 1.76 s |

**Answer:** operating time of relay 1 = 1.26 s; TMS of relay 2 ≈ 0.503 (≈ 0.5), giving t2 = 1.76 s.

## Check yourself

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

1. What is the pick-up current of relay 1 (plug setting 125%, CT 300/1)?
   A. 300 A
   B. 375 A
   C. 450 A
   D. 125 A
   Answer: B. Pick-up = 1.25 x 300 = 375 A (primary). (Key point: Pick-up current)

2. What is the plug setting multiplier of relay 1 for a 3000 A fault?
   A. 8
   B. 6.67
   C. 10
   D. 2
   Answer: A. PSM = 3000/375 = 8, which gives 3.15 s on the table at TMS = 1. (Key point: PSM of relay 1)

3. With TMS = 0.4, what is the operating time of relay 1?
   A. 3.15 s
   B. 1.26 s
   C. 0.4 s
   D. 1.76 s
   Answer: B. t1 = TMS x time at TMS 1 = 0.4 x 3.15 = 1.26 s. (Key point: Operating time of relay 1)

4. What is the approximate TMS of relay 2 (PSM 6.67, 3.5 s at TMS 1, margin 0.5 s)?
   A. 0.36
   B. 0.503
   C. 1.0
   D. 0.4
   Answer: B. t2 = 1.26 + 0.5 = 1.76 s, so TMS2 = 1.76/3.5 = 0.503. (Key point: TMS of relay 2)

## Related in Switchgear and Protection

- [Operating time of two relays (250/1, 3000 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-250-1-3000a) — Switchgear and Protection, chapter 7
- [IDMT grading: R1, R2 with 1000/5 CTs](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-r1-r2-1000-5) — Switchgear and Protection, chapter 7, asked 2×
- [Operating time of two relays (300/1, 6000 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-300-1-6000a) — Switchgear and Protection, chapter 7
- [Time-current grading for relays X, Y, Z](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-grading-xyz) — Switchgear and Protection, chapter 7
- [Design IDMT grading for relays A and B](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aradial-feeder-idmt-design-ab) — Switchgear and Protection, chapter 7
- [Minimum TMS of relay A (16000 A fault)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aminimum-tms-relay-a) — Switchgear and Protection, chapter 7

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