---
title: "Operating time of two relays (300/1, 6000 A)"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:idmt-300-1-6000a"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-300-1-6000a
site: BCT Engineering
---

# Operating time of two relays (300/1, 6000 A)

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2078 Chaitra, 8 marks
- Video: [Over current relay solved numerical problem](https://www.youtube.com/watch?v=LUQuUmMAERU) — Techspecial Information
- 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-300-1-6000a
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

The time multiplier setting of relay 1 is 0.2 and the time grading margin between the relays is 0.3 second. Determine the time of operation of two relays.
[Figure: CB – CT 300/1 with Relay 2 (plug setting = 150%) – CB – CT 300/1 with Relay 1 (plug setting = 125%) – fault, IF = 6000 A.]

| PSM | 2 | 3.6 | 5 | 9.23 | 13.33 | 16 | 20 |
|---|---|---|---|---|---|---|---|
| Time in sec | 10 | 6 | 3.9 | 2.9 | 2.8 | 2.2 | 2.1 |

## Short answer

Relay 1: pickup 375 A, PSM=6000/375=16, t(TMS=1)=2.2 s, t1=0.2×2.2=0.44 s. Relay 2: pickup 450 A, PSM=6000/450=13.33, t(TMS=1)=2.8 s, required t2=0.44+0.3=0.74 s, TMS2=0.74/2.8=0.264.

## Answer

Relay 1 (nearest the fault) operates first; relay 2 is the back-up and must operate 0.3 s after relay 1.

**Data:** CT 300/1 for both, plug setting R1 = 125%, R2 = 150%, TMS of R1 = 0.2, IF = 6000 A.

#### Relay 1

```text
Pick-up current = 1.25 × 300 = 375 A
PSM = 6000/375 = 16
t(TMS=1) = 2.2 s (from table)
t1 = 0.2 × 2.2 = 0.44 s
```

#### Relay 2

```text
Pick-up current = 1.5 × 300 = 450 A
PSM = 6000/450 = 13.33
t(TMS=1) = 2.8 s (from table)
t2 = t1 + margin = 0.44 + 0.3 = 0.74 s
TMS2 = 0.74 / 2.8 = 0.264
```

| Relay | Pick-up | PSM | t at TMS=1 | TMS | Operating time |
|---|---|---|---|---|---|
| 1 | 375 A | 16 | 2.2 s | 0.2 | 0.44 s |
| 2 | 450 A | 13.33 | 2.8 s | 0.264 | 0.74 s |

**Answer:** relay 1 operates in 0.44 s and relay 2 in 0.74 s (TMS of relay 2 ≈ 0.264).

## Check yourself

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

1. Relay 1 (125%, CT 300/1) sees a 6000 A fault. Its PSM is:
   A. 13.33
   B. 20
   C. 16
   D. 8
   Answer: C. Pick-up is 375 A, so PSM = 6000/375 = 16. (Key point: PSM of relay 1)

2. At PSM 16 the table gives 2.2 s. With TMS = 0.2 the relay 1 time is:
   A. 2.2 s
   B. 0.44 s
   C. 0.2 s
   D. 11 s
   Answer: B. t1 = 0.2 x 2.2 = 0.44 s. (Key point: Operating time of relay 1)

3. Relay 2 has a 150% plug setting on CT 300/1. Its PSM at 6000 A is:
   A. 16
   B. 13.33
   C. 10
   D. 6.67
   Answer: B. Pick-up = 450 A, so PSM = 6000/450 = 13.33 (2.8 s at TMS 1). (Key point: PSM of relay 2)

4. How is the TMS of relay 2 found?
   A. TMS2 = (0.44 + 0.3)/2.8 = 0.264
   B. TMS2 = 0.3/2.8 = 0.107
   C. TMS2 = 0.44/2.8 = 0.157
   D. TMS2 = 0.2 because both relays are the same
   Answer: A. Relay 2 must take t1 + margin = 0.74 s, so TMS2 = 0.74/2.8 = 0.264. (Key point: Grading to find TMS2)

## 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
- [Relay 1 time and relay 2 TMS (3000 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-300-1-3000a) — Switchgear and Protection, chapter 7
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- [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