# Why Protection Relays and LSIG release same LV side?

**URL:** <https://forumautomation.com/t/why-protection-relays-and-lsig-release-same-lv-side/13599>\
**Category:** Electrical\
**Tags:** protection\
**Created:** [October 3, 2025, 2:11pm UTC](https://forumautomation.com/t/why-protection-relays-and-lsig-release-same-lv-side/13599 "2025-10-03T14:11:20Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [October 3, 2025, 2:11pm UTC](https://forumautomation.com/t/why-protection-relays-and-lsig-release-same-lv-side/13599/1 "2025-10-03T14:11:21Z")

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## Why Protection Relays and LSIG release same LV side?

On the LV (Low Voltage) side of transformers (or) switchgear, protection relays and the ACB/MCCB’s LSIG (Long, Short, Instantaneous, Ground) release are employed to protect against electrical faults.

## Why are they on the same LV side?

Faults like as overload, short-circuit & earth fault are most common on the **[LV](https://forumelectrical.com/what-is-lv-winding-why-lv-winding-is-placed-near-to-the-core/)** side, where current is high.

The LSIG release (built-in trip unit for breakers) provides rudimentary protection against these faults.

 ![image](https://cdn.forumautomation.com/original/2X/5/52fa9ac8242c66cb5f2a91cb1529d705889e6c1f.jpeg)

Protection **[relays](https://forumelectrical.com/how-to-test-a-relay-checking-ssr-coil-relays/)** (such as differential, REF, overcurrent, and earth fault relays) offer sophisticated, selective & backup protection with additional parameters, logic, and coordination.

Both collaborate to rapidly detect and isolate LV faults, assuring transformer, busbar & feeder safety.

LSIG provides primary breaker protection whereas relays provide selected system-level protection. Both are necessary on the LV side wherein most protection activities are required.

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