Relay protection basics: overcurrent protection explained

What is relay protection (RPA)

Relay Protection and Automation (RPA) is a set of devices that detect faults in the electrical network (short circuits, overloads, earth faults) and disconnect the damaged section, preventing the accident from spreading.

Without RPA, any short circuit would cause a substation fire, transformer failure, and a district-wide blackout for weeks. Relay protection operates in 0.02–0.5 seconds and saves equipment worth millions.

Main types of relay protection

OCP — Overcurrent Protection (МТЗ): trips when line current exceeds a set threshold for a set time. Protects against overloads and distant faults.

IOC — Instantaneous Overcurrent (ТО): trips instantly at very high current — typically for faults close to the source.

Gas protection (Buchholz relay): for oil-filled transformers. Reacts to gas released from oil during internal faults.

Differential protection: compares current at the input and output of the protected element. If the difference exceeds the setting — internal fault. Used for transformers, generators, busbars.

Earth fault protection: responds to zero-sequence current. Critical for isolated-neutral networks (6–35 kV in Russia and India).

Calculation example: OCP & IOC for a 10 kV cable line

Given:

10 kV cable line, 2.5 km, cable 3×120 mm² Al

Maximum operating current: Iop.max = 180 A

3-phase S/C current at line end: Isc.min = 3200 A

S/C current at line start: Isc.max = 8500 A

CT: 300/5 (ratio Kct = 60)

Step 1: OCP pickup current

Formula: Ipu = (Krel × Kss × Iop.max) / Krt

Where:

Krel = 1.2 — reliability coefficient

Kss = 1.3 — self-start coefficient (for industrial load)

Krt = 0.85 — relay return coefficient

Ipu = (1.2 × 1.3 × 180) / 0.85 = 330 A

Step 2: OCP sensitivity check

Sensitivity: Ks = Isc.min / Ipu = 3200 / 330 ≈ 9.7

Required Ks ≥ 1.5 for the main zone. 9.7 — excellent, protection reliably detects faults at line end.

Step 3: IOC pickup current

IOC is set above maximum fault current at line end: Ipu.ioc = Krel × Isc.end.max

Krel = 1.3: Ipu.ioc = 1.3 × 3200 ≈ 4160 A

Step 4: IOC sensitivity check

Ks.ioc = Isc.max / Ipu.ioc = 8500 / 4160 ≈ 2.04

Required Ks ≥ 1.2 for IOC. 2.04 > 1.2 — passes.

Step 5: Secondary-side settings

OCP: Isec = 330 / 60 = 5.5 A

IOC: Isec.ioc = 4160 / 60 = 69.3 A

Standards reference

IEC 60255 — measuring relays and protection equipment

IEEE C37.112 — inverse-time characteristic curves

IS 3842 — Indian standard for relay protection

Common mistakes

Underestimated Ks — the most dangerous mistake. If Ks < 1.3, protection may fail during a real fault.

Wrong Kss — for purely cable lines without motors, Kss = 1.0, not 1.3.

CT connection scheme — star vs delta connection changes secondary current by √3.

Ignoring fault arc resistance, especially for 0.4 kV lines.

How GorkyCAD helps

GorkyCAD calculates S/C currents automatically and suggests recommended OCP/IOC settings. The engineer reviews and approves — eliminating arithmetic error risk from manual calculation.