What size switch should be used for relay protection

Adjusting a relay protection switch involves configuring key parameters such as the Plug Setting Multiplier (PSM) and Time Setting Multiplier (TSM) to ensure proper current and time coordination for fault protection. Key Parameters for Adjustment1. Plug Setting Multiplier (PSM) PSM determines how many times the actual current exceeds the relay's pickup current.

It is crucial for overcurrent relays, especially IDMT (Inverse Definite Minimum Time) types, as it directly affects the relay's operating speed. A higher PSM results in faster tripping, while a lower PSM delays operation.

PS

M settings must comply with IEC 60255-151 standards to ensure reliable operation under fault conditions and proper coordination with other relays. 2. Time Setting Multiplier (TSM) TSM scales the base operating time derived from the relay's characteristic curve.

It allows coordination between upstream and downstream relays, ensuring that the downstream relay trips first while the upstream relay provides backup.

Lower TS

M values result in faster tripping, and higher values increase delay.

TS

M selection should follow IEC 60255-3 guidelines for inverse, very inverse, and extremely inverse characteristics. 3. Overload (OL) and Earth Leakage (EL) SettingsOL (Overload Setting): Adjusts the thermal overload protection to prevent damage from sustained overcurrent.

E

L (Earth Leakage / Earth Fault Setting): Sets the pickup current for detecting ground faults. Proper adjustment ensures selective tripping without nuisance operations. 4.

Multiplying Factor (MF) MF is used for scaling metering or CT secondary currents to match the relay's input.

Correct M

F ensures accurate relay response and coordination with system measurements.

Practical Steps for Adjustment

Determine System Parameters: Identify the maximum load current, fault current levels, and feeder ampacity. Select Relay Type and Characteristic: Choose the appropriate relay (IDMT, instantaneous, directional, etc.) based on system requirements.

Set PS

M: Calculate the pickup current based on CT ratios and desired fault sensitivity.

Adjust PS

M to achieve the required operating speed.

Set TS

M: Apply time grading to coordinate with upstream and downstream relays, ensuring backup protection.

Adjust OL and E

L: Configure overload and earth fault settings according to equipment ratings and protection requirements. Verify Coordination: Plot the relay curves on a Time-Current Characteristic (TCC) diagram to ensure proper coordination with other protective devices. Test and Commission: Perform secondary injection tests or use relay testing software to validate settings before energizing the system.

Safety Considerations

Always de-energize circuits before adjusting relay settings. Follow manufacturer manuals and IEC standards for safe handling and configuration. Avoid touching internal components of electronic relays to prevent electrostatic damage.

By carefully adjusting these parameters, the relay protection switch can be sized and configured to operate reliably, discriminate between faults, and protect the electrical system effectively.