Low voltage relay protection settings are typically defined using voltage thresholds, plug setting multipliers (PSM), and time setting multipliers (TSM) to ensure selective and reliable tripping.

Key Parameters for Low Voltage Relays

1. Undervoltage (UV) Relays

• Voltage Setting (Vset): Usually set around 0.8 to 0.9 of nominal voltage (Vn) for motor protection, depending on the sensitivity required and the type of load .
• Time Delay: Short delays (e.g., 100–500 ms) are used to avoid nuisance tripping during transient dips. For loss-of-phase protection, faster and lower settings are recommended.
• Application Considerations: UV relays at incomer levels (e.g., 400V or 6.6kV) may coordinate with generator backup or grid supply changeover . 2. Overcurrent Relays (IDMT Relays)
• Plug Setting Multiplier (PSM): Indicates how many times the relay secondary current exceeds the relay pickup current. Higher PSM results in faster tripping .
• Time Setting Multiplier (TSM): Scales the base operating time from the relay's characteristic curve. Lower TSM = faster trip; higher TSM = slower trip for upstream relays to provide backup .
• Coordination: Downstream relays are set with lower TSM for faster operation, while upstream relays have higher TSM to maintain selectivity . 3. Grading and Coordination
• Time Grading: The difference in operating times between consecutive relays ensures selective tripping. Inverse time relays require longer grading times due to measurement inaccuracies .
• Backup Protection: If a downstream relay or breaker fails, the upstream relay operates after the grading time to clear the fault . 4. Current Transformer (CT) Considerations
• CT ratios are selected to match the relay input, typically 5A secondary, e.g., 600:5, ensuring the relay receives the correct signal for accurate operation .
• Multiple relays can share the same CT, but the total burden must not exceed the CT rating .

Practical Example of Settings

• Motor feeder UV relay: Vset = 0.85 Vn, Time delay = 200 ms
• Downstream overcurrent relay: PSM = 1.2–1.5, TSM = 0.8
• Upstream overcurrent relay: PSM = 1.2–1.5, TSM = 1.2–1.5
• Grading time between relays: 0.3–0.5 s for low voltage feeders

Summary

Low voltage relay protection settings are a balance between sensitivity, selectivity, and coordination. Undervoltage relays protect motors and busbars from voltage dips, while overcurrent relays with PSM and TSM ensure selective tripping. Proper grading times and CT selection are essential to avoid nuisance trips and provide reliable backup protection. These settings should always be verified against the network configuration, load characteristics, and applicable standards (IEC 60255 series).