Schweitzer Engineering Laboratories SEL-251 - Ground;Residual Overcurrent Elements for Ground Faults - See Figure 2.20; Voltage Element for Healthy;Low Voltage Indication or Internal Control (27) - See Figure 2.21; Time Delayed 52 A or 52 B Functions Handle Fuse-Saving and Inrush

2-8 Specifications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
Ground/Residual Overcurrent Elements for Ground Faults - See Figure 2.20
51NT Ground/Residual Time-Overcurrent Element
• Curve families: moderately inverse, inverse, very inverse, extremely inverse
• Time dial: 0.5 to 15.00 in 0.01 steps
• Pickup (51NP): 0.25 to 12 A secondary
• Time delay or 1-cycle reset time
• Timing: ±5% and ±1 cycle for currents between 2 and 20 multiples of pickup
• Externally torque controllable
50NLT Ground/Residual Definite-Time Overcurrent Element
• Pickup (50NL): 0.5 to 100 A secondary (for 1 £ 51NP £ 12 A secondary)
0.25 to 50 A secondary (for 0.5 £ 51NP < 1 A secondary)
0.125 to 25 A secondary (for 0.25 £ 51NP < 0.5 A secondary)
• Time delay: 0 to 16,000 cycles in 1-cycle steps
• Externally torque controllable
50NH Ground/Residual Instantaneous Overcurrent Element
• Pickup: same range as 50NLT
• Externally torque controllable
Accuracy
• Residual element pickup accuracy is dependent upon the 51NP setting. Pickup
accuracy of the 51NP, 50NL, and 50NH elements is shown below in the given
51NP setting range.
1.0 £ 51NP £12.0 A sec Pickup ±2% ±0.100 A sec
0.5 £ 51NP < 1.0 A sec Pickup ±2% ±0.050 A sec
0.25 £ 51NP < 0.5 A sec Pickup ±2% ±0.025 A sec
Voltage Element for Healthy/Low Voltage Indication or Internal Control (27) - See
Figure 2.21
• 27AB, 27BC, 27CA Phase-to-Phase Voltage Elements
• Setting Range: 0 to 250 V line-to-line secondary ±5%, ±1 V
• Two setting limits: 27H and 27L (high and low, respectively)
• 27 element asserts only if voltage is between 27H and 27L
• User selects either three-phase or phase-to-phase voltage condition
• Implement undervoltage load shedding scheme
• Internally torque control selected phase overcurrent elements
• Detect high-side transformer fuse operations
Time Delayed 52A or 52B Functions Handle Fuse-Saving and Inrush
The time delay pickup and time delay dropout settings (52APU and 52ADO, respectively) are
provided to generate the 52AT and 52BT functions. The 52AT and 52BT bits can be used to
supervise overcurrent elements for fuse saving and inrush conditions.