6.6 Overcurrent Relay Testing (50/51): Pickup, Timing, Reset, and Curve Selection

Overcurrent Relay Testing (50/51): Pickup, Timing, Reset, and Curve Selection

Quick Answer: Level II task 2.1e.2 requires "testing of basic devices, including motor overload relays, relay trip devices, over current relays, under/over voltage relays, and ammeters/voltmeters," with the graded skills including "read and interpret time/current characteristic curves" and "operate a protective relay test set." 51 is time overcurrent — pickup plus an intentional inverse delay. 50 is instantaneous — pickup with no intentional delay.

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1. The device numbers and what they do

The 50/51 combination in one relay gives a curve with an inverse portion for coordination with downstream devices and a vertical cutoff for close-in high-magnitude faults.

N versus G is a real distinction. A 51N element derives its current from the residual connection of the three phase CTs, so its accuracy is limited by CT mismatch and it can misoperate on heavy through-fault current when the CTs saturate unequally. A 51G element uses a single window CT encircling all conductors, which measures true residual current directly and can be set far more sensitively — often to a few amperes — because it does not depend on three CTs matching.

2. Pickup testing

Pickup is the current at which the element just begins to operate.

Procedure:

1. Connect the test set to the relay's current input, isolating the relay's trip output from the breaker unless a full functional trip is intended.
2. Apply current below the expected pickup.
3. Ramp slowly upward and record the current at which the element asserts. Ramping too quickly overshoots and reports a high pickup.
4. Ramp back down and record the current at which it drops out.

Dropout / reset ratio is dropout current divided by pickup current, and it is a real diagnostic. A typical electromechanical induction disc relay resets around 90-95 % of pickup. A markedly low reset ratio in an electromechanical relay indicates mechanical drag — dirty bearings, a rubbing disc, a damaged jewel, or a bent contact. Microprocessor relays have programmable reset characteristics.

Tolerance: compare against the manufacturer's published tolerance and the setting sheet. NETA's instruction, as always, is the manufacturer's data first.

3. Timing tests

Timing verifies that the relay follows its published curve.

Standard test points: 2×, 3×, and 5× pickup. Multiple points are needed because a single point can be met by a relay whose curve shape is wrong — for example a relay set to the wrong curve family that happens to cross the correct value at one current.

Procedure at each point:

1. Compute the required current: multiple × pickup × (CT ratio if testing at primary values).
2. Apply the current as a step, not a ramp — the timing clock starts when current is applied.
3. Record the time from application to contact closure.
4. Compare against the published curve at the selected time dial.
5. Allow the relay to fully reset before the next shot.

That last step is not a formality on an electromechanical relay. The induction disc physically rotates from its rest position toward the contact; if the next test begins before the disc has returned home, it starts partway along its travel and reports a falsely short time. Reset time on an induction disc can be several seconds and is itself a published characteristic.

4. Curve families and the time dial

The two effects are independent, and confusing them is a classic error:

• Tap (or pickup setting) selects the current at which the relay starts timing. Changing tap shifts the curve horizontally.
• Time dial (or time multiplier setting) scales the operating time. Changing the time dial shifts the whole curve vertically without changing its shape.

Curve families, standardized in IEEE C37.112 and IEC 60255:

Why extremely inverse coordinates with fuses: a fuse's melting characteristic is also steeply inverse — a thermal I²t behaviour. Matching the relay curve shape to the fuse shape keeps the two curves roughly parallel across the fault current range, preserving a consistent coordination interval instead of having them converge at one end.

Coordination time interval (CTI) between an upstream relay and the device below it is conventionally on the order of 0.2 to 0.4 seconds for microprocessor relays and longer for electromechanical, allowing for downstream breaker clearing time, relay overtravel, and CT and relay tolerances.

5. Instantaneous (50) testing

The 50 element is verified by applying current in steps and finding the level at which it operates. Use short applications — these are high currents and continuous application heats the relay, the test set, and the leads.

Transient overreach is the property to understand: a fault current with a DC offset can cause an instantaneous element to operate below its steady-state setting, because the offset raises the peak the element responds to. Modern relays filter for it; older electromechanical instantaneous units are specified with an overreach figure. This is why a 50 element is typically set above the maximum asymmetrical current the relay should not see — such as transformer inrush or motor starting.

6. Connections and safety

• Never open an energized CT secondary. Short the CT before disconnecting anything in its circuit. An open CT secondary on a loaded primary develops a dangerously high voltage and can destroy the CT.
• Isolate the trip circuit with test switches or shorting blocks before injecting, unless the intent is a full functional trip. An unintended breaker operation during testing is both a safety and an availability event.
• Use the test switch or FT-1 style test block where installed; that is what it is for.
• Record as-found settings before changing anything, and verify as-left settings against the coordination study when finished. As-found data is what tells the owner whether the relay had drifted or had been mis-set.

Exam trap: A question describes a technician who performs three consecutive timing shots on an electromechanical induction disc relay in rapid succession and finds the times growing progressively shorter. The relay is not degrading — the disc has not fully reset between shots and each test starts partway through the disc's travel.