Pick-Up and Drop-Out Testing - the whats, the hows, the whys

In the world of electrical protection, reliability is everything. When a fault occurs, we need relays to act; when things are normal, we need them to stay quiet. This delicate balance is managed through two fundamental concepts: Pick-Up and Drop-Out.

Understanding these thresholds is the first step in ensuring a power system doesn't experience "nuisance tripping" or, worse, a failure to trip during a fire or equipment failure.

What are Pick-Up and Drop-Out?

At its simplest, these terms describe the "On" and "Off" triggers for a protection relay.

• Pick-Up: This is the minimum value (of current, voltage, or frequency) at which a relay begins to operate. Think of it as the "Start" command. Once the input signal crosses this threshold, the relay starts its internal timer or prepares to trip a circuit breaker.

• Drop-Out (Reset): This is the value at which the relay decides the danger has passed and returns to its "shelf" or "normal" state. Crucially, the drop-out value is usually slightly lower than the pick-up value to prevent the relay from "chattering" (rapidly switching on and off) when a signal is hovering right at the limit.

Why is this needed?

Without a defined pick-up and drop-out ratio, electrical systems would be incredibly unstable. If a motor starts up and draws a brief surge of current, you want the relay to "pick up" but perhaps not trip yet. If the current then dips slightly, the "drop-out" ensures the relay resets cleanly rather than getting stuck in an uncertain state.

Typical Settings and Hysteresis

The difference between the Pick-Up and Drop-Out values is known as Hysteresis.

In a standard overcurrent relay, you might see a Drop-Out Ratio of 95%. This means if the relay is set to pick up at 100 Amps, it won't "reset" until the current falls back below 95 Amps.

Setting Type	Typical Logic
Instantaneous	Trips immediately upon Pick-Up.
Time-Delayed	Picks up, waits for a set duration, then trips if the signal hasn't dropped out.
High-Set	Used for massive faults; usually has a very tight Pick-Up/Drop-Out gap.

Testing Methods

To ensure these settings work in the real world, technicians use two primary forms of testing: Primary Injection and Secondary Injection.

1. Secondary Injection Testing

This is the most common method because it is safer and requires smaller equipment.

• How it works: You bypass the high-voltage sensors (Current Transformers or CTs) and inject low-level signals directly into the relay’s terminals.

• Testing Pick-Up: Slowly increase the current from zero until the relay "picks up" (the LED lights up or the contact closes). You record this value and compare it to the settings.

• Testing Drop-Out: Once picked up, slowly decrease the current until the relay resets.

• Why use it? It’s great for verifying the relay's internal logic and calibration without powering down the entire building.

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2. Primary Injection Testing

This is the "gold standard" of testing but is more intensive.

• How it works: You inject high current directly through the primary conductors and the actual Current Transformers (CTs).

• The Process: Since you are pushing hundreds or thousands of Amps, you are testing the entire string: the cables, the CTs, the wiring, and the relay itself.

• Testing Pick-Up/Drop-Out: Similar to secondary testing, the current is ramped up until the system reacts. However, because of the heat generated by high current, this is usually done quickly rather than with a slow ramp.

• Why use it? It proves that the CTs are wired correctly and haven't been installed backward (polarity) or saturated.

  

  Overload pickup indication on the Schneider Electric Micrologic panel

A Quick Comparison

Feature	Secondary Injection	Primary Injection
Equipment Size	Suitcase-sized / Portable Examples: EuroSMC Quasar, Omicron CMC line, Megger Freja	Portable systems like EuroSMC Raptor or CPC100, or bulk and heavy systems like Megger ODEN line
Scope	Relay logic only	Entire protection chain
Safety	High (Low voltage/current)	Moderate (Requires high energy)
Frequency of tests	Routine maintenance	Commissioning / Major audits

Why the Threshold Gap Matters

The primary reason for a lower drop-out value (Hysteresis) is to account for signal noise and transient recovery.

In a real-world power system, the current is never a perfectly smooth sine wave. It contains "noise" caused by harmonics and small load fluctuations. If your pick-up is 100A and your drop-out is also 100A, a signal oscillating between 99.9A and 100.1A would cause the relay to trigger and reset dozens of times per second. This would likely burn out the trip coil or confuse the SCADA system.

Real-Life Example: The Large Motor Startup

When a large industrial motor starts, it draws an "inrush current" that can be 6 to 10 times its rated operating current.

• The Scenario: A relay is set to pick up at 120% of the motor's rated current.

• The Action: During startup, the current spikes to 600%. The relay picks up and starts its "Time-Overcurrent" clock.

• The Drop-Out: As the motor reaches speed, the current drops back to 90%. Because the drop-out is set at 95%, the relay successfully drops out (resets its timer) before it reaches the "trip" command.

• The Failure: If the drop-out was too high (e.g., 99%) and the motor's running current was slightly high, the relay might never reset, eventually tripping the motor during normal operation.

  
  

Use Cases and Possible Overlaps

Different protection functions require different Pick-Up/Drop-Out (PU/DO) ratios based on their sensitivity requirements.

1. Overcurrent Protection (ANSI 51)

• Ratio: Typically 95%.

• Use Case: Feeder protection. High DO ratios are preferred here so that the relay resets as soon as a downstream fault is cleared by a different fuse, preventing "sympathetic tripping."

2. Under-Voltage Protection (ANSI 27)

• Ratio: Inverted logic. The relay "picks up" when voltage falls below a setpoint and "drops out" (resets) when voltage rises above a recovery threshold.

• Overlap: In weak grids, the voltage may stay low for a long time. If the recovery (drop-out) setting is too close to the pick-up, the relay might constantly trip and reconnect a facility, damaging sensitive electronics.

3. Differential Protection (ANSI 87)

• Ratio: Extremely tight.

• Use Case: Protecting transformers or generators.

• Overlap: These relays compare current entering vs. leaving. If the pick-up and drop-out aren't perfectly calibrated, the relay might "pick up" due to CT saturation during an external fault, causing an unnecessary blackout of a major transformer.

  
  

Coordination and "Race Conditions"

A common issue in protection coordination is the Reset Timer. Modern digital relays allow you to program how fast the relay "drops out" after the signal falls below the threshold.

• The Overlap Risk: If an upstream relay has a "Slow Reset" and a downstream relay has a "Fast Reset," they can get out of sync during repetitive faults (like a tree branch hitting a line repeatedly). The upstream relay "remembers" the previous heat/current and trips, even though the downstream relay was supposed to handle it. This is a failure of coordination due to poorly managed drop-out logic.

While secondary injection confirms the "brain" (the relay), Primary Injection tests the "muscles" and the "nerves" (the breaker mechanism and the CT wiring). In modern systems, this isn't just about raw power—it is a sophisticated dance with the breaker's internal logic and software.

Primary Injection & Breaker Logic

Modern digital trip units, such as Schneider’s MicroLogic, ABB’s Ekip, or Eaton’s Digitrip, are designed with protective algorithms that can actually interfere with standard primary injection if not handled correctly.

1. The "Thermal Memory" Challenge

Digital breakers "remember" the heat from a previous fault to protect cables from cumulative damage. If you run a primary injection test and the breaker trips, then you immediately try to test it again, it will trip much faster than the settings suggest.

• The Logic: The breaker thinks the cable is already hot.

• The Solution: Testing technicians must use software (like EcoStruxure Power Commission) to "Inhibit Thermal Memory" before testing.

2. Phase Unbalance & Ground Fault Logic

Most primary injection sets are single-phase. If you inject 1000A into Phase A only, a smart breaker may see this as a massive Phase Unbalance or a Ground Fault (ANSI 50N/51N) and trip instantly.

• The Logic: "Why is Phase A high while B and C are zero? This must be a ground fault!"

• The Workaround: Technicians often have to temporarily disable the Ground Fault (G) or Unbalance (U) functions in the logic to test the Long-Time (L) or Short-Time (S) curves in isolation.

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Brand-Specific Testing Examples

The methodology varies significantly depending on the manufacturer of the breaker and the test set being used.

Schneider Electric (MasterPact / PowerPact)

• Relay Logic: Uses MicroLogic trip units.

• Testing Nuance: Schneider officially prefers secondary injection for routine maintenance. However, for primary injection, they provide a "Service Interface" that puts the breaker into a "Test Mode."

  
  

ABB (Emax / Tmax)

• Relay Logic: Uses Ekip trip units.

• Testing Nuance: ABB units often require an auxiliary power supply during primary injection if the current is low. If the trip unit isn't powered up, it might miss the first few cycles of the injection, leading to inaccurate "Trip Time" readings.

  
  

Eaton (Magnum / Power Defense)

• Relay Logic: Uses Power Xpert Release (PXR) or Digitrip.

• Testing Nuance: Eaton emphasizes "Zone Selective Interlocking" (ZSI) testing. During primary injection, you verify that the breaker sends a "blocking signal" to upstream breakers to stay closed while it handles the fault locally.

  
  

Primary Injection vs. Logic: The Checklist

When testing logic through primary injection, you are looking for three specific "Pass" criteria:

1. CT Ratio Accuracy: Does 1000A injected at the bus show up as 1000A on the relay screen? (This proves the CT isn't damaged).

2. Mechanical Integrity: Does the trip unit's signal actually move the physical plunger to open the breaker?

3. Coordination Logic: If ZSI (Zone Selective Interlocking) is enabled, does the breaker trip in 0.1s (as the primary) rather than its backup time of 0.5s?

Device Brand	Logic Unit	Key Software/Tool	Critical Setting to Check
Schneider	MicroLogic	EcoStruxure	Thermal Memory Reset
ABB	Ekip	Ekip Connect	Aux Power Status
Eaton	Digitrip / PXR	Power Xpert	ZSI Jumper Status
Siemens	ETU / Sentron	powerconfig	Ground Fault Inhibit

Professional Testing Equipment

To run these tests, industry professionals use high-end primary injection sets that combine portability and power. To test not only the test points, but verify CT Ratio accuracy, EuroSMC Raptor is a perfect tool. Advantage of using the primary inejction tester instead of logic tester issued by breaker manufacturer is possibility to do.

A single unit like that can do all of the mandatory tests onsite with a minimal downtime.

In modern field testing, Primary Injection has evolved from hauling massive, multi-hundred-pound copper transformers to using modular, digitally-controlled systems. A leading example of this is the EuroSMC Raptor system.

Unlike traditional sets, the Raptor uses a "pass-through" (loop-through) secondary. Instead of connecting heavy cables to terminals, you simply run the primary conductor through the hole in the center of the unit, creating a single-turn transformer.

Testing Schneider MCCB with Raptor C-25 system

The Raptor Modular Approach (3kVA to 23kVA)

The Raptor is designed for scalability. You don't always need a massive amount of power, so the system allows you to build "blocks" of power based on the specific job.

1. The Raptor Master (3kVA)

• The Starting Point: This is the brain of the system. On its own, it provides up to 3kVA of power.

• Usage: Ideal for lower-current circuit breakers (up to 3000A for short durations) or for testing Current Transformer (CT) ratios and polarity where the burden (resistance) is low.

2. Adding Slave Units (Up to 23kVA)

If the 3kVA Master isn't enough to "push" the required current through the circuit, you can add Raptor SL (Slave) units.

• Expansion: Each Slave unit adds approximately 5kVA of additional power.

• The Configurations:

  • C-05: Master only (~3kVA).

  • C-15: Master + 1 Slave (~8.2kVA).

  • C-25: Master + 2 Slaves (~13.3kVA).

  • C-35: Master + 3 Slaves (~18.4kVA).

  • Max Config: Up to 4 Slaves can be linked to reach roughly 23.5kVA.

Why is More Power (kVA) Needed?

In primary injection, "power" isn't just about the Amps—it’s about the Compliance Voltage.

To understand why you would need to jump from 3kVA to 23kVA, you have to look at the Total Impedance ($Z$) of your test loop. According to Ohm's Law ($V = I \times Z$), if you want to push a high current ($I$) through a circuit with high resistance, the test set must be able to output a higher voltage ($V$).

Scenario A: Low Power (3kVA) is Sufficient

• The Setup: The breaker is on a bench, and you are using very short, thick copper busbars or cables (low resistance).

• The Result: The test set only needs 1 or 2 Volts to push 2000A.

Scenario B: High Power (18kVA–23kVA) is Required

• Long Cable Runs: If the test set is on the floor and the breaker is high up in a switchgear cubicle, you might need 10 meters of cable. That cable has resistance and inductance that "chokes" the current.

• High-Burden CTs: Some older protection CTs or metering circuits have high internal resistance. The test set needs more "push" (voltage) to overcome that resistance while maintaining the target current.

• Sustained Heat Runs: If you are testing the thermal trip of a 1600A breaker at 3x its rating (4800A), the cables will heat up, their resistance will increase, and the power demand will climb.

• High-Impedance Objects: Testing components like reclosers or busbar sections with many bolted joints adds cumulative resistance that a small 3kVA unit simply cannot overcome.

Key Advantage: Automatic Regulation

One of the reasons the Raptor is favored for these high-power tests is its DSP-based digital control.

In traditional sets, as the cables get hot, the current drops, and you have to manually turn a dial to keep it steady. The Raptor automatically adjusts its output to ensure the current stays exactly at the setpoint, regardless of the resistance changes in the loop.

References for Further Study

• IEEE C37.91: Guide for Protective Relay Applications to Power Transformers.

• IEC 60255: Measuring relays and protection equipment (The international standard for operating intervals and accuracy).

• Network Protection & Automation Guide (GE/Alstom): The industry "bible" for understanding pick-up/drop-out ratios in numerical relays.