Overcurrent and Earth Fault Relay Testing

How to test Overcurrent and earth fault relay testing step-by-step, including pickup, drop-off, timing tests, and broken conductor (46BC) protection.

Protection relays are the first line of defence in any electrical power system. When something goes wrong — a short circuit, an overload, or a broken conductor — it is the relay that detects the fault and sends a trip signal to the circuit breaker. But a relay that has not been properly tested is a relay you cannot trust. That is why Overcurrent and earth fault relay testing is one of the most critical tasks in substation commissioning and maintenance.

In this technical article, we will walk you through everything you need to know about testing overcurrent (50/51) and earth fault (51N) relays, including pickup and drop-off checks, timing tests, and broken conductor protection (46BC). Whether you are a commissioning engineer or a substation technician, this article will give you a clear and practical understanding of the process.

Before performing any relay testing, it is essential to have the correct settings in place — see our guide on overcurrent and earth fault relay setting calculations.

What Is Overcurrent and Earth Fault Protection?

Before you can test a relay, you need to understand what it does. Overcurrent protection is designed to detect when the current in a circuit rises above a set value (called the pickup setting). Once this threshold is crossed and maintained for a set time, the relay sends a trip signal to open the circuit breaker and isolate the fault.

There are three main types of overcurrent protection:

• Instantaneous Overcurrent (50 element): Operates with no intentional time delay. The relay trips as soon as the current exceeds the pickup set point. This is used to clear high-level faults very quickly.
• Definite Time Overcurrent (50 with time delay / DMT): An intentional time delay is added after the pickup value is reached. The relay will wait for a fixed period before tripping. This is useful for grading between relays at different protection zones.
• Inverse Definite Minimum Time (IDMT / 51 element): The operating time is inversely proportional to the fault current magnitude. The higher the fault current, the faster the relay trips. Common curve types include Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), and Long Time Inverse (LTI). IEC Normal Inverse (NI) is widely used in networks.

Earth fault protection (51N or 50N/50G) works the same way but is designed to detect single-phase ground faults. Since earth fault currents can sometimes be lower than normal load currents, the pickup settings are usually set much lower than phase overcurrent elements.

Understanding Relay Settings Before You Start Testing

Before you begin any testing, write down the relay settings from the approved setting sheet. This is a critical step that many engineers rush through — but getting the settings wrong before you start means all your test results will be meaningless.

The key settings you need to record are:

• Pickup setting (Ip): This determines when the relay starts to react. The actual pickup current is typically calculated as: Pickup Current = Setting × Nominal CT Secondary Amps
• Curve type: Make sure you select the correct curve as there can be subtle differences between manufacturers. Compare it against the approved coordination study.
• Time Dial / Time Multiplier Setting (TMS): This controls the time delay between pickup and operation for IDMT curves.
• Time delay (for 50 element): Usually set in cycles, milliseconds, or seconds.

Also confirm that the element is enabled in the relay. Many relays allow individual elements to be switched on or off. If an element is not used, it should be disabled to avoid confusion during Overcurrent and earth fault relay testing.

Pickup and Drop-Off Testing (51/51N Elements)

The pickup test confirms that the relay operates at the correct current value. The drop-off test confirms that the relay resets when the current falls below a certain level. These two tests together are sometimes called the PU/DO test.

How to Perform the Pickup Test

• Calculate the expected pickup current from the relay setting sheet.
• Apply a current approximately 5% higher than the pickup value into the relay input using a test set. For example, if the pickup is 8.00 A, start at around 8.40 A.
• Monitor the pickup indication — this can be an LED, front panel display, or output contact from the relay.
• Slowly lower the current until the pickup indication goes off, then slowly raise it again until the indication is fully and steadily on.
• Important: chattering contacts or flickering LEDs are not considered a valid pickup — wait for a stable indication.
• Record the pickup value on your test sheet and compare it against the calculated pickup current.

The purpose of Overcurrent and earth fault relay testing is not just to check the relay’s calibration — it is primarily to verify that the engineer has correctly interpreted the settings and that those settings have been entered into the relay correctly. With modern microprocessor relays, it is extremely unlikely to find a calibration issue.

Timing Tests for 51 and 50 Elements

Timing tests confirm that the relay operates in the correct time for a given fault current. This is essential for protection coordination — if the timing is wrong, the relay may operate too fast or too slow, causing unwanted trips or failing to isolate faults properly.

Timing Test for the 51 (IDMT) Element

• Select a test current that is a multiple of the pickup current (e.g., 2x, 5x, or 10x pickup). This multiple should match what was used in the coordination study.
• Connect the test-set timing input to the relay output contact that is assigned to trip when in service. This ensures you are testing the correct output.
• Apply the test current and record the time from when the current is applied until the relay output operates.
• Compare the measured time against the calculated operating time from the IDMT curve formula.
• Review relay targets to confirm the correct element operated.
• Repeat the test for each phase and the earth fault element.

Timing Test for the 50 (Instantaneous) Element

Although the 50 element is called instantaneous, many relays apply a small time delay to it. Timing tests should always be performed, even when the time delay is zero, because there is always a small inherent delay due to the relay processing time plus test-set delays.

• Apply 110% of the 50-element pickup current to the relay.
• Measure the time from the start of the test to when the relay output operates.
• Compare the measured time to the 50-element time delay setting.
• Repeat for all three phases.

Residual Neutral Time Overcurrent Protection (51N)

Residual neutral overcurrent protection calculates the unbalanced current between all three phases. Since it is set well below the phase overcurrent settings, it is generally easier to test.

When testing, follow the same steps as above but apply current in one phase at a time. It is good practice to perform the pickup test on A-phase and the timing test on B-phase and C-phase. This confirms that the relay is using all three phases correctly to calculate the residual current.

Broken Conductor Protection Testing (46BC)

Broken conductor protection is a less commonly discussed but equally important relay function. When a conductor breaks or loses continuity, it creates a highly unbalanced condition in the circuit. However, because the current does not drop to zero in radially fed lines, simple overcurrent relays may not detect this fault.

The 46BC element uses the ratio of negative sequence current (I2) to positive sequence current (I1) to detect this unbalanced condition. A broken conductor raises the I2/I1 ratio significantly. The element is also useful for:

• Detecting single-phase and phase-to-phase faults with increased sensitivity
• Identifying unbalanced loads caused by unsymmetrical feeder voltages
• Protecting electric machines during excessive unbalanced load conditions
• Detecting phase interruptions in the primary system

How to Test the 46BC Element

• Before testing, write down the pickup and time settings, and calculate the pickup current.
• Keep the positive sequence current (I1) constant and gradually increase the negative sequence current (I2) until the relay picks up.
• Note the pickup point — do not record the drop-off reading — and compare it with the I2/I1 setting.
• For timing tests, apply 110% of the I2/I1 pickup value and measure the time from start of test to relay operation.

Using Omicron CMC-356 for Overcurrent Testing

Non-directional overcurrent and earth fault testing is commonly carried out using the Omicron CMC-356 test set with the Overcurrent Test Module. Before running any tests, the general relay settings must be entered into the test object. This includes relay type, relay ID, substation details, and CT and VT parameters.

Using dedicated test software like Omicron not only makes the testing process faster and more repeatable, but also produces a professional test report that can be submitted for commissioning documentation.

Frequently Asked Questions (FAQs)

Q1: What is the difference between a 50 and 51 relay element?

The 50 element is instantaneous (or definite time) overcurrent — it operates after a fixed time delay once the current exceeds the pickup. The 51 element is an inverse time overcurrent relay, meaning the trip time decreases as the fault current increases. In practice, both elements are often present in the same relay and work together for complete protection.

Q2: Why do I need to test relay timing even if the time delay is set to zero?

Even with a zero time delay setting, there is always a small inherent delay due to the relay’s processing time plus any delay introduced by the test-set itself. Timing tests also confirm that the correct element is operating and that settings have been correctly entered into the relay.

Q3: What is pickup current and how is it calculated?

The pickup current is the actual current at which the relay begins to operate. It is calculated by multiplying the relay’s pickup setting by the nominal CT secondary current. For example, a setting of 1.75 In with a CT ratio of 400/1A gives a pickup current of 1.75 × 1 = 1.75 A secondary, or 700 A primary.

Q4: What is the purpose of the drop-off test?

The drop-off test confirms that the relay resets correctly when the current falls below a threshold. If the drop-off value is too close to the pickup value, the relay may chatter. A stable and clean drop-off is important for correct relay operation in real-life fault conditions.

Q5: Can broken conductor faults be detected by normal overcurrent relays?

Not always. In radially fed lines, the fault current during a broken conductor event may not be zero — the line may still carry partial load current. Normal overcurrent relays may not detect this condition. That is why dedicated broken conductor (46BC) protection using negative sequence current measurement is used alongside standard overcurrent elements.