Composite Voltage Blocking Overcurrent Testing: UV, Negative Sequence and Logic

Composite voltage blocking overcurrent testing proves that the overcurrent element trips only when the voltage-blocking logic permits it. It verifies the undervoltage and negative-sequence thresholds, the blocking logic, the drop-out ratio, and the overcurrent trip time. The result is a scheme that trips only when the voltage condition confirms a fault.

Key takeaways

• Test each blocking element alone. Prove the undervoltage element and the negative-sequence element independently before testing the combined logic.
• Know the logic. Some schemes enable overcurrent when voltage is abnormal; others block overcurrent when voltage is healthy. The test sequence depends on which one you have.
• Use three voltages. A negative-sequence element needs an unbalanced three-phase voltage; a single-phase source cannot prove it.
• Check the drop-out. The reset voltage and the reset time are part of the scheme, not an afterthought.
• Record the binary outputs. Use the relay’s event log or binary output contacts to see which blocking element asserted, not only the final trip.

What does a composite voltage blocking scheme prove?

A composite voltage blocking scheme combines an overcurrent element with one or more voltage-based conditions. The usual aim is to prevent overcurrent tripping during a healthy-voltage condition such as a motor start or a temporary overload, while allowing a trip when the voltage collapses or becomes unbalanced because of a fault.

The scheme may use an undervoltage element (27), a negative-sequence element (47), or both. The logic can be an AND, an OR, or a more complex arrangement with definite-time delays. The test must prove the logic, not just the individual thresholds.

How do you test the blocking logic?

1. Record the scheme. From the relay manual, write down the blocking criteria, the logic, the pickup and drop-out values, and the time delays.
2. Isolate the relay. Disable the trip output to the breaker or work on a test set with the trip circuit isolated.
3. Apply healthy voltage and low current. Confirm no trip and confirm that the overcurrent element is picked up or not, as expected.
4. Test the UV element. Lower one phase voltage until the UV element asserts. Record the pickup and the drop-out. Repeat with a three-phase balanced reduction if the scheme uses a positive-sequence UV element.
5. Test the negative-sequence element. Apply an unbalanced three-phase voltage and raise the negative-sequence component until the element asserts. Record the pickup and the drop-out.
6. Test the logic. Apply overcurrent with healthy voltage: no trip. Apply overcurrent with only UV: trip. Apply overcurrent with only negative sequence: trip. Apply overcurrent with both: trip.
7. Measure the time. At twice the overcurrent pickup, record the time from the blocking condition to the trip output.
8. Test the reset. Remove the blocking condition and confirm that the overcurrent element resets and the trip output drops out.

What settings and test points should you use?

How do you wire the VAE-430 to the relay?

What is the pass criterion?

• UV pickup: within 2 percent of the setting, or the relay manual tolerance.
• Negative-sequence pickup: within 5 percent of the setting.
• Overcurrent pickup: within 5 percent of the setting.
• Time: within 5 percent or 30 ms at the tested point.
• Logic: the trip output must follow the truth table in the relay manual for all four combinations of voltage and current.
• Reset: the trip output must drop out when the blocking condition is removed and the current falls below the reset value.

Why does the relay trip with healthy voltage or fail to trip with low voltage?

Trip with healthy voltage: the blocking contact may be miswired, the logic may be set to “block when healthy” instead of “enable when abnormal”, or the voltage input may be below the relay’s minimum measuring threshold. Check the relay metering first, then the logic.

No trip with low voltage: the overcurrent element may not be enabled, the blocking delay may be longer than the test, or the negative-sequence element may require a higher unbalance than the test produced. Use the relay event log to see which condition is missing.

No trip with negative sequence: a phase swap that looks unbalanced on the test set may produce a different sequence component in the relay if the voltage inputs are not connected in the expected phase order. Verify the phase sequence at the relay terminals.

Frequently asked questions

Can I test a composite voltage blocking scheme with a single voltage channel?

No, not completely. A single voltage channel can prove the UV element, but it cannot prove the negative-sequence element. Use a three-phase voltage source for the full scheme.

What is the difference between UV blocking and negative-sequence blocking?

UV blocking responds to a balanced voltage collapse. Negative-sequence blocking responds to an unbalanced voltage, such as a single-phase fault or an open phase. Many schemes use both because each covers a different failure mode.

Why does the overcurrent element trip before the blocking delay expires?

The blocking delay may be set to zero, or the overcurrent element may have its own definite-time delay that is shorter than the blocking delay. Check both timers in the relay settings.

Do I need to test the scheme at the actual CT and PT ratios?

Use the secondary values that the relay sees. If you inject on the secondary side, the relay settings are already in secondary amperes and volts. If you inject on the primary side, apply the ratio to the test points.

Can I use the VAE-430 for a negative-sequence test?

Yes. The VAE-430 has three independent voltage channels and three current channels. Set the phase angles to create the required negative-sequence component and verify the sequence value with the relay metering before the trip test.

Where VA-TEK test sets fit

The VAE-430 provides the three currents and three voltages needed for a composite voltage blocking scheme, plus four digital inputs for the trip and blocking contacts. Start with the relay protection testing hub for the overall workflow, and use the relay testing whitepaper for the test-record template.

Related reading: overcurrent relay testing procedure for the 50/51 element, and underfrequency load shedding testing for the frequency-based counterpart.

Standards and references

• IEC 60255-127:2010 — functional requirements for over/under voltage protection.
• IEC 60255-151:2009 — functional requirements for over/under current protection.
• IEEE C37.112-2018 — inverse-time characteristic equations.
• Relay manufacturer’s technical manual — blocking logic and truth table.