How ODES Engineered Contact-Bounce-Safe CT Test Blocks from One 500 kV Alarm

TonyZhang
Jan 5
5 min read

From a Single 500 kV Alarm to a Deeper Design Question

During commissioning on a 500 kV transmission project in 2024, a disturbance recorder captured something protection engineers never ignore: at the exact moment a test block handle was pulled quickly, the CT secondary circuit showed a transient open-circuit condition and raised an alarm.

The team did what any good commissioning crew would do:

Check wiring
Check terminal torque
Check device contact resistance

The surprise came later. When the same test was repeated under controlled conditions, several well-known international test block brands showed the same behaviour: during a very fast plug withdrawal, the CT secondary circuit briefly lost its burden, even though the devices were built with automatic CT shorting mechanisms.

The root cause was not a “bad contact” in the usual sense. It was a classic but under-appreciated phenomenon in electromechanical design:

Contact bounce in spring-loaded test block contacts degrading the intended CT automatic shorting sequence.

For a current transformer secondary circuit, even a millisecond-scale open-circuit transient can create a high transient overvoltage. That is not acceptable in high-energy systems.

Instead of pushing responsibility back to operators, ODES treated this as an engineering problem to be solved at the mechanism level. You can find more about ODES protection and test interface solutions at www.odes-electric.com.

CT Automatic Shorting vs. Contact Bounce: The Technical Conflict

Standard CT-safe test block design follows two concepts that are well aligned with IEC/IEEE guidance on CT secondary safety:

Defined contact sequence
Operational discipline

On paper, this sequence ensures that the CT secondary is either:

Connected to its normal burden, or
Safely short-circuited,

with no open interval in between.

In reality, several factors interfere:

High operating speed when the handle is pulled under time pressure
Manufacturing tolerances and wear in the plug–socket geometry
Panel vibration and slight misalignment over years of service

Under these conditions, the elastic elements in the contacts physically bounce. That bounce can partially cancel the intended pre-make / post-break behaviour, creating a small time window where:

The normal measurement path is already open, and
The shorting path is not yet fully established, or vice versa on restoration.

From an engineering standpoint this is a transient open-circuit condition in a CT secondary circuit caused by contact bounce, not by a missing shorting link. It is low probability, but in high-voltage, high-burden systems, low probability plus long lifetime still matters.

Choosing the Harder Path: Mechanism-Level Safety, Not Operator-Dependent Safety

There were two easy responses:

Strengthen operating instructions and warnings: “Do not operate quickly. Follow the procedure strictly.”
Or accept that a tiny open-circuit window exists but is “short enough” in theory.

ODES chose a third path: change the contact mechanism so that the CT secondary cannot experience an open interval, even with aggressive operation.

The development loop looked like a textbook engineering process:

Laboratory reproduction
Statistical and physical analysis
Design requirements

In other words, the goal was to turn “operator-dependent safety” into mechanism-guaranteed safety.

The Patented Contact Architecture: True Pre-Make / Post-Break for CT Shorting

Once the targets were clear, the engineering work moved deep into electromechanical detail:

Contact geometry optimisation
Redundant shorting paths
Spring force and travel tuning

It is similar in concept to pre-insertion or arcing contacts in circuit-breaker design, but applied to CT secondary shorting in a test block:

The CT shorting contacts become genuine pre-make / post-break contacts, not just “ideally faster” contacts on a drawing.

This architecture was formalized as an invention patent focused on CT automatic shorting contact structures for test blocks. The key outcome:

For the full mechanical stroke—regardless of operating speed—the CT secondary is always either:

There is no uncovered “open” window left for contact bounce to act in. From a protection perspective, this is a hardware-enforced fail-safe design.

From Patent to Standard Product: Next-Generation CT-Safe Test Blocks

A contact architecture alone is not yet a solution; it needs to be industrialised as a standard product.

In the next-generation ODES protection test blocks:

All CT circuits are implemented with the new contact architecture
Functional behaviour remains familiar to engineers
Ratings remain aligned with protection practice

For design institutes and EPC contractors, this allows the new test block to be adopted directly as a “CT secondary open-circuit risk-free” standard part in protection and control panel designs, instead of relying on detailed operating warnings in every project.

What Changes for Protection Engineers, O&M, and Owners

The practical impact of this work is straightforward but important.

For protection engineers:

CT secondary safety in test blocks is now a property of the hardware, not just of operating habits.
Secondary injection testing and scheme verification during commissioning can be performed with greater confidence, even when handles are operated frequently.

For operation and maintenance teams:

Routine testing, fault investigation and periodic relay checks become less stressful.
Disturbance recorder data shows that inserting or withdrawing the test plug no longer introduces visible CT open-circuit transients.

For owners and project managers:

The requirement “CT secondary circuits must not experience open-circuit conditions during test block operation” can be written into specifications in a measurable and verifiable way.
Adopting this test block as a standard part reduces residual technical risk across an entire fleet of substations or generating plants.

Most importantly, this type of micro-innovation accumulates into a reusable engineering standard: future projects can directly specify an architecture that intrinsically mitigates CT secondary risk, rather than re-debating operating instructions each time.

Conclusion: Craftsmanship Applied to Milliseconds

On the surface, this development targeted a small phenomenon measured in milliseconds: contact bounce in test block contacts. In essence, it demonstrates how engineering craftsmanship can turn a single commissioning alarm into a durable improvement in secondary-system safety:

Start from real disturbance recorder evidence, not assumptions.
Analyse the physics of contact behaviour instead of relying on written cautions.
Implement a deterministic, fail-safe contact architecture that removes the unsafe state from the mechanical design.

In a field as mature as relay protection testing, there is still room to refine standard interfaces. For ODES, this is a clear example of using engineering methods to solve an engineering problem—and then turning that solution into a practical, repeatable product.

If you are planning new protection panels or standardising test interfaces for transmission, distribution or hydropower projects, ODES can help you consult on test block architectures, request design notes for contact-bounce-safe CT shorting, and learn how to incorporate CT secondary safety requirements directly into your panel standards.