Preventing CT Open-Circuit Hazards with Modular Test Block Assemblies

TonyZhang
Dec 12, 2025
4 min read

One Loose Screw, One “Near Miss” – What Really Happened

During commissioning of a protection scheme, an engineer completed relay calibration and started to restore the circuits. Everything looked normal—until one phase current value refused to match expectation.

Further investigation revealed the cause: one CT secondary terminal screw had not been fully tightened, leaving that phase’s CT secondary circuit effectively open. The current circuit was abnormal, and the situation could easily have escalated into a serious incident if the issue had not been detected in time.

Stories like this are not rare in traditional commissioning practice:

CT secondary inadvertently open-circuited
PT secondary short-circuited by mistake
Unintended trip during testing
Test circuits not fully restored after work

Each “near miss” is stressful for the site team, slows down progress, and often requires multiple people to double-check circuits before closing work.

ODES designs its secondary interfaces and test equipment around a simple principle: safety must be engineered into the mechanism, not left to procedure alone. For more information on our approach to protection and test interfaces, visit www.odes-electric.com.

Why Conventional CT Testing is Vulnerable

Traditional CT and protection testing often relies on:

Directly manipulating secondary terminals with screwdrivers
Manually wiring jumpers for shorting or isolation
Relying on personal experience and “one-by-one” verbal cross-checks

In this environment, several risk mechanisms recur:

Momentary CT open circuit
Restoration not completed correctly
Human-factor dependency

Internationally, protection engineers have addressed these risks using test block assemblies—modular devices that provide a defined, plug-in interface for secondary testing. The logic is straightforward:

Insert test plug → automatic isolation and switching (with CT shorting).

The test block itself enforces safe sequences so that testing no longer depends solely on individual habits or improvised jumpers.

Taking the Concept Further: The TSB-14 Relay Protection Test Box

In practice, engineers still worry about the microscopic moment when the plug is being inserted or withdrawn. Could a CT secondary be briefly open-circuited due to mechanical vibration or contact bounce?

To address this, ODES developed the TSB-14 relay protection test box, building on the standard test block concept and adding a dedicated CT safety mechanism:

14 parallel circuits
20 A rated current
Automatic CT self-shorting
Anti-bounce CT protection (patented)
Single-operator workflow
Unified plug interface

According to customer feedback from nearly one thousand overseas projects, the use of TSB-14 has improved commissioning efficiency by more than 30 %, while providing an additional layer of CT protection that conventional test blocks do not offer.

The product has successfully passed CE certification, underscoring its suitability for international projects with strict safety and compliance requirements.

Inside the Safety Logic: CT Self-Shorting and Anti-Bounce Design

The safety behaviour of TSB-14 is not an afterthought; it is implemented in the internal contact geometry and plug mechanics: CT self-shorting before disconnection

Service restoration before removing the short
Anti-bounce contact structure

This mechanism transforms CT safety from a purely procedural instruction (“remember to short the CT before you open the circuit”) into hard-wired logic that operates the same way every time, independent of the individual engineer.

Safety and Efficiency Gains – Different Insights for Different Users

For International Users Already Familiar with Test Blocks

Engineers on international projects—whether in transmission, generation, or heavy industry—are often already accustomed to test block assemblies in protection and control panels. For them, the key benefit of TSB-14 is not learning a new interface, but gaining extra safety and redundancy on top of the familiar concept.

The CT self-shorting and anti-bounce mechanism reduce the probability of CT open-circuit incidents during harsh commissioning conditions.
A unified plug standard across modules simplifies test kits and training.
Modular 14-point layout allows complete scheme testing without additional temporary terminal blocks.

In many projects, users report that TSB-14 offers better CT protection and plug robustness than established international brands, while maintaining a compatible operating philosophy.

For Domestic Users Moving Towards Standardization

In domestic projects, the growing adoption of test block concepts reflects a wider trend: standardization and modularization of secondary testing.

The TSB-14 relay protection test box illustrates this trend in several ways:

Testing is performed through a repeatable interface instead of ad-hoc terminal manipulation.
CT safety is ensured by the device mechanism rather than handwritten notes or oral reminders.
A single configuration can be rolled out across multiple substations and units, giving commissioning teams a consistent toolset.

As more experts recognize the value of relay protection test box schemes, TSB-14 is being trialed and adopted in an increasing number of domestic projects where both safety and efficiency are top priorities.

Conclusion: Safety and Efficiency Do Not Need to Compete

The “near miss” during that commissioning test was ultimately traced back to a single, improperly tightened terminal screw—a tiny detail in a complex system. It serves as a reminder that system stability often depends on the smallest contact in a CT secondary circuit.

By embedding CT self-shorting, anti-bounce contact logic, and standardized plug-in interfaces into the TSB-14 relay protection test box, ODES helps convert high-risk manual tasks into a controlled, repeatable process:

CT secondaries remain protected, even during plug insertion or withdrawal.
Testing becomes faster and less dependent on individual habits.
Restoration is automatic and verifiable when the plug is removed.

We believe that modular interfaces + reliable self-shorting will become the mainstream approach for relay protection testing—delivering both safety and efficiency in substations and industrial plants worldwide.

For a broader perspective on ODES solutions in protection, control, and testing, visit www.odes-electric.com.

If you are planning new protection panels or upgrading existing test interfaces, ODES can help you consult on test block architectures, request TSB-14 application notes, and learn how to integrate relay protection test boxes into your standard secondary designs.