News | company news | Aug 15,2026
When designing low voltage distribution boards, the choice of toroidal current transformer is not simply a matter of adapting them to the primary current.
Electrical engineers must also take into account the current transformation ratio, secondary output voltage, accuracy class, load, window size, conductor cross-section, frequency, and the requirements of connected devices or protective equipment.
This is especially important in commercial buildings, industrial plants, infrastructure projects, and electrical distribution systems, as current transformers are integrated into low-voltage electrical panels, main distribution boards, distribution panels, and monitoring systems.
For new distribution boards and electrical switchgear, toroidal current transformers can be a practical solution if the primary conductor can pass through the current transformer during the switchgear assembly process.
This article explains how engineers can select a suitable semiconductor loop current transformer and provides several practical examples to illustrate the selection process.
A toroidal semiconductor current transformer consists of a closed magnetic core and a secondary winding. The primary winding passes through the central opening of the transformer.
A current transformer (CT) converts a high primary current into a standardized secondary current, for example, 1 A or 5 A , which can then be connected to a measuring device, a control unit, or a protection system.
For engineers, the crucial point is that current transformers cannot be selected solely based on their transformation ratio.
To perform a computed tomography (CT) scan, the following two conditions must be met:
electrical requirements
AND
Requirements for mechanical installation.
When developing new low-voltage switchgear, engineers typically need to determine the following:
Applications in the field of measurement or protection.
Let’s take as an example a typical combination of low voltage distribution boards.
The power cable can carry hundreds or even thousands of amps of current, while the measuring device connected to it can only accept an input current of 1 A or 5 A.
In this way, current transformers serve as an interface between high current power circuits and low current measurement or protection circuits.
Choosing the wrong computed tomography (CT) scan method can cause the following problems:
Therefore, the selection of current transformers should be carried out simultaneously with the planning of the distribution board and not as a separate purchasing decision.
Generally, the first parameter that engineers consider is the primary current.
However, the current conversion ratio should not be determined simply based on the normal operating current and by selecting the closest available value.
Engineers should take the following into account:
Imagine a low-voltage power cable for electrical panels that includes the following:
The possible correlations between TC values are:
800/5A
EITHER
1000/5A
The appropriate choice should be based on the specific system design and the requirements of the connected measuring devices.
Major technical problems are not limited to:
Which current transformer is closest to 800 A?
He:
What aspect ratio should the current transformer have to provide the required measurement range and comply with all the electrical properties of the power line?
The HEYI RCT series offers various primary current ratios with configurations up to 3000/5 A.
The catalog offers configurations up to 5000 A/5 A.
Larger low-voltage distribution boards can accommodate power supplies with a nominal current of approximately 1600 A.
At this stage, engineers must check much more than just the 1600/5A ratio.
The next question should be:
Does the distribution board offer sufficient space for the installation of the selected current transformer?
This is a common reason why, even with a technically correct radiation dose in computed tomography, it may prove impossible to achieve the desired end result.
The additional CT scanner must be compatible with the connected devices.
Common auxiliary parameters include:
1A，5A
The appropriate choice depends on the instruments, relays, monitoring devices, or other equipment connected to the secondary side of the CT scanner.
Let us consider the example of a distribution box where the current transformer is installed in the main distribution box and the measuring device is located some distance away from it.
Engineers can compare configurations with 1A and 5A secondary circuits based on the overall system design, secondary wiring, connected equipment, and load requirements.
The main point:
The choice of the secondary side of the current transformer should be based on the overall situation of the secondary circuit and not solely on the situation of the power supply.
The entire HEYI MR product range offers secondary winding configurations of 1 A and 5 A.
This gives electrical switchgear developers greater flexibility to adapt current transformers to different instruments and monitoring systems.
The level of precision should be chosen based on the application scenario.
The accuracy requirements for current transformers used in measurements may differ from those of current transformers used in general monitoring or protection systems.
Therefore, engineers must define the purpose of the computed tomography scanner before choosing the level of accuracy.
Here are some frequently asked questions:
Depending on the model and features, the HEYI RCT series offers level adjustments of 0.5 and 1.
Depending on the model and scale, the MR series also offers various options in terms of accuracy and load capacity.
Let’s take as an example a commercial building whose low-voltage distribution panel supplies several main consumers.
Technical requirements may include:
In this situation, the engineer must determine the following:
Primary current
According to the design specifications of the distribution partner.
secondary
→ If necessary, use a current of 5 A if the measuring device requires it.
precision
→ According to the measurement requirements.
burden
Based on the secondary circuit of a current transformer.
Window
→ Depending on the size of the cable or conductor bar.
This explains why the specifications “800/5A CT” or “1000/5A CT” do not, by themselves, constitute a complete technical specification.
In a toroidal transformer with a solid conductor, the primary conductor must be physically guided through the central opening.
Therefore, the CT scan window is a fundamental mechanical parameter.
The engineer must check the following:
The goal is not simply to find a CT value that is “high enough”.
The chosen window must offer sufficient space and be compatible with the electrical and mechanical designs.
Imagine a low-voltage distribution system with thick power cables.
The energy requirement was determined as follows:
However, the engineers discovered that installing the cables required a power transformer with a larger cross-section than originally planned for the chosen model.
At this stage, simply changing the CT scan report is not enough to solve the problem.
Engineers must select a computed tomography scanner with an appropriate window width.
During this phase, the availability of a large number of durable computed tomography devices of different sizes is very valuable.
The HEYI MR series includes several models, from the MR-28 to the MR-125 , offering different size configurations to suit various cyclist profiles.
The process of selecting current transformers (CTs) depends on the type of primary conductor.
Engineers should take the following into account:
Engineers should take the following into account:
A current transformer suitable for a single-wire configuration may not be suitable for multi-wire or busbar configurations.
Therefore, the final choice should always be compared with the actual installation plans.
Let’s take as an example a main low-voltage distribution system with a three-phase busbar system.
The initial requirements for computed tomography can be defined as follows:
2000/5A, Stage 0.5
However, the engineers still need to verify the physical dimensions of the tire.
For example, when designing a control cabinet, it may be necessary to adapt the current transformers to the following conditions:
In this case, the limiting parameter could be the CT window size instead of the current coefficient .
Therefore, when selecting a current transformer, both the single-pole circuit diagram and the mechanical arrangement of the switching device must be taken into account.
The load of the current transformer is the load that is connected to the secondary circuit of the current transformer.
A typical secondary circuit might include the following:
Therefore, engineers must take into account the entire secondary circuit when selecting loads for current transformers.
Let’s assume that the current transformer is installed in the low-voltage distribution board and that the measuring instruments are located in a remote location.
A longer cable is now used for the backup circuit.
Engineers should not select current transformers based solely on the meter’s input specifications.
Adding cables increases the total load; therefore, this must be taken into account when selecting power transformers.
This is particularly important when comparing different current transformer configurations at different load levels.
It is also used in industrial plants.
Let’s take as an example a low-voltage motor control system or an electrical distribution system that powers a large electric motor.
An engineer may be required to take an exam for the following reasons:
During motor operation, the primary current may be subject to significant fluctuations.
Therefore, current transformers must be selected according to all operating parameters and the requirements of the connected measuring or protection devices.
The engineer must check the following:
Primary participation
Does this belong in the bird feeder?
precision
Is it suitable for this use case?
burden
Can a current transformer power a connected secondary circuit?
Window
Can the motor’s power cable be passed through the current transformer?
This type of application is especially relevant for industrial distribution boxes and power distribution systems.
Imagine a commercial building that contains a main distribution panel and multiple output power sources.
As part of this project, computed tomography (CT) scans may be required for the following tasks:
Power supplies can have completely different rated currents.
For example:
| cradle | typical design requirements |
| Power supply for lighting | lower CT ratio |
| Electricity supply for heating, ventilation and air conditioning. | Preliminary report of the computed tomography scan |
| Refrigeration dispenser | The CT coefficient is high. |
| Main sources of income | High CT ratio |
Therefore, it is not an efficient approach to use the same size and the same TC ratio for all feeds.
A more effective strategy is to create a current transformer (CT) schedule table for the entire distribution system:
Power supply → Current intensity → Current transformer turns ratio → Secondary winding → Accuracy → Load → Window dimensions
This allows electrical cabinet manufacturers to standardize the selection of current transformers and reduce purchasing errors.
Both magnetic resonance imaging (MRI) and computed tomography (CT) scans consist of continuous series of ring-shaped images, but they offer different combinations of sizes and electrical configurations.
The series of randomized, controlled clinical trials includes:
The product catalog offers a variety of current conversion ratios, including configurations up to 3000/5 A , as well as Class 0.5 and Class 1 options, according to the technical specifications.
Therefore, computed tomography (CT) is suitable for cases where the project requires the use of a conventional ring CT scanner for the following operations:
The MR series includes:
MP-28,MR-40,MR-42,MR-45,MR-60,MR-70,MR-85,MR-125
This product offers various power configurations, including secondary options of 1A and 5A, as well as high current configurations up to 5000A/5A, which are available in the product catalog.
Therefore, a hybrid model can be considered if the project requires compliance with the following conditions:
More size options
The final decision should always be based on actual technical performance and not simply on choosing a series based on a higher rated current.
Suppose a telephone system manufacturer receives an inquiry with the following requirements:
Application: Low voltage installations;
Primary winding: 1600 A
; Secondary winding: 5 A
; Accuracy: Class 0.5
; Frequency: 50 Hz;
Quantity: 12 units.
A professionally organized purchasing process should not impose the following requirements on suppliers from the outset:
“1600/5A CT, 12 total.”
In addition, electrical panel manufacturers must provide the following:
The physician will then be able to determine the appropriate location for the randomized controlled trial (RCT) or magnetic resonance imaging (MRI).
This approach also makes price comparisons more meaningful, as different suppliers are evaluated based on the same technical specifications.
A practical approach involves developing a standard table for selecting current transformers for each control cabinet design.
Note the details of each automatic feeder:
| volume | technical requirements |
| cradle | Entrance and exit |
| Primary current | ___ TO |
| relationship of the TC | ___ / 1A or 5A |
| precision | ___ |
| burden | ___ GO |
| frequency | 50/60 Hz |
| Dimensions of cables/busbars | ___ |
| TC Window | ___ |
| Application | Measurement/Monitoring/Protection |
| TC model | ___ |
This simple process helps avoid one of the most common problems when buying appliances:
Although the current transformer meets the transformation ratio requirements, it is not compatible with the actual design of the distribution panel.
The CT coefficient is only one of the elements of the specification.
Current transformers must be physically designed to accommodate cables or busbars.
The secondary side must be compatible with the instruments, relays, or monitoring systems.
The entire secondary circuit must be taken into account.
Measurement, monitoring, and protection requirements must be evaluated separately.
Please check the condition of the installed equipment before placing your order.
Two current transformers with the same transformation ratio may differ in accuracy, power handling capacity, dimensions, insulation, and technical configuration.
For projects involving the installation of distribution boxes, the following procedure is appropriate:
It is necessary to compare all technical specifications and not just the unit price.
In specific projects, the information technology selection process can be briefly described as follows:
Step 1 – Determine the current in the power cable.
↓
Step 2 – Select the basic CT ratio.
↓
Step 3 – Confirm level 1A or 5A
↓
Step 4 – Determine the degree of precision.
↓
Step 5 – Calculate/Verify your tax obligation
↓
Step 6 – Measure the dimensions of the cable or distribution rail.
↓
Step 7 – Select the size of the computed tomography window.
↓
Step 8 – Confirm frequency and isolation requirements.
↓
Step 9 – Connect the power transformer to the device/relay.
↓
Step 10 – Confirmation of the final plans and specifications of the computed tomography scanner.
This method is useful for electrical switchgear manufacturers, control cabinet manufacturers, electrical installers, and turnkey project developers.
For electrical installation projects in the Middle East, the same basic principles apply for the selection of current transformers; however, there may be significant differences in project specifications and local conditions.
Typical application areas include:
Therefore, engineers must create complete technical specifications during the project procurement process before requesting a quote.
The current application form must contain the following information:
Primary current + Secondary current + Accuracy + Load + Window size + Frequency + Application + Amplitude
This allows manufacturers to offer technically comparable prices and shortens the clarification phase during the purchasing process.
Choosing a toroidal current transformer for a semiconductor drive is a technical decision that requires considering both electrical and mechanical parameters.
When designing low-voltage distribution boards, engineers must take the following elements into account:
Primary current
→ Secondary current
→ Degree of precision
→ Stress
→ CT window size
→ Cable/busbar dimensions
→ Frequency
metrology, monitoring and protection
The examples above illustrate why the appropriate current transformer cannot be selected based solely on the turns ratio.
800 A supply lines, 1600 A distribution lines, 2000 A busbar systems, and energy monitoring systems in commercial buildings may require different current transformer configurations, even if their primary applications appear similar.
Hoeyi’s RCT and MR series toroidal current transformers offer a wide range of turns ratios, dimensions, accuracy, and load combinations. The RCT series is available with current ratings up to 3000 A/5 A, while the MR series offers various frame sizes, secondary winding options from 1 A to 5 A, and high-current configurations up to 5000 A/5 A (see the product catalog for details).
For new electrical switchgear projects, it is essential to define all electrical and mechanical requirements to select the most suitable current transformer, rather than relying solely on the turns ratio.
Clearly defined technical specifications for current transformers simplify technical evaluation, reduce installation problems, and help electrical switchgear manufacturers select the right products before production begins.
Compay：HEYI Electrical Co., Ltd.
Brand: HEYI/ASCT
Contact: Bethy
E-mail: heyi@heyiele.com
Tel: 86-13968747975
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