Application and selection of current transformers for low voltage distribution boards: LMZ and LMZJ models.

Choosing a current transformer for a low-voltage distribution board is a complex task that simply involves matching the primary current to the 5 A input current of the meter. The transformer must also be compatible with the wiring, meet the measurement requirements, and be able to operate with appropriate secondary loads. Failure to meet any of these criteria can lead to problems during the installation or commissioning of the distribution board.

The Heyi LMZ and LMZJ series current transformers are used as auxiliary devices for AC current measurement in low-voltage indoor environments. These transformers are characterized by their panel-mount design, fixed mounting, and availability in various turns ratios and chassis sizes. This guide explains how engineers evaluate these current transformers and what criteria to consider when selecting a specific model.

In what situations can the LMZ and LMZJ current transformers be used?

Food measurement

In distribution boxes, current measurement allows operators to assess loads on power lines and compare phase currents. Current transformers provide current signals compatible with ammeters or compatible multifunction measuring devices.

When choosing an LMZ or LMZJ current transformer, the supply current and conductor routing are the most important criteria. A suitable current rating is only relevant if the cable or busbar can be routed through the opening and if the current transformer can be installed in the available space. Always consult the installation plans before finalizing your electrical cabinet design.

The energy consumption of factories and buildings is measured separately.

Separate electricity meters may be required for workshops, production areas, lighting, or building equipment. When used with compatible meters, current transformers (CTs) provide the necessary input current to these metering points.

This measuring device combines current and voltage measurements to calculate power and energy. Communication, data logging, and control panel functions depend on the measuring device and monitoring system. Current transformers (CTs) ensure the transmission of the current signal.

In projects with variable loads, the operating time of the circuit at a current lower than its nominal value must be taken into account. The required characteristics of the measuring instruments must correspond to these operating conditions and the accuracy requirements of the project.

Monitoring of motors, compressors and supply lines of heating, ventilation and air conditioning systems.

By measuring the current in the power supply circuits of motors, compressors, and air conditioning systems, operators can monitor loads and compare operating conditions. Engineers can evaluate LMZ and LMZJ models to determine if their electrical characteristics, permissible frequency, and installation dimensions are compatible with the circuit design.

For devices operating with a frequency converter, it is important to distinguish the AC input from the converter output. Standard industrial current transformers are not suitable for pulse-width modulation (PWM) output signals. It is imperative to always specify the target measurement point.

What are the specific provisions contained in the LMZ and LMZJ manuals?

This product catalog contains the following basic information to assist you in your initial selection. Availability and final specifications will be confirmed based on transmission ratios, accuracy classes, and load combinations.

Article	Reference information
installation environment	Suitable for indoor use
Construction and assembly	Molded construction with mounting plate.
The nominal voltage is specified in the parameter table.	0.5 kV
The diagram indicates the nominal current of the secondary winding.	5A
The figure illustrates the range of LMZ ratios.	5/5 A to 300/5 A
The LMZJ relationship group is indicated in the technical table.	5/5A to 300/5A; 400/5A to 600/5A; 800/5A to 5000/5A
The technical specifications list different accuracy options.	Steps 0.5, 0.2 and 0.2S
Displayed charge	For small groups of devices, an output power of 5 VA is used; for large groups, 5 to 30 VA.

 

The serial number and rated voltage do not replace the complete insulation specifications. Please ensure that the voltage and insulation requirements comply with the switchgear design specifications. Furthermore, the compatibility of the selected model with the required frequency must be verified.

How should engineers choose an appropriate power conversion ratio?

First, determine the normal operating range, the maximum continuous current, and the expected load fluctuations. Select the secondary winding rating based on the characteristics of the receiving device; all models in the catalog above are equipped with a 5 A secondary winding.

For a single-phase 400/5 A current transformer with a primary current of 200 A, the ideal secondary current is:

Secondary current = 200 × 5 ÷ 400 = 2.5 A

To accurately display the primary current, the measuring device must be configured according to the turns ratio of the installed current transformer. Even if the transformer and circuit are functioning correctly, an incorrect turns ratio will result in inaccurate measurements.

Do not choose an excessively high power rating for the main unit in order to increase backup power. Verify the expected low-load operating point and validate the accuracy of the data. If the power ratio required for the project is not explicitly specified, request a solution for a specific model rather than assuming that all intermediate values are available.

Why is it necessary to check the level of accuracy and the verification load simultaneously?

The product catalog lists various measurement accuracy options; however, the accuracy class must be evaluated based on the specific current range and auxiliary load conditions. A lower accuracy class alone does not indicate the accuracy of the entire measurement system.

The secondary circuit includes the device’s input, output, and return lines, as well as the connecting elements. For a circuit consisting primarily of resistors, the load at rated current on the secondary side can be estimated as follows:

Power at the terminals (VA) = Secondary current carrying capacity² x Total resistance

Let’s take this example. These are hypothetical project parameter values, not test results on LMZ or LMZJ models.

Hypotheses	Value
Nominal secondary current	5A
Meter input load at nominal current	0.5 volt-amperes
Total conductor resistance, output current and return current	0.10 ohms
Load conducted with non-nominal secondary current	5² × 0.10 = 2.5 VA
Total load expected before taking into account losses in other connections.	3.0 volt-amperes

 

A 5 VA current transformer may require additional testing. However, this calculation alone does not guarantee compliance with the stated accuracy specifications. Please verify the permissible load range, relevant power factor conditions, and stated accuracy specifications for the model in question. Where applicable, account for connection losses and losses in the test module.

For long secondary conductors, the cable resistance must be calculated based on its actual cross-section, length, and expected operating temperature. If all distribution boxes use the same conductor cross-section, the load can vary considerably depending on the conductor length.

How do window size and installation method influence your choice?

Check the complete wiring diagram for all conductors, including insulation and parallel conductors that will pass through windows. Consider mounting rails, clearance for conductor bends, and current transformer fixings.

The clearance between the window and the enclosure will be checked separately. Cables can pass through the opening; however, the current transformer enclosure may interfere with adjacent equipment or the electrical panel structure. Please refer to the drawings of the proposed model, which specify the dimensions, including the location of the mounting holes and the secondary winding entry points.

The devices listed in the catalog are equipped with weatherproof windows. During installation, please check the possibility of connection via the current transformer. For existing circuits where the wiring cannot be disconnected or routed through a window, please contact the supplier for information on suitable alternative solutions, such as detachable cables.

What should I check during installation and troubleshooting?

Qualified personnel must install and maintain current transformers in accordance with the project’s electrical safety standards. Using the approved wiring diagram, verify the primary winding direction, secondary winding polarity, and phase sequence. Incorrect polarity or phase reversal of the input current will distort power and energy measurements.

The secondary circuit of a current transformer must never be disconnected while the primary current is flowing. If disconnection of the device is necessary, follow the approved maintenance procedure and use the disconnection and short-circuit protection device prescribed for the current transformer.

During commissioning, please verify that the current transformer’s nameplate conforms to specifications, confirm the correct fill factor setting, and check the secondary winding connections. If necessary, compare the measured values to the corresponding reference values and consult the provided test documentation.

Frequently Asked Questions

Is it possible to connect LMZ and LMZJ current transformers to a multifunction electricity meter?

You can evaluate the performance of meters compatible with an input current of 5 A. Before choosing a meter, please check the current transformer transformation ratio, the meter’s input load, accuracy requirements, connection type, and electrical characteristics.

Can a class 0.2S current transformer convert the entire system to class 0.2S current?

No. System performance also depends on the operating conditions of the meters and current transformers, the secondary wiring, and the installation. The entire measurement chain must be evaluated according to the project requirements.

Is it possible to select a current transformer based solely on the cable diameter?

No. A space must be left around the window. In addition, the transmission ratio, accuracy, load, permissible voltage, frequency, and installation dimensions must be suitable for the application.

Are all these models suitable for protection relays?

The stated measurement value does not indicate the effectiveness of the short-circuit current protection. For a specific protection application, it is necessary to determine the required protection characteristics and the exact technical specifications of the current transformer (CT).

Is it possible to install these power transformers without disconnecting the existing cables?

The closed-window model shown in the diagram requires wiring. If wiring is not possible, a split-current transformer should be used. The transformer should be selected based on parameters such as the turns ratio, window size, accuracy, and load.

Please request the LMZ or LMZJ option for your group.

To receive a quote tailored to HEYI current transformers, please provide the following information: primary current range, desired transformation ratio, accuracy class, system voltage and frequency. Please also specify the device model and input load, secondary cable length and cross-section, conductor dimensions, electrical cabinet configuration, and quantity required.

This information allows us to verify electrical compatibility and suitability for your installation before providing you with a quote. To view the LMZ and LMZJ series catalogs and select specific models, please contact HEYI.

Compay：HEYI Electrical Co., Ltd.
Brand: HEYI/ASCT
Contact: Bethy
E-mail: heyi@heyiele.com             
Tel: 86-13968747975

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