In many existing systems, currents need to be recorded retrospectively. The reason is often very practical: individual machines are to be monitored, energy consumption is to be assigned more accurately, load peaks are to be detected or operating states are to be made visible. However, there is often one problem: the system is already wired, main cables are difficult to access and a complete modification would be time-consuming or involve downtime.
Current transformers offer a very helpful solution here. They allow the current of a conductor to be measured without the load current having to be routed directly through a measuring instrument. Split-core current transformers and Rogowski coils are particularly suitable for retrofitting because they can be placed around existing conductors. This makes it much easier to add measuring points in control cabinets or at machine outputs than with conventional closed-core current transformers.
This article explains which types of current transformers are available, when retrofitting is possible and what must be considered regarding space conditions, conductor diameter, measuring range, output signal, accuracy and safety clearances. It also shows why current measurement and energy measurement are not the same thing and which typical mistakes should be avoided when retrofitting current transformers.
Table of contents
- Why retrofit current transformers?
- How does a current transformer work?
- Closed-core current transformers: Precise, but often impractical for retrofitting
- Split-core current transformers: Ideal for existing systems
- Rogowski coils: Flexible solution for large conductors and high currents
- Closed-core current transformer, split-core current transformer and Rogowski coil in comparison
- When is retrofitting possible?
- Check space, conductor diameter and installation situation
- Choosing measuring range and transformation ratio correctly
- Output signal: 1 A, 5 A, mA, mV or Rogowski signal?
- Safety: Operating current transformers correctly
- Current measurement is not automatically energy measurement
- Machine monitoring and load analysis
- Typical mistakes when retrofitting current transformers
- Practical example: Recording the current consumption of individual machines
- Checklist for retrofitting
- Suitable product areas
- Conclusion: Current measurement can often be expanded without major system modifications
- FAQ: Frequently asked questions about retrofitting current transformers
Why retrofit current transformers?
In existing systems, measuring points for individual consumers are often missing. During the original installation, perhaps only the total current of the main distribution was considered. Later, however, the need arises to record the consumption of individual machines, lines, pumps, compressors, heating circuits or ventilation systems separately. This can be very helpful for energy management, maintenance, production optimization or fault analysis.
Direct current measurement by disconnecting the cable is usually not an attractive solution in such cases. Main cables are often large, difficult to access or permanently in operation. Opening cables causes downtime, requires intervention in the wiring and can create additional risks. Especially in production systems, measuring points should ideally be added without extensively modifying the existing system.
Retrofittable current transformers make exactly this possible. They detect the current via the magnetic field around the conductor. The load conductor remains connected, the circuit does not have to be routed in series through a measuring instrument, and the measurement can be added to existing conductors. Split-core current transformers and Rogowski coils are particularly suitable for such tasks.
Typical objectives of retrofitting are recording machine currents, assigning energy consumption, monitoring load states, detecting overloads, identifying load peaks or preparing an energy management system. Retrofitted current measurement can also be valuable for preventive maintenance because unusual current profiles can indicate mechanical or electrical problems.
How does a current transformer work?
A current transformer records the current of a conductor without the load current having to be routed directly through a measuring instrument. The conductor is passed through the current transformer or enclosed by it. The magnetic field of the current-carrying conductor generates a measuring signal in the current transformer that is proportional to the primary current.
In classic current transformers, the primary current is converted into a smaller secondary current, for example 100/5 A or 250/1 A. This means: at 100 A primary current, the transformer delivers 5 A on the secondary side, or at 250 A primary current, it delivers 1 A. This signal can then be evaluated by a measuring instrument, energy meter, signal converter or control system.
Other designs provide an mA, mV or voltage signal instead of a 1 A or 5 A signal. Rogowski coils generate a special signal that is often evaluated using an integrator or a suitable measuring instrument. For this reason, current transformers and evaluation devices must always be considered together.
The decisive point is: the current transformer measures the current of one individual conductor. If several conductors are routed through the transformer at the same time, the magnetic fields can cancel each other out. For normal load current measurement, the individual phase conductor or the individual relevant conductor must therefore always be measured, not the complete multi-core cable.
Closed-core current transformers: Precise, but often impractical for retrofitting
Closed-core current transformers have a fixed, closed magnetic core. The conductor must be routed through the opening of the current transformer. This design is robust, proven and can achieve very good accuracies. It is often used in new control cabinets, energy meters, current measuring instruments and permanently planned measuring points.
The disadvantage becomes apparent in existing systems. If the current transformer is closed, the conductor must be routed through the transformer for installation. In practice, this often means disconnecting the cable, sliding on the transformer and reconnecting the cable. With thick cables, busbars or running systems, this is often time-consuming or not possible.
Closed-core current transformers are therefore particularly suitable when the measuring point is planned during a new installation or scheduled modification. For simple retrofitting without dismantling the main cable, however, they are usually less suitable.
If maximum accuracy, defined installation conditions and permanent measurement are the priority and shutdown is possible, a closed-core current transformer can still be the right solution. For fast retrofitting in an existing control cabinet, however, split-core current transformers are often more practical.
Split-core current transformers: Ideal for existing systems
Split-core current transformers have an openable core. They can be placed around an existing conductor without dismantling it. This makes them particularly suitable for retrofitting in existing control cabinets, machine distributions and energy monitoring systems.
The major advantage is installation. The conductor remains connected, the current transformer is opened, placed around the conductor and closed again. This allows new measuring points to be added with significantly less effort than with closed-core current transformers. Especially in existing production systems, this can reduce downtime.
It is important that the split-core mechanism closes cleanly. Contamination, incorrect locking or a core that is not completely closed can reduce measurement accuracy. The conductor should also be routed as correctly as possible through the opening. Unfavorable positioning can also cause measurement deviations.
Split-core current transformers are available for different current ranges, conductor diameters, accuracy classes and output signals. When selecting them, it is therefore not enough to ask whether the transformer mechanically fits around the conductor. Measuring range, accuracy and output must also match the evaluation system.
Rogowski coils: Flexible solution for large conductors and high currents
Rogowski coils are flexible current sensors that are placed around a conductor or busbar. They do not have a massive iron core, but a flexible coil. This makes them particularly suitable for large conductor diameters, hard-to-reach locations, busbars or high currents.
A major advantage is flexibility. The coil can be placed around conductors where a classic current transformer does not fit mechanically. A Rogowski coil can also be a good retrofit solution in tight spaces or around large busbars.
However, Rogowski coils do not provide a classic 1 A or 5 A signal. Their signal usually has to be processed using an integrator or a suitable measuring instrument. They are also typically suitable for alternating currents and not for pure DC measurements. For energy measurement, load profiles and power network analyses, they are very useful if the evaluation device is chosen correctly.
Compared with classic current transformers, Rogowski coils are often insensitive to saturation and very well suited for high currents. At low currents, however, the resolution may be limited depending on the system. Correct positioning and proper closure of the coil are also important.
Closed-core current transformer, split-core current transformer and Rogowski coil in comparison
The following table shows the most important differences between the three designs. It helps with initial selection, but does not replace a specific technical design review.
| Design | Advantages | Limitations | Typical application |
|---|---|---|---|
| Closed-core current transformer | High accuracy, robust design, proven technology | Conductor must be disconnected or routed through for installation | New installation, planned control cabinet equipment, fixed measuring points |
| Split-core current transformer | Retrofittable without conductor dismantling, easy installation, many current ranges available | Core must close cleanly, space around the conductor must be considered | Retrofitting in existing control cabinets and machine outputs |
| Rogowski coil | Very flexible, suitable for large conductors and high currents, easy installation around busbars | Requires suitable evaluation, usually not for DC, limited at low currents | Main distributions, large consumers, temporary measurements, energy monitoring |
For typical retrofitting of individual machine outputs, split-core current transformers are often the first choice. For large busbars or high currents, Rogowski coils are very interesting. Closed-core current transformers remain useful when a new installation is planned or a very defined measuring point with high accuracy is to be built.
When is retrofitting possible?
Retrofitting is generally possible if the conductor to be measured is accessible and sufficient space is available for the current transformer. The transformer must mechanically fit around the conductor and be mounted safely. In addition, the transformer output signal must match the existing measuring instrument, energy meter or data acquisition system.
Retrofitting is particularly easy when individual phase conductors in the control cabinet are easily accessible. It becomes more difficult when cables are tightly bundled, several conductors are routed together, busbars are covered or safe clearances are not available. Installation in areas with high short-circuit power, high voltage or limited protection against accidental contact also requires special care.
Before retrofitting, it should also be clear what is to be measured. Is it only current monitoring? Should energy consumption be calculated? Should load peaks be detected? Is a measured value required for the control system? Or should data be recorded long-term for an energy management system? Depending on the objective, the requirements for accuracy, sampling rate, output signal and evaluation device change.
Work in control cabinets may only be carried out by qualified electricians or under their responsibility. Even if a split-core current transformer does not disconnect the conductor, this is still work near electrical equipment.
Check space, conductor diameter and installation situation
Mechanical installation is often the limiting factor when retrofitting. A current transformer must not only fit around the conductor, but also be mounted and wired safely. In existing control cabinets, conductors are often routed tightly, cable ducts are full, clearances are limited and busbars are only partially accessible.
The conductor diameter or busbar dimensions must match the opening of the current transformer. With split-core current transformers, there must also be enough space to open the transformer, place it around the conductor and close it safely. Rogowski coils are more flexible to install, but bending radius, closure and safe routing must also be considered.
The environment inside the control cabinet is also important. The current transformer must not obstruct moving parts, impair ventilation, make terminals inaccessible or reduce clearances below permissible limits. Secondary cables or signal cables must be routed cleanly, labelled and protected against mechanical stress.
| Check point | Why is it important? |
|---|---|
| Conductor diameter | The conductor must fit through the opening or into the split-core opening |
| Installation space | The transformer must be able to be opened, closed and mounted |
| Safety clearance | Clearances to live parts must be maintained |
| Cable routing | Secondary and signal cables must be routed safely |
| Accessibility | Maintenance, inspection and later troubleshooting must remain possible |
A technically suitable measurement can fail in practice if the installation situation has not been considered. For this reason, the mechanical check should always be carried out before selecting the current transformer.
Choosing measuring range and transformation ratio correctly
The current transformer must match the expected primary current. If the measuring range is selected too small, the transformer can be overloaded or the evaluation device can deliver incorrect values. If the measuring range is selected too large, the normal operating current lies only in a small part of the measuring range. This can reduce usable resolution and accuracy.
For machine monitoring, it is useful to know the typical operating current and possible starting or load peaks. A motor can draw significantly less current in normal operation than during starting. A heater has a different load profile than a compressor or pump. For energy monitoring, the typical operating range is often important; for protection or monitoring functions, peak values may be more important.
The transformation ratio must match the evaluation. A 100/5 A current transformer delivers a secondary current of 5 A at 100 A primary current. A 250/1 A transformer delivers 1 A at 250 A primary current. The connected measuring instrument must be set to this ratio or designed for it. Otherwise, incorrect values will be displayed.
With split-core current transformers with mA or voltage output, it must also be checked which input is available on the measuring instrument. An energy meter with a 5 A input does not automatically match a transformer with an mV output. Conversely, a signal converter with voltage input cannot easily evaluate a classic 5 A transformer.
Output signal: 1 A, 5 A, mA, mV or Rogowski signal?
The output signal is one of the most important selection criteria. Classic current transformers often deliver 1 A or 5 A as the secondary current. These signals are supported by many energy meters, measuring instruments and protection devices. For long cables and modern measuring systems, 1 A transformers are also often used because power loss can be lower.
Split-core current transformers are also available with 1 A or 5 A output, but also with mA or voltage output. These versions can be interesting for signal converters, data loggers, PLC inputs or compact energy measuring systems. It is always important that input and output match.
Rogowski coils require a suitable evaluation device or integrator. The signal of a Rogowski coil is not identical to the output of a classic current transformer. Before selection, it must therefore be clarified whether the existing energy meter or power analyzer supports Rogowski inputs or whether an additional signal converter is required.
| Output signal | Typical use | Note |
|---|---|---|
| 5 A | Classic energy meters and measuring instruments | Observe secondary circuit and burden |
| 1 A | Measuring instruments, longer cables, lower losses | Measuring instrument must be designed for 1 A |
| mA | Signal converters, data loggers, control systems | Check signal type and scaling |
| mV / voltage | Compact measuring systems, special inputs | Do not confuse with 1 A or 5 A inputs |
| Rogowski signal | Power analyzers, energy monitoring, high currents | Integrator or suitable input required |
Many measurement errors are not caused by the current transformer itself, but by an incorrect combination of transformer, output signal and measuring instrument.
Safety: Operating current transformers correctly
When operating current transformers, safety must be given special attention. Classic current transformers with secondary current output must not be operated open-circuit while the primary conductor is carrying current. An open secondary circuit can generate dangerously high voltages and damage the transformer.
Current transformers must therefore be connected correctly, short-circuited or routed via suitable short-circuit terminals when measuring instruments are replaced or cables are disconnected. This rule is particularly important for 1 A and 5 A current transformers. Before working on the secondary circuit, it must be clear whether the transformer is safely short-circuited.
For split-core current transformers with integrated electronics, mA output or voltage output, different rules may apply depending on the design. Nevertheless, manufacturer specifications, permissible loads, insulation requirements and measurement categories must be observed. Even with retrofittable sensors, installation in a control cabinet is work on an electrical system.
The correct installation direction must also be observed. Many current transformers have a marking for P1/P2 or an arrow direction. If the transformer is installed the wrong way around, the sign of the measurement can be incorrect. With pure current indication, this may not be noticed; with power measurement and energy measurement, however, it can lead to incorrect or negative values.
Current measurement is not automatically energy measurement
A retrofitted current measurement shows how much current flows through a conductor. However, the exact energy consumption cannot automatically be derived from this. For electrical power and energy, voltage, phase angle and power factor must also be considered. Especially with motors, frequency inverters or inductive loads, current alone is not sufficient to determine energy consumption accurately.
For simple load monitoring, current measurement can be sufficient. If a machine normally draws 20 A and suddenly draws 30 A continuously, this is an important indication. For an energy management system or cost center allocation, however, an energy meter or power analyzer is usually required, which evaluates current and voltage signals together.
For three-phase consumers, the current transformers must be assigned to the correct phases. L1, L2 and L3 must not be mixed up, and the current direction must match the voltage measurement. Otherwise, the energy meter can calculate incorrect power values or display negative power.
Anyone who wants to record the energy consumption of individual machines should therefore not only select current transformers, but consider the complete measuring system: current transformers, voltage inputs, energy meter, communication interface, measuring interval, data logger and evaluation.
Machine monitoring and load analysis
Retrofitted current transformers are very suitable for making machine states visible. The current profile shows whether a machine is running, how heavily it is loaded and whether the operating state is changing. For pumps, compressors, fans, conveyor belts, mills, heaters or drives, current can be an important diagnostic value.
An increasing current can indicate mechanical stiffness, clogged filters, worn bearings, blocked conveying technology or a higher process load. An unusually low current, on the other hand, can indicate idle operation, belt breakage, missing load or a failed component. In combination with operating data, current measurement can therefore be used for preventive maintenance.
Load peaks can also be detected if the measuring system is fast enough and stores corresponding values. This can help identify causes of tripped circuit breakers, overloaded outputs or high power peaks. For very short inrush currents or dynamic processes, however, it must be checked whether the current transformer and evaluation device record quickly enough.
Current measurement is therefore not only interesting for energy consumption, but also for system availability and process monitoring.
Typical mistakes when retrofitting current transformers
A common mistake is enclosing several conductors. If outgoing and return conductors are routed together through the current transformer, the magnetic fields largely cancel each other out. The displayed current is then much too low or almost zero. For normal load current measurement, one individual conductor must always be measured.
Another mistake is choosing the wrong measuring range. A transformer that is too large delivers only small signal portions in the normal operating range. This can make the measurement inaccurate or less meaningful. A transformer that is too small can be overloaded or provide incorrect values at peak currents.
Incorrect assignment of current and voltage is also critical in energy measurements. If the current transformer of L1 is calculated with the voltage of L2, incorrect power values result. Incorrect installation direction likewise leads to negative or implausible measured values.
The open secondary circuit of classic current transformers is particularly critical. If a 1 A or 5 A transformer is opened on the secondary side during operation, this can be dangerous. Short-circuit terminals, safe wiring and clear labelling must therefore be considered.
| Mistake | Possible consequence | Better approach |
|---|---|---|
| Several conductors enclosed at the same time | Measured value is incorrect or almost zero | Measure only the individual relevant conductor |
| Measuring range selected too large | Poor resolution in the operating range | Select transformer to match the typical operating current |
| Measuring range selected too small | Overload or incorrect values at load peaks | Consider starting and peak currents |
| Output signal does not match the measuring instrument | No evaluation or incorrect evaluation | Select transformer and evaluation device together |
| Incorrect installation direction | Incorrect sign or negative power | Observe P1/P2 direction and phase assignment |
| Secondary circuit open | Dangerous voltage and possible damage | Use short-circuit terminals and safe wiring |
Practical example: Recording the current consumption of individual machines
In an existing production system, the energy consumption of individual machines is to become more transparent. So far, only the total consumption of the hall has been recorded. The operator wants to know which machines draw particularly high currents, when load peaks occur and whether individual consumers continue running outside production hours.
A complete modification of the main distribution is not an option because the system only has short maintenance windows. The main cables should not be dismantled. For this reason, it is checked whether split-core current transformers can be retrofitted at the machine outputs.
In the control cabinet, the individual phase conductors of the machine outputs are identified. For each relevant machine, it is checked whether there is enough space for three split-core current transformers and whether the conductor diameters fit the transformer openings. Suitable transformers with the appropriate measuring range are then selected and connected to an energy measuring device.
During commissioning, the phases are assigned correctly, the current direction is checked and the transformation ratios are set in the measuring instrument. The measured values are then compared with known operating states. When the machine is idling, it shows a lower current. Under load, the current increases. During downtimes, it becomes visible whether consumers are still active.
After a short time, the operator realizes that a compressor regularly starts at the same time as several machines, causing load peaks. By adjusting the start sequence, these peaks can be reduced. The retrofitted current transformers therefore provide not only measured values, but also a basis for concrete energy and operating optimization.
Checklist for retrofitting
Structured preparation makes it easier to select and install retrofittable current transformers. The following checklist helps avoid typical mistakes.
| Step | Check question |
|---|---|
| Clarify measuring objective | Should current, power, energy or machine state be monitored? |
| Identify conductor | Which individual conductor must be measured? |
| Check installation space | Does a split-core current transformer or Rogowski coil fit mechanically into the control cabinet? |
| Check conductor diameter | Does the conductor fit through the transformer opening? |
| Define measuring range | Do operating current, starting current and peak current match the transformer? |
| Check output signal | Does 1 A, 5 A, mA, mV or Rogowski signal match the measuring instrument? |
| Check phase assignment | Are current and voltage inputs correctly assigned? |
| Observe installation direction | Is P1/P2 or the arrow direction correct? |
| Secure secondary circuit | Are short-circuit terminals or safe connection conditions provided? |
| Create documentation | Are transformer data, transformation ratio, measuring point and wiring documented? |
Suitable product areas
For retrofitting in existing control cabinets, split-core current transformers are particularly suitable. They can be placed around existing conductors and therefore enable current measurement without dismantling main cables. They are a good choice for machine outputs, sub-distributions and retrofitted energy measuring points.
For large conductors, busbars or high currents, Rogowski coils are interesting. They can be flexibly routed around conductors or busbars and are particularly suitable for energy monitoring, power network analyses and measurements in main distributions. A suitable evaluation device or integrator is important.
If measuring points are newly built or a very defined fixed installation is possible, closed-core current transformers remain a robust and accurate solution. They are especially useful when the wiring is being changed anyway or the current transformer can already be taken into account during control cabinet planning.
Depending on the objective, energy meters, current measuring instruments, power analyzers, signal converters, data loggers or PLC inputs can be used for evaluation. The decisive factor is that current transformer, output signal and measuring instrument technically match each other.
Conclusion: Current measurement can often be expanded without major system modifications
Retrofitting current transformers is a practical way to make existing systems more transparent. Split-core current transformers and Rogowski coils in particular make it possible to add current measurements without dismantling main cables. This allows energy consumption to be recorded, machines to be monitored and load peaks to be detected.
However, selection should be carried out carefully. Design, conductor diameter, installation space, measuring range, output signal, accuracy, phase assignment and safety must be considered together. Especially with classic current transformers with 1 A or 5 A output, safe handling of the secondary circuit is important.
Anyone who wants to retrofit current transformers should therefore not only ask which transformer fits mechanically. The complete measuring task is decisive: What should be measured, how accurate must the value be, which measuring instrument evaluates the signal and how is the measuring point safely documented? Then current measurement in the control cabinet can often be expanded easily and without major system modification.
FAQ: Frequently asked questions about retrofitting current transformers
Can current transformers be installed retrospectively?
Yes, split-core current transformers and Rogowski coils are particularly suitable for retrofitting. They can be placed around existing conductors without dismantling them. However, the installation must match the installation situation and the measuring system.
What is the advantage of a split-core current transformer?
A split-core current transformer can be opened and placed around an existing conductor. This means the cable does not need to be dismantled. This is particularly practical in existing control cabinets and systems.
When is a Rogowski coil used?
Rogowski coils are useful for large conductors, busbars, high currents or tight installation situations. They are flexible and easy to retrofit, but require a suitable evaluation device or integrator.
What is the difference between a closed-core current transformer and a split-core current transformer?
A closed-core current transformer has a fixed core and requires the conductor to be routed through the transformer. A split-core current transformer can be opened and placed around the conductor retrospectively.
Do I have to disconnect the cable for a split-core current transformer?
No. The conductor does not have to be disconnected. The split-core current transformer is opened, placed around the conductor and closed again. Nevertheless, work in the control cabinet may only be carried out by qualified persons.
Can several conductors be routed through one current transformer at the same time?
For normal load current measurements, no. One individual conductor must be measured. If outgoing and return conductors are recorded together, the magnetic fields largely cancel each other out and the measured value is incorrect.
Which current transformer size do I need?
The measuring range should match the typical operating current and at the same time take starting or peak currents into account. A measuring range that is too large reduces resolution, while one that is too small can be overloaded.
What does 100/5 A mean for a current transformer?
A 100/5 A current transformer delivers a secondary current of 5 A at 100 A primary current. The measuring instrument must be set to this transformation ratio or designed for it.
Can I measure energy consumption with current transformers?
Yes, but the current transformer alone is not sufficient. For energy consumption, voltage, phase angle and power factor must also be recorded. An energy meter or power analyzer is required for this.
Why is the installation direction important?
The installation direction determines the sign of the measurement. In energy and power measurement, an incorrect direction can lead to negative or incorrect values. Markings such as P1/P2 or arrows must be observed.
Why must the secondary circuit of a current transformer not be open?
With classic current transformers with 1 A or 5 A output, an open secondary circuit during operation can generate dangerously high voltages. Suitable short-circuit terminals and safe connection conditions must therefore be used.
Which products are suitable for current transformer retrofitting?
For retrofitting, split-core current transformers, Rogowski coils, energy meters, current measuring instruments, power analyzers and signal converters are particularly suitable. The exact selection depends on conductor size, current range, output signal and desired evaluation.