A current transformer that looks right on paper can still create problems on site. In industrial switchboards and control panels, the wrong CT ratio, class or window size can lead to poor metering, nuisance alarms or protection that does not behave as intended. This Stemar current transformer selection guide is written for engineers, OEMs and maintenance teams that need to specify the right device the first time.
Stemar current transformers are typically selected for practical plant requirements rather than theory alone. The application matters. A CT used for energy metering in a distribution board has a different job from one feeding a protection relay on a motor circuit or monitoring load in a process line. Good selection starts with the actual operating conditions, not just the nominal current on a drawing.
What to check first in a Stemar current transformer selection guide
The first question is simple - what is the CT meant to do? In most industrial applications, the answer falls into metering, monitoring or protection. That purpose drives nearly every other decision, including ratio, accuracy class, burden and whether the CT needs to handle fault conditions without unacceptable saturation.
For metering, accuracy at normal operating current is usually the priority. If the CT is feeding an energy meter, power analyser or BMS input, stable performance across the expected load range matters more than fault response. For protection, the requirement shifts. The CT must reproduce current reliably enough for the relay to detect abnormal conditions, including overloads and short circuits, without distorting too early.
There is often overlap. A single installation may need one CT for revenue-style metering and another for protection, even on the same feeder. Trying to make one CT do both jobs can work in some cases, but it depends on the relay requirements, metering accuracy target and available space.
Start with the primary current, not the breaker size
A common mistake is selecting the CT ratio from the protective device rating rather than the actual load profile. If a feeder is protected at 400 A but normally runs at 120 A to 180 A, a 400/5 CT may leave the meter operating too low in its effective range for best accuracy. In that case, a lower ratio may be more suitable if the system design permits it.
The goal is to choose a ratio that reflects normal operating current while still accommodating expected peaks. Oversizing the ratio reduces measurement resolution at the low end. Undersizing it can cause saturation or secondary overcurrent if the load exceeds expectations. For plants with variable loading, measured historical current is usually more useful than nameplate assumptions.
Ratio, secondary and burden
In any Stemar current transformer selection guide, ratio selection sits at the centre of the process. The primary-to-secondary ratio determines how the measured line current is reproduced for downstream instruments. Common secondary values are 5 A or 1 A, and the choice depends largely on the receiving device and cable run.
A 5 A secondary has long been standard in many industrial systems, but 1 A secondaries can reduce losses on longer runs because the secondary current is lower. That can help where CTs are mounted remotely from meters or relays. The trade-off is compatibility. The instrument or relay must be designed for the selected secondary, and mixed assumptions in a project can create expensive commissioning issues.
Burden is the total load connected to the CT secondary, usually expressed in VA. It includes the input burden of the meter or relay as well as the resistance of the wiring. If burden is underestimated, the CT may not maintain its stated accuracy. This is especially important in protection applications, where long cable runs and multiple connected devices can push the total burden beyond what looks acceptable at first glance.
As a practical rule, calculate burden early rather than treating it as a final check. Secondary cable length, conductor size and panel layout all affect the result. A CT that is suitable in a compact MCC cubicle may not suit the same ratio and class in a remote marshalling arrangement.
Accuracy class depends on the job
Accuracy class should match the application rather than defaulting to the highest number available. For metering, the selected class needs to support the required measurement confidence across the operating range. For protection, the class must support relay performance under abnormal conditions.
Higher accuracy or specialised protection characteristics usually come with cost and space implications. They may also be unnecessary for straightforward load indication or trend monitoring. On the other hand, choosing a basic metering CT for a protection relay can create a hidden weakness in the system. This is one of the clearest examples where cheaper initial selection can cost more in fault finding and downtime later.
Mechanical fit matters as much as electrical fit
The CT still has to fit the conductors and the enclosure. Window type, busbar type, split core or solid core construction, mounting method and clearance all need to be checked against the actual installation. Drawings often show a neat single conductor passing through a CT, but retrofit work rarely looks that tidy.
Window size should allow for conductor size, insulation, spacing and practical installation tolerance. A CT with a nominally adequate opening can become unworkable once cable lugs, heatshrink or busbar supports are considered. If the project involves retrofit installation during a shutdown, installation speed may point towards a split core option. That said, split core CTs are not automatically the best choice in every case. They can simplify fitment, but performance, mounting stability and environmental conditions still need to be assessed.
For busbar applications, dimensions and mounting orientation are particularly important. Mechanical stress, vibration and access for secondary termination can all affect long-term reliability. In mining, process and heavy industrial environments, these practical details should not be left to site improvisation.
Environmental and compliance considerations
Industrial electrical panels do not all live in clean indoor conditions. Temperature, dust, humidity, vibration and chemical exposure can influence product choice, especially in infrastructure, water treatment and process plants. If the CT is going into a harsh environment or an outdoor enclosure, confirm that the selected model suits those conditions.
Compliance requirements also need attention. Depending on the project, this may include switchboard standards, utility metering expectations, protection scheme requirements or site-specific engineering standards. A technically workable CT is still the wrong choice if it does not meet the project specification or documentation needs.
Metering versus protection in real installations
This is where selection becomes application-specific. On a motor feeder, the CT may be used by a protection relay to detect overload, phase imbalance or fault current. In that case, relay settings, fault levels and transient behaviour matter. On a plant utility incomer, the CT may support power monitoring, demand trending and energy management. There, low-current accuracy and stability over time may carry more weight.
Where both functions are required, separating them often produces a cleaner result. Dedicated metering CTs and dedicated protection CTs allow each device to be chosen for its real duty. It uses more panel space and may increase initial cost, but it can simplify commissioning and avoid compromises that show up later.
Common selection errors to avoid
Most CT issues in the field come back to a short list of specification errors. The first is choosing ratio by guesswork rather than measured or expected load. The second is ignoring burden, especially cable burden. The third is treating metering and protection as interchangeable.
Another recurring issue is secondary mismatch. A 1 A relay input and a 5 A CT will not sort itself out during commissioning. Nor will a physically awkward CT become easier to install once the shutdown window starts. The basic checks are not complicated, but they do need to happen early enough to influence the design.
It is also worth remembering that future operating conditions may differ from today’s. If a plant expansion is likely, selecting a ratio with no headroom can create a premature upgrade problem. Too much headroom is also undesirable, so the answer is rarely to simply oversize everything. Good CT selection sits in the middle - realistic, supported by load data and aligned with the actual function.
Using a Stemar current transformer selection guide in projects and maintenance
For new projects, the best approach is to review one-line diagrams, load estimates, device input requirements and panel layout together. CT selection should not be isolated from the protection study or metering architecture. For maintenance and replacement work, start by checking what the existing CT is doing before assuming an exact like-for-like replacement is correct. Many legacy panels contain CTs that were selected around past plant conditions, not current ones.
Where there is uncertainty, technical review is worth the time. A few details resolved at specification stage can prevent rework, false readings and protection concerns after energisation. That is particularly true when dealing with mixed instrumentation, long secondary runs or retrofit constraints.
The right CT is not just the one that matches the current on the label. It is the one that fits the conductor, suits the device it feeds, performs under the expected burden and supports the way the plant actually operates. If your application has more than one of those variables in play, getting engineering input early usually saves time where it counts - on site.