Tips for correctly installing current transformers to achieve maximum effectiveness
You have thoroughly studied the sub meter, current transformer, and communication options to determine the perfect equipment suitable for your project. Next, it's time to consider the installation process of the current transformer. Even if your project does not rely on accuracy in revenue levels to achieve success, you can take some simple steps to ensure smooth installation of current transformers and maximize performance.
Please remember that the installation of current transformers and meters should always be carried out by trained and qualified professionals, and comply with all local electrical regulations.
Ensure CT size is appropriate
The project parameters are constantly changing, so before installing a current transformer, it is important to verify that the existing CT is still a good choice for your application and that their size is suitable for the load you want to measure. There are two key factors in determining CT size: rated current and physical size.
The rated current represents the number of amperes that a specific CT can measure. In most cases, CT maintains a specified accuracy of 5% to 120% of its rated current. This means that if the rated current of the CT is 100A, it can be used for services ranging from 5A to 120A. When below 5A, the CT will not be as accurate, while above 120A, the CT may begin to saturate, which will once again produce inaccurate readings.
In addition to the current rating, the physical size of the CT is also important. Installing a current transformer on a conductor that is too large for the window means that the CT will not be able to close properly, resulting in incorrect
readings. It is also not recommended to have CT dimensions that are too large. Generally, the window of CT should not exceed 50% of the conductor size to maintain optimal accuracy.
Check wire polarity
After installing the CT around the wire, it is time to connect it to the power meter. If CT includes wires, color coding will indicate the polarity of the wires. Usually, the white wire is the positive pole and the black wire is the negative pole, which must be connected to the positive (+) and negative (-) terminals of the instrument respectively. If your CT uses other color schemes, such as white and brown, please check the CT's documentation before connecting.
Just like phase orientation, if the wire is connected "backwards" to the meter, the meter will measure the negative energy of that phase, so it is important to take time to make the correct connection.
Ensure that the CT is located on the correct wire
This may seem like an obvious detail, but in order for the meter to collect accurate data, the CT must be installed in the same phase as the voltage input. Mismatched CT and voltage inputs will result in incorrect measurement results, so it is particularly important to verify the installation of current transformers before leaving the work site.
Marking CT wires to ensure proper connection to the correct terminals on the instrument is a beneficial best practice. For crowded panels or switchgear, this is crucial for tracking which wires lead to which terminals. If troubleshooting is required after installation, it can also save time.
Do not excessively extend the CT wire
Generally speaking, wiring of similar specifications (usually twisted pair) can be used to extend the lead of CT. In order to minimize noise or interference to the maximum extent possible and ensure that the maximum load of the CT is not exceeded, it is important not to extend the leads beyond the actual range required for installation. In other words, if the distance between the instrument panel and the distribution board is 20 feet, 50 feet of leads are not required, which will have a negative impact on accuracy (and increase unnecessary costs). To maintain optimal performance, the shorter the cable line, the better.
Check your work
The installation process may be time-consuming, but pausing for a period of time to verify that the installation is completed correctly is the best way to prevent expensive returns to the site to correct issues. Before you leave, please check if the instrument reading is "meaningful" to the load you are measuring. This step usually reveals installation issues with current transformers, instrument settings, or other configuration traps.