Transformer

Some Tips for Choosing the Right Current Transformer You Want

Nothing is more frustrating than completing the project site to complete the instrument installation, just to realize that your kit does not have the correct current transformer. There is no tool for any job, a waste of time and money. In order to avoid this situation, some pre-planning a long way to go.

Current transformers are available in a variety of styles, sizes, and amperage ranges. They vary in output and accuracy as well. With so many variables, it is sometimes difficult to choose exactly the right CT for a project. Or is it?

Answer the Following Questions to Find the Right Current Transformer

Selecting the right current transformer can be as easy as answering a few questions about your project, the site, and the goals. You may be able to answer some of these questions before even setting foot on your project site. Others, such as knowing whether there is a space constraint in your electrical panel, are best answered after a site visit. Answer a few questions up front to save major headaches down the road.

Question 1: What type of power meter are you using?

ELITEpro XC in Electrical PanelOne thing to keep in mind is that just because a current transformer is compatible with a meter does not mean that it's the best choice. For example, did you know that all DENT CTs are compatible with both ELITEpro and PowerScout series instruments? Even though they work together, a clamp-on CT isn't the best choice to use with a PowerScout. Why? Because part of the appeal of a clamp-on CT in the first place is that it's easy and convenient to move between panels. In fact, you're paying more for that extra convenience. The PowerScout, along with other submeters in the industry, are designed to be permanently installed so why pay for the convenience of a clamp-on when you're not moving it anyway?

Some ways meter choice impacts CT choice:

    CT inputs- is your meter designed for mV output CTs or amp output? Common industry standards are 333mV, 1A, or 5A. DENT meters are compatible with 333mV.

    Will the meter be installed permanently (such as with a PowerScout or other submeter) or will you be moving the meter from location to location (such as with energy audits)?

    Does the meter have the ability to work with flexible Rogowski coils either by themselves or with an amplifier/integrator?

Question 2: How many amps do you plan on measuring?

Perhaps one of the most important questions to answer is how many amps will be measured. You will typically know this ahead of a site visit because it's typically dictated by your project goals. If your goal is to measuring a lighting load in a small office, the CT required will be much smaller than if you plan on measuring a full building load for a large complex.

Keep in mind that best CT performance occurs when the current flow is between 10% and 100% of the CT full-scale value. For example, let's say you wanted to measure four lighting circuits with #12 wires and 20A breakers. When the lights are on, the amperage measures 45 amps. The ideal CT for this example is a 50A split core.

But what about a Rogowski coil? They are easy to install and work over a broad range. Keep in mind that the best CT accuracy occurs when the load operates as close to the full rating of the CT as possible. If the load is under 20A, generally speaking a Rogowski coil is not the right choice because it is simply too big for that load. In addition, current values below 5A may cause the meter to read 0 amps.

What happens if you move the meter between many different loads? In this case, sometimes the best solution is to keep two sets of different CTs in the kit, one for small loads (for example, a set of 50A split cores) and the other for larger loads , Such as Rogowski coil. In this way, you are covered by many different environments. That’s the most important step for choosing the right current transformers.

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