By: Allan Evora
Demystifying the Confusion Around Which CT to Use.
We find that a lot of installers and end users get confused by selecting and installing current transformers (CTs). I hope to dispel some of the mystery behind CTs.
The CT is instrumental in measuring electricity. Any meter that gets installed will require you to connect directly to voltage and current. Because you can’t directly read the current, you need a current transformer to read and measure the current for you.
What Size Current Transformer Should I Specify?
CTs come in all shapes and sizes. The size is usually directly proportional to the amount of current you need to measure. The more current, the larger the CT. The window, or hole in the middle of the CT, is also an important distinction. To correctly measure current, the CT has to be able to wrap around the entire conductor of the current being measured. Typically, larger amounts of electricity require larger wires, so it’s important to keep window size in mind.
What Style Current Transformer Should I Specify?
We use three types of current transformers that come in all sizes.
Split Core CT:
A split core CT breaks in half and reattaches. This allows installation within existing, live environments without changing or unplugging wiring. Because there is a break in the coil inside the transformer, they’re not considered as accurate as a solid core CT.
Rope CT: Just like a split core CT, the rope CT detaches in half for easy retrofit installation. Because the rope is so thin, it gives you a much larger window to fit wires through, plus it’s extremely flexible. When working in tight spaces, it’s a lot easier to work with than a rigid split core CT. Retrofit situations sometimes have many wires in the distribution panel that you’re forced to route your CT behind. Rope CTs are great for those retrofit environments.
Solid Core CT: A solid core CT tends to be more accurate because there is no physical break in the coil inside of the transformer. They’re also cheaper. With no moving parts or hardware connections, they cost less to manufacture. The downside is, they’re not ideal for retrofit situations. In order to get a solid core CT around the wire, you must disconnect the wire. Disconnecting a wire also means you have to arrange for an outage, depending on the electricity, load type, and OSHA or Arc Flash ratings associated with the panel.
What CT Works Best With My Meter?
Not all CTs go with all meters. Some CTs are made where the input to the meter is measured in current (amps). Others read in millivolts. You must match whatever CT you choose to the meter you’re specifying, or already have installed.
What CT Accuracy Should I Specify?
Even within families of current transformers, there are different accuracies. For example, some CTs are specifically meant for metering purposes, and others are meant for protective relaying.
- A metering CT will be more accurate across the full range of the instrument, meaning it will be accurate at both low current readings as well as mid and high current readings.
- A CT designed for relaying purposes may not be as accurate at the lower levels. Typically, protection schemes protect against overcurrent. So, CTs meant for protective relaying are more accurate around higher ampacities or higher loads.
We see a lot of instances where an engineer specifies a revenue-accurate meter, but doesn’t specify CT accuracy. Unfortunately, in the construction world, not specifying a piece of equipment means the cheapest one will be purchased. That means if you spend the money on a revenue-accurate meter, you’ll waste it with a less accurate CT. Just remember, the CT accuracy will be the limiting factor on your measurements.
Remember: There’s a Right Side and a Wrong Side on a CT
The last thing I’d say about current transformers is that they are directional. If you look at most CTs, there will be a label that will tell you which side should point toward the source of power. The opposite of the source of power is the load, or the consumer of the power.
A simple example is a lamp plugged into the wall. The light is the load and the receptacle in the wall is the source. The CT will state that a certain side should be toward the source, so make sure you’re pointing the CT toward the source of power, (in this case, the receptacle in the wall) which means the opposite side should be directed toward the load (in this case, the lamp.)
If you install a CT backwards, your meter readings will be incorrect. If your meter readings are incorrect, and your system is already installed and working, that could mean an outage. An outage would mean downtime and additional cost on your projects.
We’d be happy to help you if you have any questions in specifying or installing CTs. Remember, making sure you get it right the first time can save you a lot of time and aggravation in future reporting and M&V efforts.
Allan D. Evora is a leading expert in control systems integration and president of Affinity Energy with over 20 years of industry experience working in every capacity of the power automation project life cycle. With a background at Boeing Company and General Electric, Allan made the decision to establish Affinity Energy in 2002. Allan is an alumnus of Syracuse University with a B.S. in Aerospace Engineering, graduate of the NC State Energy Management program, and qualified as a Certified Measurement & Verification Professional (CMVP).
Throughout his career, Allan has demonstrated his passion for providing solutions. In 1990, he developed FIRST (Fast InfraRed Signature Technique), a preliminary design software tool used to rapidly assess rotary craft infrared signatures. In 2008, Allan was the driving force behind the development of Affinity Energy's Utilitrend; a commercially available, cloud-based utility resource trending, tracking, and reporting software.
Allan has been instrumental on large scale integration projects for utilities, universities, airports, financial institutions, medical campus utility plants, and manufacturing corporations, and has worked with SCADA systems since the early ‘90s. A passion for data acquisition, specialty networks, and custom software drives him to incorporate openness, simplicity, and integrity into every design in which he is involved.