What Do I Need to Know When Selecting a Current Transformer

A Current Transformer is used to measure alternating electrical currents.  The word Current Transformer is abbreviated as (CT); there are two basic physical types of current transformers Split Core Current Transformers and Solid Core Current Transformers. A current transformer is used to measure AC load currents going through a conductor such as a wire or bus-bar. The Split Core CT opens up and fits around the conductor and then is closed such that the load conductor doesn’t need to be disconnected. A Solid Core CT requires that the load conductor be disconnected and placed through the center of the Solid Core CT.

A Solid Core CT should be selected, if your CT needs are for new construction and there is a need for better cost, reliability and accuracy. A Split Core CT should be used in existing installations, which would be difficult and costly to remove the load conductor.

In general it is expected that you will see 0.5% to 2% accuracy in mid range Split Core and Solid Core CTs. It is necessary to select a CT according to your desired overall metering accuracy needs. For example some power meters installations require ANSI C-12.1-2008 this requirement sets a minimum of +/-2% accuracy, were as ANSI C12.16 requires a CT that is +/-0.6% accurate, with an overall +/-1% system accuracy. Not all installations have ANSI requirements, however be aware that there are differences in current transformer accuracy and you will usually pay a premium for higher accuracy transformers.

There a several different common outputs available from current transformers used for mid range power meters, power monitors, power quality monitors, and current meters. These outputs are 333mV, 1V, 100mA, 1A and 5A. The 333mV if becoming more and more popular in the United States were as Europe is commonly using the 5A CT. When selecting for your needs make sure that your metering device supports your current transformers output type. Remember that many current transformers can be built to order, so asker your current transformer distributor to see if they can provide you with whatever custom solution you may need. The costs of making a custom solution are usually 2-5 hours or engineering time.

Temperature range of many Current Transformers manufactures is commonly not specifically stated however most are designed to operate from -20 to 60 degrees Celsius. Check with your current transformer distributors for more details. The temperature range is of more importance for CTs that are of type current output; 5A output CTs for example will be subject to an internal temperature rise were as 333mV will not. Many CTs are not designed for outside use, it is important if outside use is needed to verify with your CT distributor that an outside weather package can be applied.

You should know your maximum load. The closer you select a CTs nominal load to the actual load the better your overall accuracy will be. For example if your loads maximum current will be 93A a 100A CT would be a much better fit than a 200A CT. Nominal load current for mid range CTs range from 1A to 5000A or more.

The tighter the fit around the conductor the better the accuracy, this implies that when you select a CT you should know the size of the conductor to ensure that you get the best fit, it would be unwise to use a CT with a 3 inch x 5 inch opening to for a 1 inch x 1 inch conductor.

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