CT’s, or current transformers, and PT’s, or potential transformers are used in metering to step down current and voltage to safer and more manageable levels. Many people want to know what is a current transformer and potential transformer. Here I will try to demystify the CT PT confusion. One thing that I want to note also is that CT rated meters are not only used as a secondary electric meter, they are also used as a primary electric meter as well. CT rated meters are also typically demand meters as well.
When CT’s and PT’s are used in a metering installation, the installation is known as being transformer-rated. Some people refer to the meters that use a CT PT combination or just CT’s as a current transformer meter. Transformer-rated services run in parallel with the service. This means that unlike self-contained services the customer’s power is not interrupted when the meter is removed. The reason that they are needed is that either the current and/or voltage of the service to be metered is too high. This also depends on the policies and procedures of the utility. For example, some utilities require anything over 480v to be transformer-rated. While other utilities do not.
Also, some utilities do not use PT’s in 480v services at all. I recommend against this practice for the safety of the meter tech or lineman who may need to install or remove these meters from service. Read why you should be using PT’s here.
So, what do CT’s do? As stated before they serve to step down high current to a safe a manageable level. Revenue grade CT’s are engineered to produce 5 amps when the amps on the service are at the rated value. For example, a typical installation in a 120/208 service 400 amp service contains 200:5 CT’s. When 200 amps are flowing through the primary side of the CT, 5 amps are coming out of the secondary terminals.
CT’s have nameplates and ratings just like any other piece of electrical equipment. The most important things to note on the nameplate are the ratio and the rating factor. The ratio will be printed in large letters on the side of the CT. Typical ratios are 200:5, 400:5, 600:5, 800:5 and so on. Again, what this means is that when the stated value of amps is flowing through the primary side of the CT, 5 amps is flowing through the secondary side.www.ctsensorducer.com