Current transformers are critical to both the safety and reliability of an electrical network. CTs are used for two key purposes in any electrical network
- Step down the high current coming from transmission lines and make it suitable for measuring my meters
- Isolate and protect the relays and meter circuits from high-voltage current in the power systems.
Explore the basic construction and working of current transformers
Depending on the two applications, the current transformers are classified into two major types i.e. measuring CTs and protection CTs. The basic design of the two transformers is the same, but there are certain differences based on the application.
What are the differences between the measuring CTs and protection CTs?
| Measuring CTs | Protection CTs |
| Application
Measuring CTs are used to step down high current from power lines to lower value suitable for measuring instruments such as meters. |
Application
Used in association with protective devices such as relays to monitor fault currents during electrical faults. |
| Design
Metering CTs are designed to have a high measuring accuracy of up to around 125% of the rated current for all loads. |
Design
Protection type CTs are designed to operate smoothly in high current up to around 10-20 times of the rated current under fault conditions. |
| Performance
The performance of the metering CT is important only during normal conditions. It has significant errors during fault conditions |
Performance
The performance of a protection type CT is important during fault conditions. It may not deliver accurate performance in power measurement. |
| Core
The core of measuring type CTs is typically made of nickel-iron. Theis core has a low saturation point so that the core can saturate and prevent fault current from reaching the meters. |
Core
The core of protection type CT is made of cold-rolled silicon steel. This core has a high saturation point so that it keeps working even in a high fault current. |
Explore more information about the materials used for the current transformer core on our blog here.
The right choice between durability and accuracy
As they are designed to withstand fault currents, protection-type current transformers tend to be more durable than metering current transformers. This aspect of durability could tempt many users to connect protection-type CTs instead of metering-type CTs to measuring devices. In some developed countries, there exist current transformers that can provide both measurement and protection. However, it is advised that metering and protection CTs should be used for the applications they are meant to serve. Exchanging these current transformers could lead to electrical mishaps and serious fire incidents. To explore what would happen if a protection-type CT is used for metering and vise-versa, read our blog here.