Relationship between rating of relay ct and breaker

A very specific and technical question!

In a power distribution system, a relay CT (Current Transformer) and a breaker are two essential components that work together to protect the system from overcurrents and faults. Here's the relationship between their ratings:

Relay CT (Current Transformer) Rating:

• The relay CT is designed to measure the current flowing through a circuit and produce a secondary current that is proportional to the primary current.
• The CT rating is typically specified in terms of:
  • Primary current rating (e.g., 100 A, 200 A, etc.): The maximum current that the CT can handle without saturating or becoming inaccurate.
  • Secondary current rating (e.g., 5 A, 10 A, etc.): The maximum current that the CT secondary circuit can handle.
  • Turns ratio (e.g., 1000:1, 2000:1, etc.): The ratio of primary to secondary current.

Breaker Rating:

• The breaker is designed to interrupt the circuit in case of an overcurrent or fault.
• The breaker rating is typically specified in terms of:
  • Interrupting capacity (e.g., 100 kA, 200 kA, etc.): The maximum current that the breaker can interrupt.
  • Rated voltage (e.g., 230 V, 400 V, etc.): The maximum voltage that the breaker can operate at.
  • Rated current (e.g., 100 A, 200 A, etc.): The maximum current that the breaker can carry continuously.

Relationship between Relay CT and Breaker Ratings:

• The relay CT rating should be matched to the breaker rating to ensure accurate fault detection and reliable tripping of the breaker.
• The primary current rating of the CT should be equal to or less than the rated current of the breaker.
• The secondary current rating of the CT should be equal to or less than the rated current of the breaker.
• The turns ratio of the CT should be chosen such that the secondary current is proportional to the primary current and can be accurately measured by the relay.

In summary, the relay CT and breaker ratings should be matched to ensure accurate fault detection and reliable tripping of the breaker. The CT primary current rating should be equal to or less than the breaker rated current, and the CT secondary current rating should be equal to or less than the breaker rated current.