How to Determine if a Current Transformer is Suitable

I. Selection Parameter Matching Judgment (Basic Judgment)
First, verify that the core selection parameters match the actual scenario requirements. Any mismatch indicates unsuitability:

1. Current Ratio Matching: The long-term operating current of the circuit under test should reach approximately 2/3 of the transformer's rated primary current, and the maximum continuous operating current should not exceed 1.1 times the rated value, and the short-term overload should not exceed 1.2 times (duration not exceeding 1 hour). If the operating current is significantly lower or higher than the rated range, it will lead to decreased accuracy, indicating unsuitability.

2. Accuracy Class Matching: For metering scenarios, the accuracy should be ≥0.2 class; for routine measurements, ≥0.5 class; and for protection scenarios, the error should meet the protection class requirements (e.g., 5P/10P). Failure to meet these accuracy requirements indicates unsuitability.

3. Rated Load Matching: The actual total load of the secondary circuit (instrument internal resistance + line resistance) must be less than or equal to the rated load of the transformer. If the actual load exceeds the rated range, it will lead to excessive measurement error and be deemed unsuitable. A typical example: using a 5VA, 200/5A transformer for a 200-meter transmission distance, the actual load reaches 70.5VA, far exceeding the rated capacity, and is directly deemed unsuitable, requiring replacement with a 200/1A specification.

4. Voltage Insulation Matching: The rated voltage and insulation class of the transformer must be greater than or equal to the nominal voltage of the circuit being measured; otherwise, there is a risk of insulation breakdown, and it is deemed unsuitable.

II. Professional Judgment of Protection-Type Transformers (Instrument Verification) For current transformers used for protection, their conformity must be verified through 5%/10% error curves:

1. Determine the corresponding error curve according to the transformer nameplate markings: if the nameplate indicates 5, check the 5% error curve; if it indicates 10, check the 10% error curve (e.g., 10P20 corresponds to the 10% error curve). 

2. Calculate the secondary rated load of the current transformer: Rated load = secondary rated capacity / (square of secondary rated current). For example, for a 15VA, 5A specification, the rated load is calculated as 15 ÷ (5 × 5) = 0.6Ω.

3. Comparison and judgment: Under the rated current multiple, as long as the secondary load allowed by the error curve is ≥ the calculated rated load, the current transformer can be judged as qualified and suitable; otherwise, it is unsuitable.

III. Installation condition compatibility judgment Inadequate installation conditions will also cause the current transformer to malfunction. The following verification is required:

1. Structural space matching: The window size must match the busbar specifications (if the size is too small, installation is impossible, and it is considered unsuitable); new projects should use closed-type transformers, and renovation projects should use open-type transformers; mismatched types are considered unsuitable.

2. Environmental condition matching: In special environments such as high temperature, high humidity, strong electromagnetic interference, and outdoor conditions, the current transformer's protection level and temperature resistance characteristics must meet the requirements; otherwise, it is unsuitable. 

3. Wiring compliance judgment: The polarity connection is correct (P1 in, P2 out, S1 corresponds to P1), the secondary side of the high voltage transformer has one and only one reliable grounding point, the S2 open circuit protection device is normal, and the wiring does not meet the requirements, which will lead to measurement errors or even safety risks. It is determined that the installation is not appropriate.