Thinking Accuracy with Current Transformers

Accuracy is, and probably will always be, a contested subject. Even the readings on the most sensitive gauges ever made will be subject to factors beyond anyone's control. This is why it's reasonable to consider a margin of error every time you take a measurement, especially when it comes to currents.

Given that current transformers are always on the lookout for unusual energy spikes, accuracy must be their most defining feature. They don't have to be perfectly accurate, just accurate enough to confirm the readings and help managers make sound decisions. However, no one-size-fits-all transformer exists at present.

There are several accuracy classes, depending on who you ask. IEC 60044-1, the more widely used standard, enumerates 11 accuracy classes. IEEE C57.13 enumerates 10 classes. For the purposes of clarity, take IEC 60044-1 as an example. By order of highest accuracy, current transformers are usually concerned with only the four topmost classes.

• 0.1 – 0.1 to 0.4 ratio error, 5 to 15 minutes phase displacement
• 0.2 – 0.2 to 0.75 ratio error, 10 to 30 minutes phase displacement
• 0.5 – 0.5 to 1.5 ratio error, 30 to 90 minutes phase displacement
• 1.0 – 1.0 to 3.0 ratio error, 60 to 180 minutes phase displacement

Some might think that it's simple to get a 0.1-class transformer, but this choice depends on the nature of the application. The less accurate classes, for instance, are primarily used for surge protection. The 0.1-class transformer, on the other hand, is necessary for laboratory use due to its low ratio error.