When we talk about the rated secondary current, we’re simply referring to the nominal current output that the CT is designed to provide at its secondary terminals when its primary is carrying rated current.

For instance:

A CT with 1000/5 A means that when 1000 A passes through the primary conductor, the CT will supply 5 A on the secondary side.

Similarly, a 300/1 A CT will give 1 A on the secondary side when the primary is at 300 A.

In practice, the secondary current is proportional to whatever is happening on the primary. So, if only 500 A flows in that 1000/5 A CT’s primary, you’ll get about 2.5 A on the secondary.

Why 5 A and 1 A?

The majority of the CTs sold in Australia are rated at either 5 A or 1 A secondary current. These two ratings have become the standard in the industry because they are both practical as well as safe.

5 A CTs have historically been the most used. They’re tough, easily found anywhere, and most equipment is built to be compatible with them.

1 A CTs are commonly used where there are long cable runs. Since the current is less, there are less losses (I²R losses in the cable), thus preserving accuracy over distance.

So the choice often comes down to your installation: if the meters and relays are close to the CT, a 5 A CT is fine. But if you’ve got a long run—say, from a main switchboard out to a remote meter—a 1 A CT may be the smarter choice.

The Role of CT Ratio

The rated secondary current is only half the picture. You also have to consider the CT ratio—that’s the ratio between the rated primary current and the rated secondary current.

A CT labeled 200/5 A has a ratio of 40:1. That is, for every 40 A in the primary, you’ll get 1 A in the secondary.

The ratio is important for measuring scaling. If you install a 200/5 A CT but not program the ratio into your meter, your measurements will be far from the correct value. For example, a meter directly wired to the CT secondaries will read 5 A at full load. But actually, that’s 200 A on the primary.

Accuracy and Burden

This is where things become interesting. A CT’s capability to accurately replicate the main current on the secondary side relies on more than its ratio. The precision of a CT is coupled with two primary factors:

• – Accuracy class – This is the maximum error percent the CT can have in normal conditions. For instance, a Class 0.5 CT should not exhibit more than 0.5% error.
• – Burden – Total load associated with the CT’s secondary. This encompasses the resistance of your relays, meters, and connecting cable.

Now, the rated secondary current comes into play here. If you overburden the CT secondary with excessive load, the CT might not provide an accurate secondary current anymore. That’s why we need to keep the cable runs short, utilize the appropriate cable size, and select CTs with sufficient VA rating.

Safety Considerations

One of the most critical things to keep in mind with CTs is this: never have a CT secondary live with the primary energized.

Here’s why: a CT acts like a current source. If the secondary circuit is open, the CT will continue to attempt to drive the rated secondary current through it. With nowhere for the current to flow, the voltage on the secondary terminals can soar—sometimes to several kilovolts. That’s both harmful to people and equipment.

So, whether you’re dealing with a 5 A or 1 A CT, always short the secondary or keep it connected to a proper burden when the primary is energised.

Practical Example

Let’s say you’ve got a 600/5 A CT installed on the incoming supply to a commercial switchboard. The CT ratio is 120:1.

If the load is drawing 300 A, the CT secondary current will be:

(300 ÷ 600) × 5 = 2.5 A

If the load increases to 600 A, the secondary will supply 5 A.

Now consider this CT wired through 50 metres of 2.5 mm² copper cable to a meter. With a 5 A secondary, cable losses will be apparent. But if instead you had ordered a 600/1 A CT, the secondary current would be less and the cable heating losses (I²R) would be very much lower. That’s the sort of design choice that can make all the difference in actual installations.

CTs for Metering versus Protection

It’s also important to remember that CTs are not created equal.

Metering CTs are made with precision accuracy, particularly with lower currents. They are tasked with ensuring your billing meters and monitoring systems provide accurate measurements.

Protection CTs, however, are designed to deal with short-circuit scenarios. They can possibly not be as precise for small loads, but when a fault happens and the currents jump, they must accurately reproduce that current in order for relays and breakers to trip appropriately.

Both types use their rated secondary current as the basis for reference, but application determines how they’re ordered.

1 A or 5 A: A Decision

So, which one do you use in your project? Here’s a quick guide:

Use 5 A CTs when:

• – Your meters and relays are nearby the CT.
• – Cable runs are short.
• – You’re dealing with older systems where 5 A inputs are typical.

Use 1 A CTs when:

• – You have long secondary cable runs.
• – You want to minimize copper losses.
• – Your instruments are 1 A input compatible.