Instrument Safety Factor, or ISF, is one of those CT concepts that often gets mentioned but not always fully understood.

In simple terms, ISF defines how much current a current transformer will allow to pass to its secondary before it starts to saturate and limit that current.

For example, if a CT has an ISF of 5, it means that under fault conditions, the secondary current will be limited to approximately five times its rated value. Beyond that point, the CT core saturates, and the secondary current no longer increases proportionally.

This is not a flaw in the design—it’s intentional. ISF is built into metering CTs to protect connected instruments.

Why do we need ISF in the first place?

To understand ISF, think about what happens during a fault.

In normal operation, current levels are within expected limits, and meters operate safely. But during a fault, current can rise to many times the normal value in a very short time.

If that full fault current were transferred directly to the CT secondary, it could easily damage meters, ammeters, or energy monitoring devices.

This is where ISF comes in.

By forcing the CT to saturate at a certain level, the secondary current is effectively limited. Instead of allowing extremely high currents to pass through, the CT “caps” the output, protecting the connected equipment.

So ISF is essentially a built-in safety mechanism.

How is ISF defined?

ISF is defined as the ratio of the primary current at which the CT starts to significantly saturate to its rated primary current.

In practical terms, it tells you how many times above rated current the CT can go before it limits the secondary output.

For example, consider a 500/5 CT with an ISF of 5. This means that once the primary current reaches about five times its rated value, the CT will begin to saturate and prevent further proportional increase in secondary current.

The key idea is that ISF controls how much fault current is “allowed through” to the instruments.

How does ISF relate to CT saturation?

ISF and saturation are directly linked.

Saturation is the physical phenomenon where the CT core reaches its magnetic limit. ISF defines when that saturation is expected to occur.

A lower ISF means the CT will saturate earlier, limiting current more aggressively. A higher ISF means the CT will allow more current to pass before saturating.

So when you choose a CT with a specific ISF, you are effectively deciding how early or late saturation should happen under fault conditions.

For metering CTs, early saturation is desirable because it protects instruments. For protection CTs, the opposite is true—you want the CT to remain linear as long as possible, which is why ISF is not typically used for protection CTs.

What are typical ISF values?

In most metering applications, ISF values are relatively low.

Common values include 5, 10, or sometimes 15. An ISF of 5 is often preferred because it provides strong protection for connected instruments by limiting the secondary current more quickly during faults.

Higher ISF values allow more current to pass before saturation, which may be acceptable in some applications but offers less protection.

So in general, lower ISF values mean better protection, while higher values mean less limitation on current.

How does burden affect ISF?

This is where things get interesting—and where many mistakes happen.

ISF is not a fixed value in practice. It depends on the actual burden connected to the CT.

When the burden increases, the CT requires more voltage to drive the same current. This pushes the CT closer to its saturation point, which can cause it to saturate earlier.

But here’s the important part: if you select a CT with a higher rated burden than what is actually connected, you effectively increase the saturation voltage. This means the CT will take longer to saturate, which increases the effective ISF.

In other words, overestimating the burden can result in a higher ISF than intended.

This is exactly what you demonstrated in your earlier article. A CT that was supposed to have an ISF of 5 ended up behaving like it had an ISF of 15 because of incorrect burden selection.

That’s a big problem, because it allows much higher currents to reach the instruments during faults.

What happens if the ISF is too high?

If the ISF is too high, the CT will not saturate when it should.

This means that during a fault, the secondary current can rise to dangerously high levels. Instead of protecting the connected meters, the CT allows excessive current to flow through them.

The result can be overheating, insulation damage, or even complete failure of the instrument.

In short, a high ISF defeats the purpose of having a metering CT in the first place.

What happens if the ISF is too low?

At first glance, a very low ISF might seem ideal because it provides strong protection.

However, there is a trade-off.

If the CT saturates too early, it may limit current even under conditions where accurate measurement is still needed. This can affect the performance of the system, especially if the CT is used for both monitoring and control purposes.

So while low ISF improves protection, it must still be balanced with the need for accurate operation within the normal current range.

How is ISF different from protection CT characteristics?

This is an important distinction.

ISF is mainly used for metering CTs. It focuses on limiting current to protect instruments.

Protection CTs, on the other hand, are designed to remain accurate during faults. Instead of ISF, they use parameters like Accuracy Limit Factor (ALF), which defines how much current the CT can handle while maintaining accuracy.

So while ISF encourages early saturation, protection CT design aims to delay saturation as much as possible.

This difference reflects the completely different roles of metering and protection CTs.

How do you ensure the correct ISF in practice?

Getting ISF right comes down to proper system design.

First, you need to calculate the actual burden accurately. This includes not just the connected instruments, but also the resistance of the cables.

Next, you select a CT with a rated burden that closely matches the actual burden. Avoid the temptation to oversize the burden “just to be safe,” because this can lead to unintended changes in ISF.

You also need to choose an appropriate ISF value based on the level of protection required for your instruments.

Finally, always consider how the CT will behave under fault conditions, not just during normal operation.