CT Ratio

[Truemanator]

I'm a new engineer and I've just been asked if I can find out the CT ratio used when collecting power quality data with a Fluke 1750.

I can probably find the information and provide the number, but I'd rather know what it means - might save me some time in a book.

What is a CT ratio and what significance is it?

 

[rockman7892]

The initials CT stand for Current Transformer. These devices do exactly what the name implies, they transform one current magnitude into another. Most of the time when measuring current the devices used to measure current cannot handle the magnitude of the current being measured to pass directly through the measuring equipment so a CT is used to measure a ratio of measured current.

Most of the time CT ratios will be in a ratio of X:5 where X is the primary rating of the CT and the 5 represents the 5A that will be measured at your measuring equipment at the rated X current.

For example lets say you have a 100:5 CT. This means that when measuring current in a cable and the current is 100A this 100A will be transformed into a 5A current that will be seen by the measuring device. The device is programmed internally to take the value of current it sees (5A) and represent it as a current value on the wire. In order to do this, the correct CT ratio has to be inputed into the device. If you were measuring 50A on your cable then the CT would output 2.5A to the device and the device would therefore display the 50A that you were measuring.

How these CT's work are based off of EMF's coming from the cable or device which current is passing through.

 

[Truemanator]

Okay, thanks! That makes sense. I'm assuming a PT does a similar thing, but steps down the voltage for measurement instead.

 

[oldfieldguy]

True--

You got the "PT" thing nailed down, too.

They're simple devices on the surface, but he fun come sin when you start connecting a few of them for measurement or protection one more complex systems.

old field guy

 

[stevenal]

"The Recorder automatically detects the values of attached current probes. If you have connected a 1750 Power Recorder to a PT (potential transformer) or secondary CT (current transformer) to step down values for measurement purposes, you can enter voltage and current ratios in the 1750 Setup screen."

So if directly using the Fluke probes on the primary circuit, the calculations are behind the scenes for the user. If the probes are connected to instrument transformer secondaries, the ratios can be found from the instrument transformer nameplates or system drawings.