CT's

"1000/5" is the ratio, meaning for a 1000A current flow through the CT, the secondary (output) current will be 5A. You could also say the ratio is 200:1, but that can be misleading. The second "/5" is just redundant, meaning it is a 5A maximum continuous current device. So for instance you cannot use it on a 2000A continuous circuit and expect to get 10A out of the CT. The 200:1 ratio would be the same, but the CT is not rated to deliver 10A continuously (and accurately).

I have found CT data plates showing 400/200-10/5. This means that it is a multi ratio CT (either 200A or 400A primary) with a secondary current output of 5A. The 10A shows the rating of the test winding.

CTs also have a "RF" or "rating factor". Some manufacturers call this value "Continous Thermal Rating". This value talks to the current handling rating of the device. For example...

A 400 amp CT with a RF of 1.5 can operate properly on a continous basis, at 600 amps. This device is considered having a maximum electrical rating of 600 amps.

A 400 amp CT with a RF of 2.0 can operate properly on a continous basis, at 800 amps. This device is considered having a maximum electrical rating of 800 amps.

As a rule of thumb... CTs accuracies are based on the full current rating of the device. Here is a link...

http://www.itron.com/asset.asp?path=/products/specsheets/itr_008572.pdf

Can you give more specific about the nameplate of the CT?.. In my opinion, it has two secondaries... it gives 5A on one winding and 5A on the other one.. If only you could give the actual nameplate.. it would clear things up..

Dear Sirs,

The 1000/5/5A CT indicate two cores CT in one unit with 5A output respectively. If your CT is designed for protection Class (as shown in simplified single line diagram the CT class is 5P20) that means we can use for protection.

However, USA CTs can be used for both protection and measurement.

In your case, you have to check the CT class either for protection or measurement. Each core will generate 5A at secondary CT when 1000A flows in the primary CT.

Yes it is, 5A is IN, but the relay setting (if the CT is for protection) is depend on your equipment to be protected and coordination study.

Regards,

<--However, USA CTs can be used for both protection and measurement.-->

I would like to disagree on this one. We, in our company, are using the ANSI standards, still CTs are classified whether it is protection or for metering.

Protection CTs goes in the following annotation: C100 or the like

while on metering CTs: 0.3B1.2

We are not using metering CTs on protection aspects and vice versa.

Dear Hkian,

Thanks for your correction.

In our plant using CT protection for both protection and metering which are C classes with minimum voltage classes 100 Volt and above. For example C100 CT can deliver terminal voltage 100 V to standard burden (B-1, B-2, B-4, and B-8) at 20 times rated secondary current without exceeding 10% ratio correction).

The lower voltage classes of 10, 20, and 50 with standard burdens of B-0.1, B-02 and B-0.5 at 0.9 PF are primarily for metering service and should be used cautiously for protection.

Those are information that I used for supporting my previous comment.

Sorry I am not familiar with 0.3B1.2. Would inform us?

Regards,

Hi,

Allow me also to make my correction, according to ANSI C57.13-2008:

0.3B1.8 - it means as follows: 0.3 means 0.3% error at 100% rated current (taking into consideration the RF factor of the CT)

next allow me to correct myself, it is not 1.2 but 1.8: B-1.8 means the burden of the metering CT at 0.9pf

I know that your reference is from Blackburn's book, but it is still C57.13-1978.

Yes, at some point, I would agree, you could use protection CT for metering purposes. But the essence is that, we use metering to be as accurate as possible, thus we have to use CT for metering purposes. If the protection CT is used for metering purposes, we have to see to it that the meter itself can carry this 100A (taken from 5 A nominal secondary times 20) during fault. If we are using metering CTs, we are also protecting the meter, since during high fault currents, the metering CT will easy saturate thereby reducing the current to flow towards the meter.

The concept is this, protection CTs are used to replicate high current as high as possible to provide good information on protection aspects but it may not need to be very accurate.

On the other hand, metering CTs are used to be accurate as much as possible.(especially if we are talking about revenue).

I will welcome any comments.

hkian