Using a standard "donut" CT (say 100:5 2va burden) how do I determine the maximum distance I can run the secondary output before losing accuracy?

 

Responding to M. Schultz's 25-Mar-08 (21:43) query... Accuracy is primarily (excuse the pun) affected by burden and the CTs secondary-winding impedance:

Assume your CT conformed to USA ANSI C57.13, then its Burden Designation would be B-0.2, which means the burden should not exceed 0.2 Ohm. This corresponds to a maximum resistance and reactance of 0.180 and 0.086 0hm, respectively.

If additional detail is required, let me know!

Regards, Phil Corso (cepsicon @ aol. com)

 

So Phil,

Being unfamiliar with the ANSI reference, are you telling us that the burden rating of a CT is meaningless? If so what is the point of having CTs with different burden ratings? I would have thought you could apply Ohm's law to get an impedance of 0.08 Ohms.

I have sometimes seen a CT in the MCC connected to an Ammeter on the control panel several hundred feet away, typically 0-1 Amp from memory. I assume that under startup conditions the burden is exceeded but from a Ammeters point of view this would be a good thing, i.e. pegged to the stop but not by too much.

Your ANSI reference, does that refer to metering, protection or both?

Regards,
Roy

 

Roy, answering your questions in the order presented:

1) What did I write that led you to believe I said that burden was meaningless? The R and X values I provided are the maximum burden for the CT cited in the example! As a matter of fact its rating was given as 5.0 VA!

2) The number of CT burdens is limited to a few values.

3) Application of Ohm’s law yielding 0.08 Ohm would provide a burden 2 and 1/2 times the recommended value!

4) What was the point you were making with your ammeter example? That one shouldn't use Standards?

5) The Standard I referred to covered metering CTs!

Regards, Phil