Harris Institute of Technical Training

How valid are Burden tests on lightly loaded / no load CT's? We currently do an Admittence test on all installations but are beginning to burden test. Everything I have read seems to imply that unless a CT is 50% loaded the test won't give valid results.

Hey Neil, thanks for contacting me and for the question.

Metering CTs have an accuracy rating of .3% at rated load (200 amps on a 200:5 CT) and at the nameplate burden rating of the CT. Metering grade CTs have a .6% accuracy rating at 10% of rated load and (also) at the burden rating of the CT. So, if you are using metering grade CTs, you should be able to perform a burden test with as low as 10% of rated current flowing through the CT primary winding (e.g., 200:5 would be 20 amps) and expect .6% accuracy (with any current value less than 10% there is no requirement to meet the .6% accuracy rating).

The .3% and .6% accuracy ratings for metering CTs requires that the primary to secondary current values of the CT marked ratio to be within .3% or .6% respectively. Also, the .3% and .6% accuracy ratings for metering CTs requires the primary to secondary phase angle relationships from the CT to be within .3% or .6% respectively.

As a side note the .3% accuracy rating for metering CTs is also required through the rating factor of the CT. For example: a 200:5 CT with a rating factor of 4 must remain within the .3% accuracy limits with 800 amps flowing through its primary winding and 20 amps in the secondary winding and secondary circuit.

Another side note is an error that is made during CT burden testing, when metering professionals put too much additional burden into a CT secondary circuit which can cause a perfectly good CT to fail a burden test. From my experience, most secondary CTs have a burden rating of .5 ohms of total secondary circuit burden and the primary CTs have burden ratings somewhere between 1.5 and 2.0 ohms of total burden. I recommend that whatever device you are using to do your burden testing with should be set within these limits.

Finally, a burden test device adds additional burden to the CT secondary circuit. If you have long CT secondary circuit wire runs then there will be some amount of inherent burden in these circuits. As you add in additional burden, this could again cause a perfectly good CT to fail a burden test. CT secondary circuits with long wired runs can provide relatively high inherent burdens, but you should still test the CT secondary circuit (add in burden) to see how well the connected CT is performing.
Brad

As for the equipment we use to do Admittance and Bureden testing is 505 UTEC Transformer Analyzer.