Who is using high accuracy CTs? We switched awhile back to .15S accuracy CTs at Minnesota Power a number of years back. There are alot of good advantages. You need to carry less stock. A handful of different ratios can cover most all installations.
At MP we use a 600:5 bar type (2 bolt ansi pattern) CTs on services under 1200 amps (mostly wall mount CT cabinets). For over 1200 amp or pad pound connection cabinet we use 1000:5 and 2000:5 CTs. Anything under 2000 amps we install the 1000 units and then up to 4000 amp services we use the 2000:5.
The .15S accuracy class CTs are very good accuracy down to 1% of name plate so a 600 is very good down to 6 amps primary. The older CTs usually lost the .3% accuracy at about 60% of name plate and down below 30% of name plate could be off by a considerable amount (full 1% possibly or more). On smaller loads a high accuracy 600:5 CT will provide better registration than a standard accuracy 200:5 in most cases.
Great Topic Jason –
We have been using the GE Encompass CT’s (JAB-OW) or (JCR-OW) in the 250/5 Bar Type and 500/5 Window Type and also in the 1000/5 Window Type. These are High Accuracy to 4% of nameplate. These ALL having a Rating Factor of 4 so you can push your CT’s amperage up higher.
We also will mix in the Astra Eliminator line (AP5Y) of 500/5 and 1000/5 which are the same accuracy of 4% of nameplate. These also having a RF of 4.0 and are High Accuracy/High Temp depending on what application you are looking for it may make a difference.
Ritz has come a long way in their CT’s and their DCDW are High Accuracy with rated burden down to .2 and Accuracy of 1%. Pretty solid CT if you ask me with a .15 accuracy. The only downside for our utility is we like to keep our Multiplier within reason. So if we install a 250/5 CT in a 480 volt service with a PT added, the Multiplier is X125 vs a Multiplier of X300 with the Ritz 600/5. All a matter of preference in my opinion. I think Ritz finally has a RF of 4.0 which for as long as i can remember it has been 3.0. We push our RF pretty good which is a practice everyone should try and do!
Most High Accuracy CT’s are pretty comparable, every company should definitely make the switch and stop using anything other than High Accuracy.
I agree it is mostly preference for the CT ratio. Personally I don’t think having larger multipliers is much of an issue. Most newer meters give you several decimal places on the KW readings so you can be billing pretty accurate. We have actually seen where some older 200:5 CTs were changed out to the newer 600:5 Ritz units and saw an increase in registration. Several of these are on accounts that the customer never even comes close to 50 amps and really shouldn’t even be a CT rated setup but we have to work with what was existing there. Pushing the CTs to have more secondary current isn’t nearly as critical as it used to be with the old .3 accuracy class CTs. Most of our in house testing we have done with an Omicron CT Analyzer has shown that the CTs are really just as accurate at 5% of nameplate as they are at full RF. Paired with a good electronic meter registration should be just fine, a mechanical meter could end up with some registration issues with very low currents from the CTs though. We all know the mechanical meters tend to fail first on light load testing.
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