CT Sizing
CT Sizing
(OP)
Im curous about CT sizing for a 347/600V service entrance. It being 600A 100% rated, would a 600/5 CT become saturated near the full 600A mark? Would this application require a 800/5 CT instead?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: CT Sizing
RE: CT Sizing
RE: CT Sizing
Upstream breaker = 600A @ 100%
80% Load = 480A
Next high available size CT = 600:5
RE: CT Sizing
CTs have a rating called "rating factor"...official name continuous thermal rating factor. This is how many times the rated primary current that the CT can operate at continuously without thermal damage.
For metering CT, it also defines the top limit of the accuracy class.
For instance, a 600:5A CT rated RF=2.0 and accuracy of 0.3B0.5 (IEEE rating) is guaranteed to maintain a 0.3% accuracy performance from 600A to 1200A (rated current up to rating factor) and 0.6% accuracy performance from 60A to 600A (10%rated to rated current). For metering applications, it's a common mistake to use to high of a ratio. 600V CTs for metering normally come with rating factors of 3.0 or 4.0. If the max current is 600A, the best way to meter is with a 200:5A CT with a rating factor or 3.0.
If the CTs are for protection, then you need to educate yourself on the protection ratings of whatever standard region you're in. But in short, no a protection-rated CT won't saturate at or near rated voltage, assuming the rated burden is not exceeded.
RE: CT Sizing
Interesting post scottf, Im interested in reading more regarding this. Mostly for metering as all of the protection relays have been specified by OEM.
RE: CT Sizing
RE: CT Sizing
In the IEEE world, most meters used are Class 20 meters, meaning rated for 20A continuous input.
I'm not sure, but I think in the IEC world, with 1A rated CT secondaries, the meters are Class 2.
RE: CT Sizing
RE: CT Sizing
The PM175 specifies a burden of 0.1VA. I was wondering what voltage the PM175 would see at its input if 600:5 ct hooked up assuming primary current did get as high as 600amps? 20mV?
The other thing is i have had some guy tell me that a split core is available but isolation level is only 7kV - can someone guide me here - the cable it is going around is insulated etc photo attached
Thanks in advance ...