CT/PT Metering

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dvcraven0522!

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Electrical Design
I understand the concept of current transformers (CT) relative to metering installations. What don't know is when are potential transformers required?

I have seen some service with CT/PT and some not. Mostly services rated 480V. Are PT's used only for 48oV services?

Dan Craven
 
Our utility only uses PTs where the service voltage is 480 volts and higher, however they do permit a self contained 200 amp 480 volt meter.
 
Generally, since isn't too expensive to build voltage measuring equipment that can work with 600V directly, you can directly measure the voltage with no need to step it down. Unlike working with hundreds of amps, where it adds significant expense to make circuits that can directly handle it, such that it is much more cost-effective to use CT's to step it down. Even self-contained metering has a built-in CT.

As for whether or not PT's are required, it depends on the specifics of what the equipment requires. Some devices can handle 480V or 600V directly (usually metering), some equipment has limitations that require 2:1 PT's to reduce it, or a similar small ratio. One such example is relays that require PT's for 277V line-to-neutral circuits, so you can program overvoltage settings that otherwise would be above its internal maximum of 300V.

For medium-voltage, you are pretty much guaranteed to require both PT's and CT's, to step down both the voltage and current to reasonable representative values for the metering equipment to handle.
 
With the new meters, PT’s are becoming a thing of the past. Just did a 1600 amp 480 volt service, and they didn’t use PT’s. The meter accepted the 480 directly. I had a generator install on a store in Virginia, we got everything swapped over in time that night, but when the utility company came out to make it hot, they decided to upgrade the meter. Store was supposed to open at 7:00 am, but poco didn’t turn it back on until almost 9:00 am. Store manager was mad! Lost business and having to pay employee’s to stand around outside. Poco would not allow us to run the store on generator even though they were totally isolated.
 
hillbilly1 said:
With the new meters, PT’s are becoming a thing of the past. Just did a 1600 amp 480 volt service, and they didn’t use PT’s. The meter accepted the 480 directly. I had a generator install on a store in Virginia, we got everything swapped over in time that night, but when the utility company came out to make it hot, they decided to upgrade the meter. Store was supposed to open at 7:00 am, but poco didn’t turn it back on until almost 9:00 am. Store manager was mad! Lost business and having to pay employee’s to stand around outside. Poco would not allow us to run the store on generator even though they were totally isolated.
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Why would the POCO care?
 
One reason to use PTs is if the meter is on a door that might get opened frequently. Running line voltage across a door hinge adds a risk of the wire getting pinched or the insulation getting worn and potentially energizing the door. A lot of specs limit the voltage on any door mounted device to 120VAC or less. The secondary of a PT is typically 120V.
 
hillbilly1 said:
They are nervous when generators are running, probably had bad experiences with improper installs. Easy enough to check. Can’t really blame them. I’ve seen plenty of bad temporary and permanent installs.
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How would this be different from any other time that the grid is failing to provide power to the building?
 
In Lowes, the fixtures have split power, half the fixture is on generator powered panel, while the other half is on utility only panel. Both circuits are on the same phasing in their respective panels. I had a store in Virginia that was complaining a bunch of breakers would trip when on generator. Found the ballast changing monkeys were tying both circuits together when they changed the ballasts. It would go unnoticed until they actually lost power, and the generator would backfeed the utility until all of the affected circuits tripped. Found at least 20 circuits that way. The lighting maintenance company had to come back and go through all of the fixtures.
 
FYI - PT is not the IEEE listed reference. You can call it a potential transformer or a voltage transformer but the correct reference is VT. This was a change that took place many years ago.
But then, everybody still says 277/480V or 120/208V, which are incorrect as well. (480Y/277V and 208Y/120V)
 
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