CT orientation question

shockking

Member
Location
Sacramento, CA
Occupation
engineer
In general, if one CT in a three phase system is installed backwards, arrow pointing the wrong direction, can you simply switch the leads at the meter? This reverses the polarity and the meter readings seem fine (power and current are positive). Does this affect the PF or other measurements? Can this damage the CT?
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
In general, if one CT in a three phase system is installed backwards, arrow pointing the wrong direction, can you simply switch the leads at the meter? This reverses the polarity and the meter readings seem fine (power and current are positive). Does this affect the PF or other measurements? Can this damage the CT?
Shunt the CT, probably has a shunting bar with it. Swap the X1 & X2 on the CT or at the meter like you suggested.
No damage and readings are all correct. We have several like that over the years.
if it ain’t broke, don’t fix it.
 

Carultch

Senior Member
Location
Massachusetts
That's a hard pass.... I'll have to say nope......
As long as the current thru the CT core is zero while you execute the swap. there's no issue with swapping the leads to correct for the orientation being backwards. Since a CT converts a large current into a small current, the voltage drop across that 1 inch of conductor length is amplified by the reciprocal of the current ratio, in the event that the CT output is opened under load. Get your clamp-on ammeter and confirm that there is zero current in the circuit being measured, before reworking the output leads.

It certainly is a cleaner install to get it correct the first time, to avoid confusing future users.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
I was told the same thing but wasn't told why, because I didn't ask.
What happens?
I was told, in theory that when rated current (400A for a 400:5 CT) is flowing thru a CT and the secondary wires are open-circuited, the terminal voltage rises to infinity. I never measured it but many years ago I got knocked into a substation chain link fence when I opened a CT sec circuit testing a protective relay. It made me a believer for sure! o_O
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The voltage doesn't go infinite, just very high.

A 400:5 CT is a transformer with 1 primary turn and 80 secondary turns. In other words an 80x step up transformer.

In normal use the secondary is almost shorted, either through a low value current shunt resistance or through an analog meter current coil. The low secondary voltage is reflected as a minuscule primary voltage drop, perhaps a few millivolts.

If you open the secondary the secondary voltage can climb to 80x the primary voltage drop, which is limited by the primary operating voltage and by the saturation of the transformer core.

Hundreds of volts is likely, thousands possible.

Jon
 
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