CT Placement
CT Placement
(OP)
We know that when placing a window CT around a conductor that the conductor should be centered in the window and continue straight for some distance beyond the CT.
Is there some rule of thumb or standard that addresses what "straight for some distance" means?
Is there some rule of thumb or standard that addresses what "straight for some distance" means?






RE: CT Placement
MikeL.
RE: CT Placement
(OP)
1. In my 50 years of working experience on LV systems, I have NOT noticed any practical significant error irrespective of whether the conductor is placed perfectly in the centre and continue straight for some distance before or beyond the CT.
This can be easily demonstrated by moving around the jaw of the portable clamp-on ammeter and flex the conductor before and after the CT. There would be no "practical significant error" when not under high precision laboratory measure.
2. There is no such requirement(i.e must be placed centrally and a certain straight length before and after)in IEC CT, switchgear and installations standards.
3. For good workmanship and appearance but not technical reasons, it is desirablebut not mandatory to place the conductor centrally and maintain straight before and after the CT.
RE: CT Placement
I hope you aren't conducting those clamp-on ammeter tests with significant fault current flowing in the conductor under consideration.
RE: CT Placement
RE: CT Placement
In my experience, for currents up to about 5000A, the placement of the primary conductor in the window doesn't have a big impact on accuracy. This goes for round window profiles and for CTs with a decent core cross-section (I know that's not specific). For rectangular windows with flat bar conductors, it's a little more tricky and sensitive to placement.
Return conductor placement is a little more difficult to define. I'm not sure I understand exactly where the rule of thumb that catserveng mentions comes from. Since core cross section has a lot to due with localized saturation effects and phase cross-talk effects, in some sense the smaller the CT height (and width) the longer the conductor needs to be "straight" after it passes through the CT.
For generator CTs with high current ratios, they should employ some method of magnetic shielding to minimum the impact of non-centers conductor and return conductor effect. Normally, the vendor will specify the phase-to-phase spacing and conductor routing limitations (I know we do) and this varies from design-to-design. These limits come from heating and accuracy concerns.
RE: CT Placement
Respectfully
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: CT Placement
RE: CT Placement
RE: CT Placement
I have been burned by right angle bends in the busbar beside the CT. The application was 3000:5 CT's on a busbar and we saw about a 5% error that went away when the CT was relocated about 200mm away from the bend.