Grounding CTs and PTs

[Mila2015]

I have an installation where CTs and PTs are installed, but will not be used (not wired for metering nor protection). We will be connecting all phase CT terminals to ground to avoid an open CT situation, but the field contractor wants to also ground phase PT terminals. Is that typical? Those terminals will be reading 120V, so would grounding them cause damage?

 

[davidbeach]

One secondary PT terminal should be grounded, but not both. Connect the three in wye and ground the wye point and you'll be good. If you don't ground you risk that secondary voltage float up toward phase voltage due to capacitive coupling across the windings.

Ideally the CTs would have all terminals shorted together and then a single ground from that.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mila2015]

Thanks davidbeach. See attached file for actual PT configuration. This has dual secondary windings (Y and Delta) as you see. So, you're saying for the Y secondary, I connect A1X11, B1X11, and C1XII together, and ground middle wye point only correct? What about the delta winding?

 

[davidbeach]

The ground shown on the delta winding is necessary.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jghrist]

But add a ground to the neutral point of the wye connected secondary.

 

[RRaghunath]

Mila2015,
"So, you're saying for the Y secondary, I connect A1X11, B1X11, and C1XII together, and ground middle wye point only correct? What about the delta winding?" -
No, for star winding, you shouldn't connect A1X11, B1X11 and C1X11 together. Leave them open as they are. ust connect the star point of PT Y-secondary windings to earth.
Regarding Delta secondary, one of the terminals of broken delta is already connected to earth and that is adequate.