CT grounding 1

[tulum]

Quick question...

Does it matter which leg of a transformer is grounded?

Since the current "flip-flops" back and forth 120 times a sec (60hz) I would not think so.

I am not an expert though...

Regards,
TULUM

 

[davidbeach]

A single CT, not interconnected to any other CT, can be grounded on either side. Once you have more than one CT, the grounding point in the circuit can make a difference. Multiple grounds are BAD.

 

[tulum]

Thanks David,

So if we are consistant there should be no headaches...

Regards,
TULUM

 

[veritas]

Tulum

Agree with davidbeach. Never earth both side of the CT simultaneously. This effectively shorts out the CT load. With threephase systems, the CT's are commonly shorted at the neutral point. Again only earth at one point. The idea of earthing after all is to prevent the secondary wiring insulation being stressed beyond its intended design insulation voltage.

Regards.

 

[RalphChristie]

Although it does not matter which side you ground, I was learned (old practices?) that:


To prevent secondary circuits from attaining dangerously high potential to ground, these circuits are to be grounded. Connect either S1 terminal or S2 terminal to ground.
For protective relays, ground the terminal that is the nearest to the protected objects.
For meters and instruments, ground the terminal that is the nearest to the consumer.
When metering instruments and protective relays are on the same winding, the protective relay determines the point to be grounded.
if there are taps on the secondary winding which are not used, then they must be left open.
If more than one current transformer are galvanic connected togher they shall be grounded at one point only (e.g. differential protection)
If cores are not used in a current transformer they must be short-circuited between the highest ratio taps and shall be grounded
It is dangerous to open the secondary circuit when the CT is in operation. High voltages will be induced.

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[tulum]

Thanks guys...

Is there any harm in just always grounding the x1 terminal of the ct? We sometimes ground the x1 and we sometimes ground the x2... However, since ct's such as the hammond CT200 comes with leads where the white is the x1 and the black is the x2... if we ground the white it stays consistant with what we do on PT's.

No electrical logic, just easy for the guys in the field

Any thoughts?

Regards,
TULUM

 

[jghrist]

tulum,
Sometimes in 3ø applications it is necessary to use X2 as common. In this situation you cannot ground X1.

 

[tulum]

Can you expand jghrist?

 

[jghrist]

If the CT is used for directional relaying or power measurement, then you have to use the correct terminals as the common neutral point. Depending on how the CT primaries are connected, this might be X2, not X1.

 

[davidbeach]

Also, in a differential application, you will have multiple CTs connected together and grounded at one location, most likely at the relay. I don't think you can come up with any single rule that would apply at all locations other than "It depends".

 

[stevenal]

Suggestion: Make the grounding point obvious, both on the drawings and in the field. Some wiring I've seen seemed designed to mask a grounding point, thereby making a second ground likely (guess they forgot to ground this). A good reference on the subject is IEEE C57.13.3.