CT Grounding at One Point

HowStuffWorks "Grounding, Electrical"

Principles of Electrical Grounding (pdf)

And if the CT circuit is grounded at more than one point, there is the possibility that current flow from the CT in the ground loop that is formed would bypass the protection or metering equipment it is designed to feed. Especially with protective relays, you want to be sure they see an accurate current value so that they do their job, otherwise they won't do their job.

how would you verify single point grounding on CT

During testing/commissioning (de-energized!), you disconnect the one known ground point (at the place you want it grounded) and measure the resistance of the CT circuit to ground. If it still shows grounded, then you know you have another ground connection. Search for that grounding point and disconnect it. Continue the process until you have no other ground connections, then reconnect your original, desired single-point ground connection.

Note that IEEE Std C57.13.3 recommends "The circuits shall preferably be connected to ground at the first point of application of the circuit (switchboard or relay panel)."

I hope this is helpful.

yes helped me a lot thank you

Raja,

"Why we earth CT's star point at one point..." I understand that your question is that why we earth one of the secondary terminal of CTs at one point.

There is no harm if you earth the same secondary terminal at any other point in the run of the wiring.

I have seen on High voltage CTs coming from Europe having one earthing screw per CT. This is a special Brass Screw with sharp pointed bottom end so that when screwed in fully in to one of secondary terminals (S1 or S2) in directly connects, that terminal to base plate of CT thus connecting that terminal to earth. Normally the manufacturer applies Red Paint to head of this screw to distinguish from other terminal screws.

Besides above earth when we bring wiring to Test Type CT terminals forming star point, we make ensure that Start point is formed at same terminal which is earthed at CT. Also we connect earth wire at star point.

Hence you will find that same secondary terminal gets earthed at two points in the run of wiring by default.

Question is why we earth the secondary of terminal - if we realise this then it will make it clear of there is any harm of multiple earthing or not.

Reason 1: If we test CT for Insulation Resistance between Primary and secondary, though this value will run in Mega Ohms, however large it may be, there is small amount of leakage current between primary and secondary (Ohms law). We must provide some path for this leakage to flow to earth for safety of CT and Low Voltage Circuit.

When we earth one of secondary terminals the path of leakage current is:

Primary conductor -> Secondary conductor through insulation between primary and secondary -> secondary conductor to earth.

Thus above we are providing a path of least resistance to leakage current to flow to earth - thus saving our equipment.

If we do not earth one of secondary terminals, the path of leakage current is:

Primary conductor -> Secondary conductor through insulation between primary and secondary -> secondary conductor to earth via insulation of Secondary conductor earth.

From above we find that by not earthing secondary is dangerous, depending upon ration of Primary to secondary insulation to secondary to earth insulation (two resistance in series), The voltage of Primary (HT) to earh will divide proprotinal to two insulation resistances and can reach dangerous level for low voltage secondary circuit (say dust accamulation on HT terminals of CT).

I had explained above in relation to relative values of Insulation resistances of Pimary to Secondary and Secondary to Earth. Same also applies to Capacitance values (voltage will divide between two capacitance in serics inversaly proprotional to their value).

2. Second Reason for earthing secondary at CT (I explained above that Earthing Screw is provided on CTs) is that in case of any breakdown of insulation between Primary and secondary - the fault current eaisy flows to earh without damaging any LV equipment.

For this reason when we High Pot CT, all secondaries are shorted and connected to earth. This is to save LV equipment, God forbids, the CT fails. As an additional protection if any relay is drawout type that is racked out.

Hence from above we conclude that for safety of equipment, we must earth one terminal of CT Secondary to Earth as closest as possible to CT and there is no harm if we earth same terminal at second point for safety.

Regards,

Ramesh

Dear Friend

Glad to see your response, I liked the way you explained. Aconlytually I did not get an exact answer to my question, yet. I am working on one and half scheme i.e two busbars and three breaker. In which for Transformer metering the sum of CT4 and CT2 has been done. Now designer has make only one point for grounding at CT4's star point not at CT2's star point.

The astonishing thing is that we can only have to ground one CT's star point, despite that CT4's grounded star point electrically connected with CT2's star point, as both wires meet for sum and goes to control room for metering.

As you said we can ground at any point of that grounded wire runs, so why not in above case?

Thanks

Raja,

Could not understood your comments. Is it possible to attach a small sketch of drawing what you are trying to explain to me?

Thanks

Ramesh

If the current transformers are grounded at multiple ground points, the normal current does not create any problem. However, during a fault condition, the magnitude of potential rise at the different ground points of the current transformer will be different. The rise in ground potential may not be an exact representation of primary current and the relay may trip even there is no fault in the protection zone. Tripping a differential relay due to a fault external to the zone of protection is one of the more popular nuisance trips.