current transformer grounding? 2

[sparkyMV]

Can someone help. we are having a debate if the secondary of a current transformer needs to be grounded?
I have never seen this done, all I have ever seen is just two leads coming of the CT going into a terminal strip or device(meter etc)
I am thinking it may be a difference between european and north american code.

 

[Skogsgurra]

"Everything electric shall be grounded in some way or another" seems to be the Europen norm. There are even grounding wires going from a trnsformer core to ground in most installations.

The grounding of CT secondaries is done for many reasons. One is that capacitive coupling between a HV busbar and the CT secondary is a reality. In some transformers, the capacitance is quite high and the resulting common mode voltage voltage on the secondary side can be higher than the insulation was designed for. Especially if attached cables are short.

In MV installations, the voltage usually is not very high, but grounding is still done. CT in LV installations are also grounded, in Europe. It is only when CT:s are used for actual current measurement in drives that the secondaries are not grounded. But those CT:s and their wiring are then an integral part of the drive and not accessible from outside.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

In my experience a CT, or more often a group of CTs, should be grounded at one, and only one, point in the secondary circuit. Multiple grounds can cause some peculiar relay behaviour which can be tricky to diagnose. I didn't think North American electricity was that far different to the type we use in Europe, so I suspect CTs over the pond are also grounded at one point only. One of the experts from over there will no doubt confirm my assumption one way or the other.


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Sometimes I only open my mouth to swap feet...

 

[sparkyMV]

Thanks, good advice but again my question is then why on doughnut type CT;s there are only two wires coming off the CT and no point to attached a ground. I know you could ground a leg where you terminate the wire but again I have rarely seen this.
The reason this question came up is that we have a CT at the primary of a 3phase ungrounded Y,33KV / 690V phase shifting transformer used on a Robicon VSD. All three phases and the equipment bond/ground cable are through the CT which is a 50:1 doughnut type CT. I assume this is for grounf fault dedection(zero sequence)There is no ground point on the output of the CT or where the two wires terminate at the relay/swicthgear. The relay it self is grounded, perhaps internally the CT inputs are grounded, I don't know as we don't have the internal drawings.Any thoughts are more than welcome, just wanting to understand this. The person that originally asked this question is of european decent, I am from Canada and the project is in Africa.

 

[GTstartup]

Why would you assume that there should be a separate terminal for grounding the secondary? As Scotty stated the return wire of the CT should be grounded, normally at the closest point to the CT usually on a terminal strip.

For an example, go to the junction box of your main transformer and look at the CT terminal strip, there you will see the CT secondaries grounded. This is common in North America.

 

[sparkyMV]

I still don't see a lot of CT's grounded either at the terminal strip or anywhere else, The only time I see them grounded is when they are not being used, "shorted & grounded"
I have spoken to a couple of electrical engs. and they also agree they do not need to be grounded. According to the CEC if the secondary is less than 150V and limited to 5amps you don't have to ground and with the NEC if they are less than 1000V, no live parts and only accesable to qualified personal you don't have to ground.
still not 100% sure on which is the right answer.

 

[GTstartup]

This might help

http://www.netaworld.org/files/ItemFileA877.pdf

 

[LionelHutz]

I see them grounded all the time. They should be grounded.

Look in the installation instructions for Multilin or Schweitzer Engineering Laboratory protection relays because they all show the CT installation including a ground connection.

 

[Skogsgurra]

Hey Sparky! You see them grounded when not used?

Qoute: The only time I see them grounded is when they are not being used, "shorted & grounded"

Are you shure about that? Grounded when they rest in their boxes?

Why are you so unwilling to accept a simple fact of electrical engineering life?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[thinker]

As per NEC-2005, Article 250-170 secondary circuits of PT's and CT's shall be grounded (if primary voltage exceeds 300V to ground). This should be the end of discussions.

 

[ScottyUK]

Are people who don't use or care about the NEC allowed to continue the discussion? It is by no means the final word everywhere - although in this instance it certainly seems to be in line with accepted practice pretty much everywhere else.


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Sometimes I only open my mouth to swap feet...

 

[thinker]

Scotty, I would like to apologize for my last sentence. I did not mean to shut down discussion, that was just improper usage of English (my only excuse is that English is not my native language).

 

[ScottyUK]

No offence taken, Thinker. Your English is very probably better than mine, although as I am a Geordie many people would say that English is a second language for me too!


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Sometimes I only open my mouth to swap feet...

 

[sparkyMV]

you are all right, ct's & pt's should be grounded accordeing to NEC and other documentation out there if they are under a certain voltage. what I meant by seeing ct's shorted and grounded is where they have been pre-installed a system but are NOT being used and NOT in a box, I hope you know Skogsgurra that ct's are too be shorted when not being used or connected to a burden/device/meter. I have also seen many other drawings similiar to multilins etc that do NOT show a ct grounded anywhere. I think there is some open room when it comes to ct's as they are not physically connected to the cables/busbars that they are measuring and in different code books it states they do not need to be grounded if they are inaccessable to un-qualified people. But I agree with you all that grounding a CT is never a bad idea, it is just I have seen many doughnut type ct's where the two secondary leads are just going into a meter etc, with relays it is diferent as many have internal grounding points, thank you all

 

[waross]

The Canadian Electrical Code, 2006 edition, Rule 10-116 is basically the same wording as the NEC rule.
Bear in mind that many of the rules in the CEC and the NEC are written in response to real world accidents and issues experienced by many professionals over the course of many years spread over the United States and Canada.
Not understanding the reason for a code rule is rarely an acceptable justification to ignore it.
One reason of many to ground CTs is to prevent common mode voltages caused by capacitive coupling between the primary conductors and the CT secondary circuit.
Your 33 KV primary is capable of inducing common mode voltages even though all phases and the neutral/ground are present.
Currents will generally cancel but if two phases and the ground/neutral are bunched in the center of the CT window and one phase is close to the edge of the window it will induce a higher capacitively coupled voltage than the other conductors.
Re- seldom having seen CT secondaries grounded. This is probably a reflection of the part of the industry in which you have been working. This is not a put down or a criticism.
There are interesting differences of experience on the forum from time to time between members, for example, who work on diesel generators in the 15 to 1500 KVA class, those who work on diesel generators in the over 2 MVA range and those who work in really large power plants. It is not a question of right or wrong, just different exposure and experiences.
I think that the members here who have a lot of exposure to systems such as you have described expect and depend on secondary grounding of CT circuits.
respectfully

 

[LionelHutz]

So, I'm guessing that you're the one arguing against the need to ground the CT secondary in the debate you mentioned?

 

[Swervy]

HI,I WORKED FOR A PANEL MANUFACTURER FOR YEARS, AND WE ALWAYS GROUNDED THE SECONDARY ON ONE SIDE.THIS WAS TO STOP THE TRANSFORMER BURNING OUT IF ACCIDENTALLY LEFT OPEN CIRCUIT.ALSO HIGH VOLTAGES WOULD BUILD UP IF CT LEFT OPEN, INSTEAD THOUGH IF LEFT OPEN THE SIDE GROUNDED WOULD PROTECT THE CT.

 

[itsmoked]

As the voltage should still skyrocket if one side floats why does grounding one side only help?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Swervy]

I was led to believe that under normal operation the current would circulate through circuit of lowest impedance ie meter etc, earthing one side would drain any build up to ground ie not let voltage increase build into destruction mode!!

 

[GTstartup]

You were led to believe wrong. Open circuit is open circuit, grounded on one side or not - As Keith alluded to.

Shorting the CT across it's termninals will prevent overvoltage.