current transformer grounding?
current transformer grounding?
(OP)
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.
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.





RE: current transformer grounding?
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: current transformer grounding?
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Sometimes I only open my mouth to swap feet...
RE: current transformer grounding?
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.
RE: current transformer grounding?
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.
RE: current transformer grounding?
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.
RE: current transformer grounding?
http://www.netaworld.org/files/ItemFileA877.pdf
RE: current transformer grounding?
Look in the installation instructions for Multilin or Schweitzer Engineering Laboratory protection relays because they all show the CT installation including a ground connection.
RE: current transformer grounding?
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: current transformer grounding?
RE: current transformer grounding?
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Sometimes I only open my mouth to swap feet...
RE: current transformer grounding?
RE: current transformer grounding?
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Sometimes I only open my mouth to swap feet...
RE: current transformer grounding?
RE: current transformer grounding?
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
RE: current transformer grounding?
RE: current transformer grounding?
RE: current transformer grounding?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: current transformer grounding?
RE: current transformer grounding?
Shorting the CT across it's termninals will prevent overvoltage.
RE: current transformer grounding?
RE: current transformer grounding?
so in answer to original question ,what is the correct answer!! about one side being grounded, and yes we used to short out any un-used ct secondarys to prevent high voltage if the circuit was to be broken at all.Instead of whats wrong, what is correct answer? Please.
RE: current transformer grounding?
Unused CTs should always be shorted.
CT secondaries should generally be grounded (in one place). This becomes increasingly important as the primary voltage increases. At high primary voltages an ungrounded CT secondary is a safety hazard.
RE: current transformer grounding?
RE: current transformer grounding?