Hello to all!.Am trying to find code article reference for above question. It is going to be on test. I can not find this anywhere! I know that one side of the control Xfrmr,s secondary can be grounded, and that the Xfrmr may have secondary ungrounded( floating system) I just can,t find a specific reference.This question will be on my motor control test. Thanks for any help!
How are control transformers grounded in accordance with the N.E.C?
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Control Xfrmr grounding question
Control Xfrmr grounding question
benaround, thanks for your input,On my shop job, I was asked to make control Xfrmr drawings with connections for (240 to 120 volts) 480 to 240,& 480 to 120. which I did. Xfrmr has dual winding primary & dual winding secondary. I just don,t know if this is a bad question on test or not. The specific question they want me to answer is:How are conrol Xfrmrs grounded in accordance with the N.E.C. Make code article reference. Should This question be treated as a regular Xfrmr question? Where does it say that a Xfrmr may have one side of the secondary grounded and where does it state that Xfrmr may be ungrounded(floating ground)?Thanks Much!!
Control Xfrmr grounding question
benaround, thanks for your input,On my shop job, I was asked to make control Xfrmr drawings with connections for (240 to 120 volts) 480 to 240,& 480 to 120. which I did. Xfrmr has dual winding primary & dual winding secondary. I just don,t know if this is a bad question on test or not. The specific question they want me to answer is:How are conrol Xfrmrs grounded in accordance with the N.E.C. Make code article reference. Should This question be treated as a regular Xfrmr question? Where does it say that a Xfrmr may have one side of the secondary grounded and where does it state that Xfrmr may be ungrounded(floating ground)?Thanks Much!!
Volta
Senior Member
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- Columbus, Ohio
You can find some guidance in 250.20(B) and (B)(1), as Benaround showed.
That will show you what you need to achieve when grounding the system. As it it Separately Derived, 250.20(D) points you to 250.30(A). That will show you the methods required / allowed to ground the system.
Do you know where, on your drawings, you would ground the secondary to comply with 250.20(B)(1) in each configuration?
- Location
- Illinois
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- retired electrician
In general 250.20(B)(1) applies. In some cases you may be able to apply 250.21(A)(3).
Control Xfrmr grounding
Control Xfrmr grounding
Volta, Thanks for reply. The question they gave us on our shop job is not directly related to the Xfrmr diagrams i drew, they are simply asking for a specific code reference as to how control Xfrmrs are grounded. The exact question is:How are control transformers grounded in accordance with the N.E.C. (Make code article reference) I just can,t find any specific reference to this question. I have spoke to a few other students from class and they say the same. We may just challenge the question. I just wanted to get support online from this site. there is much knowledge here! I just don,t know what answer to give for this question, if any at all! Thanks again everyone
Control Xfrmr grounding
Volta, Thanks for reply. The question they gave us on our shop job is not directly related to the Xfrmr diagrams i drew, they are simply asking for a specific code reference as to how control Xfrmrs are grounded. The exact question is:How are control transformers grounded in accordance with the N.E.C. (Make code article reference) I just can,t find any specific reference to this question. I have spoke to a few other students from class and they say the same. We may just challenge the question. I just wanted to get support online from this site. there is much knowledge here! I just don,t know what answer to give for this question, if any at all! Thanks again everyone
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- Bremerton, Washington
All transformers per Art 250 (not autotransformers) require a system bonding jumper and a connection to a grounding electrode system. The first is for bonding, so a line to case fault can find its way back into the transformer windings. The second is to stabilize the voltage and protect against overvoltage in a lightning strike.
250.30(A)(1) Ex 3 only requires the sbj to be a min of 14 AWG or not small than derived phase conductors
250.30(3) Ex 3 gives an exception for a grounding electrode conductor for 1 KVA transformer.
250.30(A)(1) Ex 3 gets the fault current back into the windings and stabilizes the voltage.
250.30(3) Ex 3 does not require the GES, perhaps its just a practical exemption and a small transformer if damaged.
Please read both of the exemptions in their entirety.
I worked on a control system once with an ungrounded transformer secondary. The voltages were all over the place. A sbj solved that.
250.30(A)(1) Ex 3 only requires the sbj to be a min of 14 AWG or not small than derived phase conductors
250.30(3) Ex 3 gives an exception for a grounding electrode conductor for 1 KVA transformer.
250.30(A)(1) Ex 3 gets the fault current back into the windings and stabilizes the voltage.
250.30(3) Ex 3 does not require the GES, perhaps its just a practical exemption and a small transformer if damaged.
Please read both of the exemptions in their entirety.
I worked on a control system once with an ungrounded transformer secondary. The voltages were all over the place. A sbj solved that.
The exact question is slightly misleading. Control transformers are grounded just like any other non-control transformer per Art 250, and Art 450. See especially 250.20 (A),(B) to see if the current and sizing require grounding. There are exceptions that permit ungrounded systems.
"Control Circuit Transformer" is defined in the NFPA79 as a transformer for - you guessed it - control circuits. It then sends you back to the NEC for grounding, bonding, and OCPD.
Ask your prof where his working definition is located.
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