Transformer as an SDS
- Thread starter A-1Sparky
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petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
I don't see how it could not be an SDS.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Just about the only transformers that are not SDS's are autotransformers.
petersonra
Senior Member
- Location
- Northern illinois
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- Semi-retired engineer
forgot about them. don't use them real often.
- Location
- Illinois
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- retired electrician
I think there is a lot of confusion becuase a transformer will have a direct physical path between the grounded conductor of the primary and the grounded conductor of the secondary, assuming that both are grounded systems.
The text in blue was added to the 2011 code to make it clear that you can have a connection between the primary and secondary via the EGCs and still have an SDS. Prior to that change, many code users skipped over the fact the the definition said you can't have a direct connection of a circuit conductor to the other system. The EGCs are not circuit conductors.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
I agree that the change in 2011 gives more clarification. With or without the change the way to tell is to ask yourself will the circuit still function as intended (disregarding equipment grounding) if any connection between primary and secondary is opened? If the answer is yes - then it is a separately derived system.
Take a magnetic HID ballast - they are all autotransformers and disconnect the primary and secondary leads that are tied together - the unit will not operate as intended.
Take a 480 x 120/240 single phase transformer and omit the equipment grounding conductor with the primary supply - use non metallic wiring methods to ensure there is no equipment ground. Is the operation of the 120/240 secondary effected in any way by the missing EGC on the primary- absolutely not.
Buck/boost type are basically an isolation transformer connected as an autotransformer... and as such would not be an SDS.
One example I can think of that is neither an autotransformer nor an SDS is a pad mount "service" transformer. To clarify, the transformer is consumer owned, so technically not a service transformer... but it is connected the same as a service transformer, where the primary and secondary neutrals are connected.
kwired
Electron manager
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- NE Nebraska
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- EC
The primary and secondary neutrals are connected but not dependent on each other for the operation. If you had a 7.2 kV x 120/240 unit and removed the bond between primary and secondary, and still supplied the primary with 7.2 kV the secondary will still output 120/240 and will not care what or if anything is grounded.
Quite true... but that has nothing to do with the requirement. The primary neutral is a circuit conductor as is the secondary neutral conductor. Connected to each other disqualifies it as an SDS.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
Probably the correct answer. Most of us will not run into this installation, but is quite common for POCO transformers, but as long as it is POCO owned transformer the secondary conductors are service conductors to us.
Industrial establishments that use medium voltages will have to watch this more carefully than most others.
- Location
- Illinois
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- retired electrician
I really don't see the elecrical difference between two white wires that are connected together and two green wires that are each connected to their associated white wire and then connected together.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
There is no difference. The difference when talking SDS is the intended use of those conductors.
I am not 100% sold that a 7.2 kV primary with one conductor grounded, and a 120/240 volt secondary with center tap grounded is not a separately derived system, but I may be able to be bought. Again like I said when this does happen it is almost always POCO owned and to most of us we are talking service conductors and not conductors of separately derived systems. Not all that familiar with practices at places that do use medium voltages but for it to even be an issue, the neutral of the primary needs to be utilized as a current carrying component of the transformer primary - if it is an equipment ground only then the secondary is definately a separately derived system.
This would be no different than a 480-120/208 transformer, typically they are delta primary. If it was wye primary and actually connected neutral of the wye back to supply then it would be more questionable.
Electrical color blind
:lol:
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Electrical color blind
Make one white and one gray or white with stripes and now you have more of a difference:lol:
iceworm
Curmudgeon still using printed IEEE Color Books
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- EE (Field - as little design as possible)
Yes. Those sections (including additional associated sections) are majorly screwed up. JMO Consider if the install is grounded Y, industrial and includes a ground grid. I'm thinking there is likely more neutral current in the ground grid than in the neutral. hummm ... I wonder if this is a safety issue. I wonder if this is considered objectionable ground current.
ice
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