Logic of 240.21(c)(1)

[JNMoffa]

Hello all,

Does anyone understand the reasoning behind 240.21(c)(1) of allowing single phase 2-wire and multiphase delta-delta 3-wire transformers to have the primary OCPD also provide protection for the secondary transformer conductors but not other transformer configurations?

Thanks

 

[LarryFine]

Each section of the secondary acts a a 1ph transformer because only one primary winding supplies it.

 

[JNMoffa]

If that were true wouldn't a center-tap delta delta transformer be included as well under this section?

 

[steve66]

No, I believe a fault involving the center tap would only cause half the current to flow in the primary, compared to a line to line fault.

This is because the voltage to the center tap is only 1/2 the line-line secondary voltage, so the power delivered to a fault would only be 1/2 that of a line to line fault, and since the power is 1/2, the line current on the primary would only be 1/2 because the line-line voltage is a constant.

 

[synchro]

To expand on what Steve said, the ampere-turns (i.e., amperes x number of turns) on a transformer primary is equal to the ampere-turns on the secondary (except for a few percent more on the primary for the magnetizing currrent). This is because the ampere-turns determines the amount of magnetic flux that flows around the core through both the primary and secondary windings. And so if you have only L1-N loads then just 1/2 of the secondary turns are active. Therefore this half of the secondary winding must conduct 2 times more current to produce a given amount of ampere-turns than if the whole winding was used. But only the total amount of ampere-turns are reflected over to the primary winding, and so the primary current will be the same whether there's 2x current in a half-winding or if there's 1x current in the whole winding.

And so with the center-tap there is not the one-to-one correspondence between primary and secondary currents that is necessary to protect secondary conductors (and secondary windings) with only primary OCDs.

 

[Carultch]

Hello all,

Does anyone understand the reasoning behind 240.21(c)(1) of allowing single phase 2-wire and multiphase delta-delta 3-wire transformers to have the primary OCPD also provide protection for the secondary transformer conductors but not other transformer configurations?

Thanks

On transformers with qualifying topologies, overcurrent on each secondary winding will be in direct proportion to a corresponding overcurrent on the primary winding. I call this protection by proxy. Given a 480V:240V transformer, a 200A breaker on the primary effectively acts as a 400A breaker on the secondary, in a topology that qualifies. Single phase transformers that are 2-wire on both sides, and three phase delta-to-delta 3-wire to 3-wire transformers are the topologies that qualify.

In transformers of other topologies, such as anything with a centertap, or anything with a WYE system, there is no guarantee that overcurrents will line up winding-to-winding. An overcurrent on the secondary could be redistributed among all phases on the primary, so that it appears to the primary OCPD to be ordinary levels of current that is "business as usual", thus not trip the primary OCPD.

 

[JNMoffa]

That makes sense, thank you all for the replies.