Transformer Secondary Protection Clarification

[xptpcrewx]

Three questions:

1. What exactly does the NEC consider to be transformer secondary protection?

2. Is this ANY OCPD on the load side of a transformer that complies with 240.21 and 450.3?

3. If so, does this imply overload protection is the only thing required, since short-circuit protection cannot be achieved by 240.21(C) with a secondary protective device?

Thanks in advance!

 

[electrofelon]

Three questions:

1. What exactly does the NEC consider to be transformer secondary protection?

2. Is this ANY OCPD on the load side of a transformer that complies with 240.21 and 450.3?

3. If so, does this imply overload protection is the only thing required, since short-circuit protection cannot be achieved by 240.21(C) with a secondary protective device?

Thanks in advance!

I think it would help to note that 450.3 is ONLY about transformer protection, and 240.21(C) is ONLY about transformer secondary conductor protection. They are two completely separate things, although often we meet both with the same OCPD.

As you can see from tables 450.3, there are various combinations of primary and secondary protection. You are correct that secondary protection only provides overload protection.

 

[xptpcrewx]

Thanks, electrofelon, That covers item 3, but how about items 1 and 2 specifically?

 

[electrofelon]

Thanks, electrofelon, That covers item 3, but how about items 1 and 2 specifically?

I guess I don't really understand what you are asking in 1 and 2. It would be a fuse or circuit breaker with a setting matching the scenario given in tables 450.3.

 

[don_resqcapt19]

Yes, the secondary protection is the first OCPD on the secondary side of the transformer. If 450.3 requires transformer secondary protection, the device selected in accordance with 240.21(C) to protect the secondary conductors, actually provides both the transformer secondary protection and the secondary conductor protection.

 

[electrofelon]

Yes, the secondary protection is the first OCPD on the secondary side of the transformer . If 450.3 requires transformer secondary protection, the device selected in accordance with 240.21(C) to protect the secondary conductors, actually provides both the transformer secondary protection and the secondary conductor protection.

I am not seeing anything that says the first OCPD is the qualifying one or that any maximum distance is given. Say I use really big secondary conductors and thus a large 240.21(C) OCPD that exceeds the 450.3 value. Wouldnt it still be compliant as long there was a 450.3 compliant OCPD somewhere down the line, or even multiple OCPD's per note 2?

 

[don_resqcapt19]

I am not seeing anything that says the first OCPD is the qualifying one or that any maximum distance is given. Say I use really big secondary conductors and thus a large 240.21(C) OCPD that exceeds the 450.3 value. Wouldnt it still be compliant as long there was a 450.3 compliant OCPD somewhere down the line, or even multiple OCPD's per note 2?

No matter what size secondary conductors you use, the length is limited unless the conductors are outside.
I guess if you have multiple OCPDs in series with no taps between them, it could be a downstream OCPD that protects the transformer secondary....but that would be a really stupid installation. I would be very likely to red tag it just for that reason

 

[electrofelon]

No matter what size secondary conductors you use, the length is limited unless the conductors are outside.
I guess if you have multiple OCPDs in series with no taps between them, it could be a downstream OCPD that protects the transformer secondary....but that would be a really stupid installation. I would be very likely to red tag it just for that reason

NEC 113.27 - Installations shall not be stupid

It is hard to imagine such an installation. Perhaps something like a secondary system, designed around a future larger transformer.

 

[xptpcrewx]

Good discussion. Overall, "Transformer Secondary Protection" is a poor term. Anytime equipment protection is concerned the full damage curve should be considered rather than just the LT characteristic. If overload protection is the only thing required, then I suppose its not really being protected.

 

[paulengr]

Good discussion. Overall, "Transformer Secondary Protection" is a poor term. Anytime equipment protection is concerned the full damage curve should be considered rather than just the LT characteristic. If overload protection is the only thing required, then I suppose its not really being protected.

Not really. And typically you would not do this. Short circuit protection is normally done on the primary side and due to the fact that trying to provide “tight” protection does not work because of magnetizing inrush. Since it offers little secondary protection that makes secondary protection mandatory in most cases.

 

[xptpcrewx]

Not really. And typically you would not do this. Short circuit protection is normally done on the primary side and due to the fact that trying to provide “tight” protection does not work because of magnetizing inrush. Since it offers little secondary protection that makes secondary protection mandatory in most cases.

Actually, the inrush point is below the transformer damage curve, so when coordinating primary protection this should be taken in to account. I don't see this as being "tight" protection. My point is that something doesn't really qualify as being "equipment protection" if it's only partially protected.