Effect of Buck Transformer on SCCR?

[Shackled Designer]

I am reasonably certain that I could eventually get in touch with one of the manufacturer's transformer engineers and get a value for the impedance of the BB transformer itself. Since the transformer's secondary was originally supposed to have an electrical socket attached, though, the overall current limiting could theoretically change depending on the device that gets attached to the receptacle. Knowing what (little) I do about capacitive and inductive reactance, I would expect that certain devices that get plugged into the socket might actually lower the overall circuit impedance and thereby negate the transformer's current limiting.

Maybe such a loss of impedance is atypical in most situations, but then again, we are never very certain what a user might actually plug into a socket labeled, "Programming Port Only."

On paper, a loss of current limiting can happen in any transformer, isolating or not, but with an isolating transformer, the secondary is at least normally insulated from the panel feed's available fault current.

Thank you, templdl and kwired, for your continued input in this discussion.

Kind regards,
The Shackled Designer

 

[templdl]

How long has it been since your wife left you?:lol:

Sorry, that may not be very funny to some people, but also may be hilarious to others.

Excellent point. Thank you kwire for the heads up as the rest of us may benefit by your sharing you personal experience with the rest of us.
I'm still trying to tame my darned NEXUS7 when it automatically changes the words that I am typing such as changing 'similar' which it had changed to 'simulate.' It just doesn't make a suggestion, it just makes the change.

 

[kwired]

I am reasonably certain that I could eventually get in touch with one of the manufacturer's transformer engineers and get a value for the impedance of the BB transformer itself. Since the transformer's secondary was originally supposed to have an electrical socket attached, though, the overall current limiting could theoretically change depending on the device that gets attached to the receptacle. Knowing what (little) I do about capacitive and inductive reactance, I would expect that certain devices that get plugged into the socket might actually lower the overall circuit impedance and thereby negate the transformer's current limiting.

Maybe such a loss of impedance is atypical in most situations, but then again, we are never very certain what a user might actually plug into a socket labeled, "Programming Port Only."

On paper, a loss of current limiting can happen in any transformer, isolating or not, but with an isolating transformer, the secondary is at least normally insulated from the panel feed's available fault current.

Thank you, templdl and kwired, for your continued input in this discussion.

Kind regards,
The Shackled Designer

I can't recall doing any inductance related calculations since college, but remember a short circuit will have a tendency to draw as much current as is available to draw, loads like motors will actually contribute to the available current, but when you are talking high current the impedance of a transformer is likely higher than when there is little load. Maximum available fault current also depends on limitations of the supply circuit. Many simple fault current calculations ignore the supply circuit so the result assumes unlimited capacity from the supply circuit, this is more true for utility transformers than for premises wiring transformers, as we often really don't know what the limitations may be on the utility feed. Exact same install in two different locations can have different utility supply limitations, even when the same utility transformer is used.