What does surge current look through large 30a transformer?

[rstofer]

I thought I saw in home depot the circuit breakers are rated for 10kA breaking.

Yes, some but not all.

Given a choice, I would buy the higher rating.

[Doctorandus_P]

A long time ago I bought an 750VA toroidal isolation transformer (230Vac - 230Vac).
I also added a low Ohmic shunt resistor for current measurement and a voltage divider, so I could both measure current and voltage with my scope without too much hassle.

Every now and then the transformer hums very loud (for a transformer) for a few seconds when plugged into mains. Even without a secundary load.

I put the shunt measurement box in front of the transformer, instead of behind it.
According to the shunt / scope combination I measured 300A+ peaks.
On my scope I see very high peaks (over the shunt) when the transformer goes into saturation.

It has something to do with remanent magnetism in the transformer, and the moment it is pluged in to mains. If this combination is bad, then the magnetic field in the transformer goes into full saturation and the only thing limiting the current is the Ohmic resistance of the primary winding and the power cord. (and my shunt resistor.

The humming becomes inaudible after a second or so when the thing stabilizes.

For audio amplifiers it is very usual to put a 10 to 20 Ohms resistor in series with mains, and to short that resistor with a relay for normal operation afther the big Elco's are charged, and the transformer has stabilized.

[Berni]

Yes this is particularly a problem for large toroidal transformers.

Its a combination of the magnetization left in the core when its unplugged and the transformer being plugged back in outside of zero crossing when the voltage is the same direction. The two add together to make a stronger than normal field in there and it goes straight into saturation. Why its so pronounced likely has something to do with the particularly low leakage inductance of toroidal transformers where the field has nowhere to escape out of the core so once one part of the core goes into saturation it suddenly throws all the rest of the core into it.

I found it common to sometimes see the lights dim for a very brief moment when plugging in large toroids.

Some of the Keithley 2000 series multimeters have a bit of a concerning way of doing automatic 110/220V selection using a transformer. By default the thing powers on with the 110V winding but there is a resistor in series with the mains to limit the current. Then once the circuitry gets powered up it senses the mains voltage and upon deciding it is indeed getting 220V it flips a relay that changes the transformers primary onto the 220V tap. As a result powering up the meter results in half a second of angry transformer noises before you hear the relay click and the transformer goes silent.