Inductive Kick from Transformers

Voltage is the inductance multiplied by the rate of change of current, so, yes, there will be a spike. How large it is depends upon whatever else the transformer is connected to at the instant the current changes and how the change in current is reflected on the primary circuit.

Ground/earth has little relevance, as it only exists to carry fault current in correctly-designed systems and there is no fault in this case.

Thanks for replying. I guess what I am looking for is would the voltage continue to rise until there is a arc across the secondary open circuit terminals. I am thinking of how fluorescent bulbs work with the inductive ballast and the starterswitch.

When one switches anything off that is running on an AC supply, there is usually an arc at the switch. Does that answer the question?

even if the AC supply is cut off only at the secondary of a transformer? Wouldn't the stored energy in the magnetic field be able to release its energy through the primary winding since its connected to a voltage source?

The inductive kick is the analog of a water hammer in pneumatic circuits. The faster you stop the flow, the higher is the force to keep it flowing, be it water pressure or voltage.

In water circuits they install small compressible air columns to absorb the kick, in electrical ones, they install capacitors, in both cases, good enough to resist the peak potential.

Remember, with an inductor and a flowing current, you have stored energy and the product stays the same (less circuit losses - resistance etc), so as you try to make current go to zero, the voltage tries to go to infinity, usually something breaks, a spark or ??

Yes there is an inductive kick when the transformer primary is opened and there is no load or a light load on the transformer secondary.

A capacitor across the transformer primary will reduce the voltage kick.
My boss said that if the capacitor current was 1/2 the transformer exciting current the voltage kick would be twice the peak AC voltage.
If the capacitor current was equal to the exciting current, there would not be a voltage kick.

Sorry, but I was talking about when the transformer secondary is opened and the primary is on a voltage source. Similar to open circuit tests performed on transformers.

1. Assuming the voltage source is an AC (50 ro 60Hz) the opening of the secondary side circuit breaker will normally break the current as the current sine wave goes through zero. In which case there will be no voltage spike. If the current is chopped by the circuit breaker as sometimes happens with Vacuum cbs (that is the current goes to zero before the normal sine wave goes through zero) then a voltage spike is probable.

2. Transformers have a shunt impedance (look at the equivalent diagram of a transformer) which will prevent the spike reaching "infinite" voltage.

Anytime the flow of current through an inductor (coil) is interrupted, a spark or spike of current will be experienced. This is true for both the secondary and primary sides of your transformer. The generated opposing energy is due to the (L di/dt), where the initial magnitude is normally determined by things such as; initial power applied, frequency of interruptions, number of turns, size of wire used in that particular coil or inductor, etc. Then the energy amplitude will gradually diminish or decay as a factor of your coil's reactances.