For an inductive load, the steady-state condition is for maximum current to coincide with zero voltage and maximum voltage at zero current. When you suddenly connect an AC voltage source to an inductor, your current initial condition is zero amps. If this happens at maximum voltage, then steady-state conditions are obtained right away and there is no current transient which must die-down before steady-state is achieved. In the worst case, where you close the switch at minimum voltage, it will take some time for the current to transition from the initial zero to a sinusoid which lags the voltage by 90 degrees. This can be modelled as the sum of the steady-state current plus an exponentially-decaying transient current. As the resistance in the circuit goes toward zero, the peak value of this sum approaches twice the steady-state peak value. The math involved can be seen here (no guarantee given that there are no errors !): https://ibb.co/bDkORL So, a 2x transient may not seem that bad but consider what happens if your inductive load is close to saturation in steady-state operation. Twice the current may put it well into saturation, and then the model I gave is no longer true, since the inductance decreases during the transient, and the final peak current may be 10x or more times the steady-state value. This is especially a risk with transformers because the flux in their core is typically maximum in the unloaded state (secondary open). Their core size is determined primarily by heating from losses and in their desire to minimize the size, weight, and cost of the transformer, they typically have only a slight margin below saturation in the unloaded state. Transformers routinely experience large transient current surges when suddenly connected to the AC mains at a random phase. Sean On Sun, Oct 21, 2018 at 9:41 PM RussellMc wrote: > With a capacitor it is intuitively 'obvious' that maximum AC voltage poin= t > is the worst for closing a contact. > With an inductor "this is not the case". I am not going to risk a one > sentence explanation here :-) - rushing off to do "stuff" - so others may > wish to comment on the joys of closing and opening switches in AC loads. > > > Russell > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .