Electronic – Contact sparking even with flyback diode

There could be some small back EMF effect when the slug moves. However, I seriously doubt that is what is causing the high voltage on closing. There may be other effects:

1. Relay contacts bounce. That means the solenoid will be disconnected multiple times even during a overall "on" operation. These short disconnects that happen after some current has built up could cause high voltage for a short time.

2. Ringing. There is inevitable capacitance in the system accross the coil. When the coil is switched on, it is like energizing a tank circuit. In ideal conditions, this could ring up to twice the input voltage especially with contact bounce. In practise, the DC resistance of a solenoid is usually substantial enough to damp the system well enough, and the R and L of the solenoid dominate.

3. It's not really there. The scope may be showing you things that aren't really happening at transients and especially with poor probe grounding.

I don't know what exactly is happening, except that I'm quite skeptical the EMF is really going to 200 V. I also don't like the PTC fuse being in series for testing these things. Try shorting it out and see what that does. Also try putting a reverse diode immediately accross the solenoid, not at the other end of a wire or on the other side of the PTC fuse. This should be a fast diode.

While rerouting wiring to avoid noise coupling is definitely a good idea, I would also recommend adding a RC (for AC loads) or diode (for DC loads) snubber at your load, i.e. putting it across the terminals of the electric lock itself -- cleaning up the load's inductive spike at its source also avoids radiating noise from the wiring that could interfere with other devices.