>
> The lowest spikes I measured with the NPN-transistor switching the relay,
> especially when I put 330pF between base and ground (22 Volts spike).


Slowing down the turn-off will do that, at the expense of heating the
transistor some while it's in the linear region.

I had a similar problem on an SMPS once, 60V transistor was showing 58V
spikes, but the factory told me the transistors were tested to 70V.. Turned
out that when I put a 1000V mosfet in there, the spikes were about 200V!
The original transistor was getting into avalanche far faster than the scope
could see.  Lesson learned, make a check with a very high voltage part!

Solved that one by combination of light snubber network and slightly slowing
the turn-off time.
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