Something interesting just happened to me about 10 minutes ago.
I am frequently having to bounce between 120V and 240V. So what I did was install 2 power conditioners in the same rack and just use whichever one I need. I was testing my 240V conditioner with a step-up transformer just to make sure everything was working properly. Afterwards I powered down and unplugged the 240V conditioner and back into the 120V. I plugged the 120V conditioner into a 120V wall outlet. Once I flipped the power on I heard a pop inside the conditioner and assumed it was an internal fuse. So I just took it apart and sure enough I popped a disc fuse, but my outlets are still working.
Any idea why this could have happened? My only thought is that I had just ran some gear on 240V and when I plugged that same gear back into the 120V conditioner maybe some capacitors sent some stored energy backwards through the conditioner and thats what popped it. But I don't know.
As Ron said, you popped an MOV, not a fuse. It is supposed to take the "bad shit" and pass it to the Neutral (or ground, i forgot) so it bypasses your gear. This is why the gear still worked. They wear out. When going bad, they will also add noise to your system, and trip GFCI breakers.
Personally, i have UPS in the system, and power bars. No damn "conditioners"
Chris.
You can turn a "conditioner" into a power bar by clipping out the MOV.