As with the arming propellor on aerial bombs, I don't think it was possible for it to explode until the fuse was screwed all the way in. I'm not sure exactly how it worked, but it was a fairly basic mechanical devise.

Then again, so was the impact pistol, and they had problems as well. :-?

Hum. So there are no precedents of torpedoes blowing up inside their tube or torpedo rooms, then?