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blown NST's

Hi all
	For around 4 years I've used 3 NST's (12/30 each) hooked up
in parallel, using a static gap, with no problem. Recently I build a rotary
gap..... now all three NST's seem to have blown !
They are all shorting to the metal case internally.... one of them has a
real bad DEAD short, and knocks the trips out immediately.
The other two seem to knock the trips out after a minute or so, they
have a fairly high leakage to the cases, but it's not a dead short.
Now, I've read all about this business of de-potting NST's, but these
ones seem to be potted in resin ! At least , it looks like resin to me.
It's very hard to even scratch, and the stuff is a pale yellow colour, so
I reckon it's impossible to de-pot these. They are "RICCI" units.
Can someone confirm that these are resin-potted ?
Further to this, if I can't de-pot these, is it not possible to used them if
A. Use them without the cases earthed (Dangerous ?)
B. Cut the cases off with tin-snips, and peel the metal off (Also dangerous
These units are housed inside a cabinet, and could be mounted on rubber
mountings to isolate them from ground, thereby getting rid of the tripping
problem, but will they work properly if they are not earthed ?
						Richard Barton.