ground connection on transformer secondary blowing fuse?

[ftransform]

So I have a isolation transformer, and I wish to add a switch to ground the transformer 2ndary so it is not floating. However if I do ground one of the transformer secondaries (to earth ground that is) the fuse just blows. Why is there a rush of current created by grounding one of the transformers secondaries?

[ftransform]

oh fuck me, i am using a autoformer..................................................................................... I thought it was a 110:220V transformer...................... 

[IanB]

Yes, 99.9% of step up/step down voltage converting "transformers" are autotransformers.

[duskglow]

Correct me if I'm wrong, but that means it's not an isolation transformer either, right?

[IanB]

No, absolutely not an isolation transformer. If you test the (unplugged) transformer with your meter you will find continuity between the input and output sides.