We want to install an isolation transformer to protect our devices and operators,so we bought 100VA,220vac isolation transformers and installed them on the line protected by 20A fuses but the transformer burnt. So then we changed the fuse to 2A fuses but the other transformer also burnt.Now we're thinking of using 1A fuse. But my main concern still is,How do I determine the right fuse for this application?

If my math is correct, and this is at 240 volts, it may be less than half an amp.
Where do you live? and what do you want to isolate?

JUst found this:
File:Trenntransformator.jpg - Wikipedia, the free encyclopedia
100/220= .454545 Amps

P = V * I

VA is also a unit of power, so transformer is rated for 100 VA or 100 Watts.

I then equals P/V, So fuse should be no bigger than 220/ 100/220 or .45 A.

The question still remains; what is the ratings of what you are trying to protect? What are you trying to protect?

I can offer better solutions than an transformer if I know what your trying to protect from what?

The transformer failed by one of two reason, either not rated for the 220 volts, and/or the 100 VA rating is not sufficient.

The issue is with the transformer, not the fuses.

I live in Lesotho and I want to isolate a lamp testing station (just a lamp holder or a holder with a device the shows the parameters of the lamp being tested) from the main supply.