2 x 5kVA 230/110V single phase AC transformers connected in series ( cascaded), would the inrush current (5X) be double or just the same as one? or somewhere inbetween.

Question

I have asked this in some other forums but did not get a conclucive answer. 
 Its for the protection, there would be no load at switch on, curretly the designers has used a 25A BS88 fuse . from what I have discussed with others, the inrush current would be the same but for a longer duration as there is no electrical coupling between the two transformers. i.e. the second cannot magnitize until the first has saturated.

mapj1 · Accepted Answer

So double isolation - in effect the first transformer is loaded by the other. Unusual, rather than a single 1:1 transformer, - I assume you have a high voltage platform to supply or something. In any case, the situation will be similar to the parallel case - there is almost no delay - the first half cycle on the primary of T1 will be the first half cycle on transformer T2 as well. You may expect more or less twice the peak in-rush current, but probably not quite due to the conversion losses in T1.

mapj1 · Answer

The fuse current rating should not be based on inrush anyway - except that what you do not want is a 'fast blow' fuse. Now some of the fuses in BS88 are real slow and 25A seems about right for 5KVA max load. 
 I have highlighted the 25 amp curve in the set below and unless you think the inrush will take more than half a second or so, you should be good for well over 100A.. and at that level the dip in the supply voltage may well help you.

Mike

2 x 5kVA 230/110V single phase AC transformers connected in series ( cascaded), would the inrush current (5X) be double or just the same as one? or somewhere inbetween.

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