Transformer inrush and fuse protection

[TheAmmoniacal]

I have a variable (0-270 VAC @ 3 A) isolated/isolation transformer (skyddstransformator) which has a DC resistance of just 2.3 ohm on the primary (2.467 H @ 100 Hz). As you can imagine it trips my circuit breaker every time I plug it in due to the high inrush current, anyone know the most appropriate NTC to use here? Some specific recommendations would be appreciated.

(Putting a 4 ohm NTC from my 'salvage bin' (small and wimpy, unmarked, unbranded and unspecced) in series with the live mains has fixed the inrush issue (I don't know why?). Don't feel confident drawing much current while using this.)

The label on the transformer also recommends fusing both the input and output, I assume saturating it could be pretty bad? Which fuse would be the best here for avoiding damaging it? 3 A fast-acting? 2 A just to be safe? Anyone know some good in-line fuse holders for regular fuses? How fast is fast-acting and how fast would it be damaged if the secondary winding got shorted?

I already have some in-line fuse holders for the ATO automotive fuses, but these are generally only 24V rated so I'm wary of using them, anyone seen one of these burn out on 230 VAC?

[sarepairman2]

don't do that. get a proper fuse, for safety sake

fuses are complicated well engineered parts that require some thought for specifying. if you are gonna whack some random shit in there, at least try to get it half right.

littlefuse has fusing guidelines for transformers.
http://www.ttiinc.com/attach/2257144.html?type=support&primitive=0

This will at least get you in the ballpark.

To properly select a fuse, you must

1) select select a mean time between failure you are happy with (lets say 10,000 on/off cycles).
2) measure inrush currents, operating currents, overload currents, and use fuse manufacturer tables to select a proper fuse.


The more cycles you have, the less protection it offers. The less cycles you have, the more protection it offers.  . It's not so bad as it looks though, if you really do some engineering this might not hold true.

Luckily, for transformers, you often don't need to do all the measuring, so long you have the transformers specs. Simply google fusing transformers and there will be plenty of informative material.

As for your inrush current, since you are building your own device, i recommend a time delay relay, it is a much more robust circuit then a NTC.