fuse for primary or secondary? Is mains breaker will be ok for LOW voltage app?

[001]

Hi!

I use 2x24vac@20a transformer
What the best way to add fuses? Primary or secondary windings?
Can I use automatic mains breaker like this in LOW voltage secondary?

[Ian.M]

As long as its before a full wave rectifier so it only 'sees' AC it will work safely, but if its a thermal or thermal-magnetic breaker, the voltage drop across its series heater element at full load current may be excessive - what's acceptable as a percentage drop from a 240V supply, is probably *NOT* going to please you when the same voltage drop is a ten times higher percentage of a 24V supply.

*NEVER* use an AC mains breaker for DC!
Rare Exception: Unless it actually has a DC rating and type approval.

[001]

*NEVER* use an AC mains breaker for DC!
Rare Exception: Unless it actually has a DC rating and type approval.

Yea
I`m understand it 

The main question is minimal breaker coil voltage drop at rated current and time
And breakers off not at rated currents but at x2 x3 overload
How to select right nominal range?

[Ian.M]

Consult its datasheet!

[David Hess]

There should always be a fuse on the primary side to handle transformer shorts which can otherwise be very bad news.

A fuse on the secondary side is optional.  The only time I commonly see this included is *after* the big input filter capacitors when used in combination with a crowbar circuit.

[Siwastaja]

Many transformers have internally fused primary, and they don't absolutely require an input fuse - consult the datasheet. If it says nothing about this, an external fuse is a very good idea. Estimating the inrush current may be difficult if not specified; a slow-blow fuse should be used, and you can try finding the lowest value which doesn't blow.