For a small 24V 1A transformer where should I place the fuse? Secondary 1A fuse or primary 100mA fuse (or above accounting for inrush current)? Mains is 230V. Also, the smallest fuse available in the market seems to be 200mA.
Also, how about a 30V 6A transformer? I guess for a 12V 500mA transformer the answer is obvious, as I have no access to 50mA fuses for the primary. But how useful is a primary side fuse? It's not like the transformer will short out on its own, and it's not like the utility voltage will go up to extremely high levels for any length of time. So I am really placing the fuse to protect the transformer from a shorted load placed on the secondary (including shorted bridge rectifier), right?
Added: Thanks for all the replies. These transformers are for one off projects. They are all EI core. So a primary side fuse with 150% rating at 230V would be:
transformer required fuse actual fuse 24V 1A 156mA 200mA 30V 6A 1.17A 2A 12V 500mA 26mA 200mA
Is that correct? The minimum fuse I can get is 200mA. And above 1A is usually in 1A steps. I might not be able to get a 1.5A fuse. Also, I have no idea if these are slow blow fuses. The cartridge fuses seem to be tinned copper in a glass tube. I could potentially test the fusing current and time for the fuses just to be sure (fuses are cheap).
How much current might a transformer draw at startup? I found somewhere (lost the link) that it may be a lot more (like upto 35A for a 5A transformer, so it is a bad idea to repeatedly switch on and off a transformer e.g. with a contactor).
The small 12V transformer is possibly inductance limited. I was unable to draw more than 650mA from it at the rated voltage (12VAC). Is this how a class 2 transformer should behave? Or should 500mA have been the short circuit current?
If I understand correctly, a primary fuse will protect the transformer from shorts in the winding. Shorts in the winding can arise due to overheating. Overheating can occur due to high current draw in the primary, or inadequate cooling. There are 2 reasons for high current draw in the primary: 1. repetitive inrush current due to repetitive switching on and off the primary side, and 2. high current draw on the secondary side. If I operate the transformer only with a manual toggle switch (not a doorbell), and if I have a suitable fuse on the secondary, then I have none of those problems. That leaves only inadequate cooling which can destroy the transformer winding. Is my thinking correct?
Is it correct that the main reason to place a fuse on the primary is that a fuse on the secondary is no more necessary to protect the transformer? A shorted secondary will draw enough current to blow the primary side fuse. But this will not happen for a class 2 transformer. So a class 2 transformer might still burn up because it was operating for an extended time at slightly over maximum rating, if it lacked a secondary fuse.
[Sorry for so many questions. I am definitely confused about this. My gut says to just go with 2 fuses and avoid headache... but on some sites they even say 2 secondary fuses!]
added Thanks again for all the replies and clarifications. I found in my hands yesterday a brand new 30V 6A transformer. As soon as I plugged it in (no secondary load), the fuse on the primary side blew. It was rated 5A. Thinking it may be an old fuse, I replaced it with a new one. It blew again. The transformer as well as the mains wire to it was already warm. So I guess that answers my own question. Now for the hassle of returning the transformer...