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by GScrimES - no comments
What a great question: Protection for transformers.

Do you protect the primary or secondary?

In perspective: Yes there are different types of transformers like say auto transformers –

but in any and now that I think about it – I am sure this will apply to all types of transformers, and yes there might be the safety occasions when it is unsafe to have protection devices in circuit and so forth

– the specialist should take care of that – this information is in general.

The short answer is: Protect the input from short circuits and the output from overload.

So what should this overload be?

One manufacturer says: Just put the rated current of the transformer or lower.

Well if you put the rated 10Amp fuse in a 10Amp transformer will it ever trip?

The fuse its rated for 10Amp so it will never trip…and I have not even mentioned the temperature affects etc.

Why? It will never trip! Depending on the amount of queries on this I will do fuses too. 

This is what I do – before I even started doing this product assessment forum thing –

Calculate the nominal actual maximum load current the transformer will supply.

Overrate the transformer by a minimum of 20% to 30% that’s ideal – bigger the better – whatever you can afford.

Add 10% to the actual maximum current -that’s it, protection rating. DONE

Ahhh, but at 10% we came to the same protection rating…really?

What does one achieve: A design built in safety factor. 

What about the primary?

The primary is normally protected by the SCPD(Short Circuit Protection Device) that supplies the circuit…just make sure you wire it with correct sized components – wire, switches etc…then it will last forever.

Though if you have a worry-bug and you want to install a fuse:

Protecting the primary is one has to account for the high start-up inrush currents…:

And I can see the questions coming –

One is:- Why don’t you just use a circuit breaker on the primary and when the load current increases it will trip?


Why do you use a fuse and not a circuit breaker as the protection on the secondary?


Lets cover this here, with manufacturers / standards recommendations and all.

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