Dear All,

When I tried to protect my transformer (step up), which when the secondary is shorted, the primary draws 300mA, I put a quick acting fuse 300mA, bur I saw that it takes a lot of time to blow. So I had to put a 150mA fuse, which is not correct. I was wondering then what is the usefullnes of the fuses, and how I could protect my transformer, from overheating of the primary if the secondary was shorted

many thanks for your help

 

Typically a fuse of the proper rating should below before the transformer overheats to the point of damage or failure.
For a shorted secondary the fuse should blow in much less than a second.

 

Can you tell us a little bit about the circuit, for example:
What kind of transformer is this?
What is driving the primary, at what frequency and what kind of waveform

Why do you think the transformers are overheating? Many transformers are designed to run very warm and to survive being hot.

Are you putting the fuse in the primary of the secondary?

Are you using a quick-blowing fuse?

You can sometimes select the correct fuse by consulting manufacturer's blowing time charts, such as the chart below. Most manufacturers publish charts like this or provide some data points to indicate the fuse's blowing time under varying conditions.

 

The transformer is a step up transformer giving 7kV at the secondary when you have 225V at 50hz at the primary. For safety reasons these type of transformers use magnetic shunts, to limit current at the secondary at 5mA. So when the secondary is short circuited, you do not have any load, and the mA at the primary is 300mA. The primary is getting hot, not a lot, but I though to put a quick blowing fuse of 300mA, for extra safety. It took a lot of time to blow, that's why I asked the question. To be honest and thank you very much, I didn't know the existance of the fuse graph, which actually can give you info on what fuse you can use. Except the fuse is any other way to limit the current ?

Many thanks for the support

 

It might be that your transformer is "impedance protected", meaning that it is designed to run with the secondary shorted and not need a fuse. It is best to check the specifications.

Yes, there are more complicated ways, but a simple solution does not occur to me off-hand. Maybe somebody else can provide other ideas.

Fuses are tried and tested solutions. More complicated solutions are likely to be more difficult to get working correctly and introduce more potential points of failure.

 

So your fault current is 300 mA. What is the normal opperating current? I would choose a fuse rated somewhere between these two limits.

A fuse running at it's rated fault current is not supposed to trip. Look at Dick's graph and note the lines converge slightly ABOVE the 100% operating point.

Say your normal current is 100mA. A 200mA fuse would still be opperating at 50% of max, but see a 50% overage at the fault condition.

 

Many thanks for your help, I think it is clear now.

 

The transformer is a step up transformer giving 7kV at the secondary when you have 225V at 50hz at the primary. For safety reasons these type of transformers use magnetic shunts, to limit current at the secondary at 5mA. So when the secondary is short circuited, you do not have any load, and the mA at the primary is 300mA. The primary is getting hot, not a lot, but I though to put a quick blowing fuse of 300mA, for extra safety. It took a lot of time to blow, that's why I asked the question. To be honest and thank you very much, I didn't know the existance of the fuse graph, which actually can give you info on what fuse you can use. Except the fuse is any other way to limit the current ?

Many thanks for the support

The best way to think about a fuse rated for 300 mA is not that it is designed to blow at 300 mA, but that 300 mA is the maximum current that it is guaranteed to NOT blow at. That means, to meet that guarantee, it probably won't blow at 301 mA, either. In practice, there will be some actual maximum current somewhere above 300 mA that a particular fuse (not the one in the box next to it) can conduct without blowing. As you move above the current, however, the fuse will not instantly blow. It will take time. If you are just a little above it, it might take hours. But that time drops rapidly as the current increases. How rapidly it decreases determines if it is a fast-acting, a slo-blo, or something in between.

One rule of thumb (for low voltage applications, which yours is not) is to use a rated current that is 125% of the circuit's expected maximum load current.