Series Connected Fuse for XFMR Protection

[ElecDan]

Two Questions:
For transformer protection:
1) If I need to use series connected two fuses for transformer primary protection, can I use one fuse on the primary side and on the secondary side the other, or I must use both of them on the primary side in order to call it series connected?

2)If I need to use a current limiting fuse (CLF)for transformer secondary side bus Arc-Flash protection purposes - what location I should be using the CLF - primary side or secondary of the XFMR?

Thanks for any comments.

 

[davidbeach]

It depends. Far more information would be required to begin to figure out the answer. Why on earth would you need two fuses in series anyway?

 

[ElecDan]

Hi David,

Thanks for your comments.
Basically primary is 25kV and secondary side is 600V. Fuse suggested in design is rated for less than 25kV and fuse manufacturer suggests to use another series connected fuse in order to meet the voltage requirements.

The concerned XFMR is a station service one with a capacity of 225kVA.

 

[ScottyUK]

Fuses with adequate voltage rating are available - why not just fit the correct fuse?

 

[davidbeach]

What Scotty said. I've seen the results of that sort of nonsense, back when the use of 34.5kV on wind farms got ahead of the available fuses. It's ugly when the wrong fuse blows first. If it is at all possible to do it with one fuse, that's the way to go. The apparent higher first cost is far less expensive than the fix after a fuse actually blows.

 

[ElecDan]

I totally agree with you Scott and David. On that note I would like to add that sometimes it's game a between the business and technology/science, and in that sometimes you end up having only 20% technology or science and 80% business!

In fact a world known manufacturer has stated in fine-print in their fuse data sheet something like this, if you use this fuse after certain voltage level, then use a second fuse in series in order to meet voltage requirements.

Say someone bought all these fuses as mentioned above without a proper understanding of their power system need, then what? Finding remedies are probably necessary!

In my own design, I would probably use a fuse at the primary of the transformer for short circuit purposes with appropriate ratings and preferably a CLF to limit fault current if that is important. And a low voltage overload breaker on the secondary (600V) side of the XFMR.

One thing I am not sure though, if you use one fuse on the XFMR primary and other one on the secondary side, then you still can call them series connected.

 

[davidbeach]

No, they'd both have to be on the primary. What you're hoping to prevent is catastrophic failure of the transformer for what would otherwise be a minor failure. If it is an oil filled transformer you want to keep the oil from igniting; a fuse on the secondary won't help.

 

[ElecDan]

Thanks David. That's what I was proposing in order to be called series connected.

 

[unclebob]

Fuses alone on the secondary side won't protect the transformer from overload.