BIL rating of fuse. 1

[uts123]

Dear All,

Are fuse rated for BIL?
I haven't seen in manufacturer's catalog that it is rated for XXX BIL. I saw fuse is rated for current and system voltage.

So if we are using fuse for 95kV BIL system, the fuse should be rated for 95kV BIL. Am I right?

And there is no BIL declared in fuse manufacture's catalog. how I know that fuse is rated for which BIL ?

Please reply.

 

[jwparker]

BIL is usually left for transformers, switchgear etc. since a fuse really isnt going to take a impulse the same way. You would just specify the AIC, or amps interrupting capacity. If you're trying to design around a bil then look into the instantaneous time curves.

 

[cranky108]

BIL is for the fuse holder. Current and voltage is for the fuse.
However, there are deratings for both for tempeture, and alitutude.

 

[jwparker]

Sorry that was a bit vague,
BIL is used to determine voltage spikes.
AIC is used to determine amperage spikes.

Voltage spikes dont matter too much to fuses as they are designed as a current limiting device.

 

[jwparker]

How do you edit posts??

Anywhoo, ignore the part where I say BIL and instantaneous time curves, I havent had my coffee this morning, and that statement is completely wrong.

 

[dpc]

This is the manual typewriter of forums - no editing here.

If you really regret a post, you can red-flag it and ask that it be removed or corrected.

 

[uts123]

Dear all,

Still I have confusion.
Transformer, Switchgear (Breaker, Instrument transformer, Insulators) are rated for BIL and they are rated for system voltage as well. As these equipments are seeing voltage so rated for system voltage and BIL.

So Fuse is rated for system voltage because fuse will see the system voltage. Now when fuse see the system voltage and rated for system voltage then why it is not rated for BIL ?

Please give your view.

Thankx.

 

[davidbeach]

BIL determines how much overvoltage to ground the device can safely be exposed to. In the case of a fuse, it is the fuse holder that has the insulation that much withstand any overvoltage. The fuse itself is expected to be held away from any grounded surface (or other phases) by the fuse holder. Therefore it is only the fuse holder that needs a BIL rating. The fuse then "takes on" the BIL rating of the fuse holder in which it is installed; a given fuse can have multiple BIL ratings as part of a fuse/fuse holder assembly, but by itself has no BIL rating.

 

[RRaghunath]

Another point about voltage rating of fuse is - when the fuse blows and opens the circuit, the gap between the live parts inside the fuse cartridge should offer sufficient insulation to prevent restrike. This is the reason why the fuse length has to increase with its voltage rating.
With regard to BIL, I agree with davidbeach - is not relevant for fuse as it is an in-circuit element and not connected between phase and earth.

 

[uts123]

Hi David,

I have learnt a lot from your response in Eng Tips. But still I am not convinced with your view in this issue.

You are saying that Fuse Holder will take care about over voltage to ground and fuse has nothing to do with voltage. If it is so why fuses are rated for voltage as well, like 5kV,5A 15kV,5A 25kV,5A. And we can't use 5kV,5A fuse for 15kV system.

Since either end of fuse will exposed to system voltage when there is L-L or L-G faults so we need to put rated kV fuse.

So there is no doubt that fuse has to takecare of voltage indepndently of fuse holder. So why it shouldn't takecare of BIL.

Please give your opinion.

 

[cranky108]

The voltage rating on a fuse is a measure of the voltage breaking level of the fuse. Or basicly the length of the fuse tube, or cord.

 

[stevenal]

Perhaps the confusion is over the BIL units; volts. But BIL is a measure of insulation. Why would you expect a normally conductive fuse to achieve a specific insulation level across its length after it is melted? It will only melt until conduction ceases, and this a random type of event. You will not be depending on the melted fuse to provide isolation for deenergized work. And the fuse is already worthless, no need to protect it from damaging impulses. They are rated for voltage, but not for insulation.

 

[davidbeach]

One last try: Things that have BIL ratings have parts that are connected to line voltage and things that are connected to ground. Fuses, like open conductors, are only exposed to line voltage and rely on something else for insulation from ground, usually air. Fuses need voltage ratings because there will be a voltage across an open fuse (a good fuse is all at a single voltage) and you don't want a fuse to fail to interrupt because it can't draw the arc out long enough for it to extinguish.

 

[rbulsara]

Fuses essentially becomes part of the conductor and not insulation. BIL apply to insulating capacity. Fuse voltage rating is for what davidbeach said in his last post. While conducting, the voltage across the fuse is essentially zero.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Voltage rating of fuse is more relevant for the need for fuse link with adequate number of locations to break to help extinguish the arc during its operation. Higher voltage requires more link breaks, longer fuse link and more extinguishing material.

Rafiq Bulsara

http://www.srengineersct.com

 

[uts123]

Last Try!

I came to know that BIL is apply to the equipment whose one part is connected to voltage and other part is connected to earth.

Now suppose a star connected PT (grounded) of 25kV system. If all 3 fuses of PT blows out, then one end of fuse will exposed to voltage and other end of fuse will exposed to ground through star point.

Suppose now voltage spike come in the system due to lighting. is there chance of flashover across the fuse? As other part of switchgear is propery BIL rated and air gap between two points of melted fuse is not sufficient for BIL withstand.

 

[waross]

Well think about an actual fuse. In a fused cutout, the type that is ubiquitous in distribution systems and most likely to be exposed to transients in the BIL level. When the fuse blows, the spring clip pulls the link out of the fuse body to aid in interrupting the current. Now the air gap in the fuse is basically the same as the air gap of the fuse holder. A transient high enough to jump across a blown fuse may may also be high enough to jump across the fuse holder without a fuse installed.
BUT, that is not how BIL is measured or tested. It is measured to ground.
In your example, if the PT had a proper BIL rating before the fuse failed it would still have the same rating after the fuse failed.
The same argument could be applied to every distribution transformer on some four wire distribution systems. The argument would still be invalid.
Yes, a transient approaching BIL levels may jump across an open fuse or across a fuse holder with the fuse removed. That is one of several reasons that many utilities will allow their crews to commence work on any transformers before the primary safety grounds are installed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

Suppose now voltage spike come in the system due to lighting. is there chance of flashover across the fuse?

Yes, and so what? Then there will be a surge voltage on the load side of the fuse. As there would be if the fuse were not blown. The VT on the load side has adequate BIL to withstand the surge or it has surge arrester protection.

 

[davidbeach]

Bill, I think you left out a "NOT", added below.

That is one of several reasons that many utilities will not allow their crews to commence work on any transformers before the primary safety grounds are installed.

 

[LionelHutz]

Why not look at it another way? There is no requirement to do the BIL testing with a blown fuse. Equipment is not purposely operated with blown fuses.

It's not very likely that you will pass the BIL test with a line to neutral connected transformer but say the transformer does pass with a good fuse. Then, what difference does it make if the blown fuse will isolate the 60kV or not? If the blown fuse passes the voltage to the tranformer then the transformer will still pass the test just like it did with a good fuse.