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How to select proper rating fuse!!

paddy

Sep 11, 2012
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Hi,

Greetings of the day!!!!

I have a circuit which operates on 24V DC supply and operating current is around 150mA. I want to know what are the parameters that needs to be considered while selecting suitable fuse. Can only operating current of a circuit is sufficient to select the fuse?
Does a 24V DC/500mA slow blow fuse will do a job?
Please guide.

Regards,
Paddy
 

Arouse1973

Adam
Dec 18, 2013
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Depends on the maximum current the citcuit will use. This will effect the type of fuse to use also. If there is a large inrush current then I would go for an anti surge type. Try a fuse with 25% over head to start with and see how you go.
Adam
 

Colin Mitchell

Aug 31, 2014
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A 500mA slow-blow is far to high to provide protection.
The only time this type of fuse will blow is when the project has short-circuited.
In fact you cannot protect a 150mA circuit.
Even a 315mA will only blow when the circuit has shorted.
Any small rating fuse will just bend when the circuit is turned on and eventually fail through fatigue.
 

paddy

Sep 11, 2012
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Thanks for advice.
I have to certify the circuit as an intrinsically safe circuit ("ia") in accordance with IEC 60079-11. Keeping in mind that thing can you advice any specific fuse?
 

Colin Mitchell

Aug 31, 2014
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It depends on the current capability of the transformer.
If the transformer is capable of delivering 500mA, the fuse will never blow.
You have to fit a 315mA fuse and turn the circuit on and off 100 times with 5 minutes between.
Look at the fuse and see if it "dips."
 

Merlin3189

Aug 4, 2011
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I'm far from expert in this field and having looked at IEC 60079-11, I think you need some specialist expert advice. An understanding of failure modes and critical parts of your specific circuit/device would be needed.
That said, I had a few observations.

Fuses operate by melting metal - not what you want in an explosive atmosphere, so if the fuse is part of your device, you need special safe fuses. Try looking at the info provided by a manufacturer of such fuses.

It always takes some time to melt a fuse, so even if the fuse is in a remote power source, you need it to melt before any part of your circuit gets hotter than the specified temperatures. Looking at your worst case failure, how long does the critical component take to reach that temperature? Then you know how fast your fuse needs to blow. If that time is too hard to meet, then you need to uprate the critical components so that they heat up more slowly.

For over-Voltage protection, people use solid state 'crowbar' circuits to create a safe short circuit to quickly blow the fuse.
That may not be what you want here as you are concerned about excess current. So maybe a foldback current limiter could be used to prevent large over-current and trigger a crowbar fuse-blowing circuit.
 

profbuxton

Nov 22, 2014
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This is for an intrinsically safe application???. From my experience these applications require strict limits on the energy which can be supplied into the hazardous area. I don't recall the energy limit, its not very high,it needs to be below the energy required to set off some particular level of explosive atmosphere.

I don't know if you are designing an intrinsically safe device or not, if you are then you should have enough expertise to do so, otherwise I suggest you use one of a number of devices available which have been certified for use in a hazardous area.

These devices usually have a current sensing resistor to drive a "crowbar" SCR which will blow an internal nonreplaceable fuse. Then the complete unit must be replaced, not repairable.

PS: From your post I will assume its for some sort of current loop transmitter. Be aware that some transmitters when used with sensitive intrinsically safe barriers can "trigger" the "crowbar" due to switch on surge. Been there, it happened.

PPS: the intrinsically safe barrier needs to be mounted in a safe(non-hazardous) area
 

paddy

Sep 11, 2012
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Hi all,
Thanks a lot for your suggestions!!!!

Hi profbuxton,

yes it is current loop transmitter used for gas detection. The electrical rating of 2 wire transmitter (used for detection of toxic gas) is 24V/25mA. And for 3 wire transmitter (used to detect explosive gas) the electrical rating is 24V/150mA.

As suggested by you we generally used barriers in safe area for protection but I am trying to implement it within the circuit to avoid the use of barrier.

Please advice.
 

profbuxton

Nov 22, 2014
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I don't know how you can avoid use of a intrinsically safe barrier if used in a hazardous area. Surely this is a closely regulated safety issue. If you are designing a circuit of your own then I would expect it to be subjected to stringent test by the appropriate authorities before allowing it to be used.
 

davenn

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Sep 5, 2009
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I don't know how you can avoid use of a intrinsically safe barrier if used in a hazardous area. Surely this is a closely regulated safety issue. If you are designing a circuit of your own then I would expect it to be subjected to stringent test by the appropriate authorities before allowing it to be used.


Agree totally
 

paddy

Sep 11, 2012
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There are several battery operated devices available that can be used in hazardous area. So i am just trying to achieve it in current loop transmitter. Surely we are going to test the circuit from certification body and for that purpose only I am trying to comply the clauses given in standard.
 

Rory Starkweather

Nov 13, 2014
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I don't think anyone makes fuses that small, and the 'authorities' probably know that.

Add a huge flashing red LED, speech synthesizer and audio amp set to say "She can't take much more, Captain."
 

Calmore

Jan 19, 2015
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Many, many moons ago I used to manufacture 'fireproof' systems for use in explosive atmospheres and they were essentially hermetically sealed enclosures made from cast metal. They were even pressure tested to ensure no gas could leak inside.

I haven't studied the regulations but suspect you *could* use fuses, but only if other specific precautions are undertaken. As others have said I think you need someone with knowledge of the regulations to advise further.
 

profbuxton

Nov 22, 2014
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It is possible to fuses in very low current ratings. I have used 50mA fuses in cards used in computer systems to protect analog input circuits.
 
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