Why 250mA fuse for 4-20mA analog input?

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Thread Starter

Mohamad Ashour

Hey guys,

I see a lot 250mA fuses put inline with each Analog 4-20mA I/O. The thing is I don't know why they use 250mA. My thought is if you have 4-20mA loop, the highest continuous current will be 20mA. Per NEC, the fuse size is 125% of continuous current plus 100% of noncontinuous current. So fuse size will be 20mA * 1.25 = 25mA (don't know noncontinuous current).

So where the 250mA fuse came from?

Also related to this topic, I have a client saying they have a standard to have 250mA for inputs (Analog or Digital/PLC relay outputs) and 500mA fuses for outputs (Analog or Digital) and I don't why either. To me Analog Outputs should be the same as Analog Inputs and Digital inputs consume about 10mA which is less than 20mA (10mA * 1.25= 12.5 mA) and Relay output depends on load which might be solenoid with 1-2A consumption.

Can somebody clear my confusion?

Sorry for the long text. that's what happened when you are confused:)

Mohamad Ashour
 
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Bob Peterson

NEC fusing is not really applicable to this kind of thing as these are neither feeder or branch circuits.

Even so, you would be misapplying the rules anyway.

The fuse is there to protect the wiring. It is sized to the ampacity of the wire.

The wiring is what is sized to a 125% of the load, not the fuse.

Further, the code specifically permits one to up size a fuse or circuit breaker to the next highest standard size (as long as that is 800A or less), and 1A is the smallest standard size of fuses that is listed in the standard size fuse chart in 240.6(A).

The fuses don't really serve much of a useful purpose IMO anyway. At one time people used them for isolation and to try and protect the I/O but fuses don't really work all that well at either.

--
Bob
http://ilbob.blogspot.com/
 
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Bruce Durdle

Many 4-20 mA transmitters will pull 25 mA in the event of an internal fault, or if a seriously overrange input is applied. And don't forget that a fuse running too close to its operating point will get hot and is more likely to fail if vibrated or shocked.

The fuse is not to limit the current to the transmitter or receiver but to protect the wiring and power supply in the event of a fault. What's the limiting current of your signal wiring?

Mu minimum value would usually be 100 mA - but 250 mA isn't excessive unless the conductors used are very small diameter.
 
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Mohamad Ashour

Thanks Bob & Bruce for your answers.

Bruce,

its clear from your and Bob answers that the fuses are for protecting wiring and power supply.

what do you mean by the limiting current for signal wiring? does that mean the ampacity of wiring which is in my case is 18 AWG? 18 AWG carries around 7 Amps (somewhere in 400 NEC articles mentions that) which is much more than 100mA or 250mA.

My question is there a calculation or rule of thumb to get these numbers or it was decided long time ago based on experience that 100mA/250mA is the highest current should be in the wiring and kept using these values ever since.

In other words are these values arbitrary or there is calculation or rule of thumb to get to these values whether they are 100mA or 250mA?

I know I am asking why like 6 years old but I am just curious. you are welcome to point me to a source or standard where I can find the answer.

Thanks,
Mohamad Ashour
 
Are you sure about 250mA fuse is in line with 4 ~ 20mA signal or you have a module which has no of channels and this fuse is for protection of Module?

Please update even if I am wrong.
 
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Wiwi Hartono

Hi,

Could it be 250ohm resistance?
If it is, the purpose would be to convert to 1-5VDC from 4-20mA which is very common.
 
I suspect that a fuse installed in a 4-20mA loop is an attempt to protect an analog input on a 2 wire, loop powered circuit, and that the fuse is not there to protect the wiring.

The fault condition that can damage a 2 wire loop powered analog input is a short between the two wires going out to the transmitter, because that short puts the power supply voltage (typically 24Vdc) directly across the analog input (+) and (-) terminals. I don't know what style modern day internal AI dropping resistors are, but they're not definitely not power resistors.

Ohms Law arithmetic (E2/R) shows that a 10 ohm resistor won't last very long dissipating 57 watts, nor will a light duty 250 ohm resistor last very long dissipating 2.3 watts.

The cost of a bad AI due to a burned out dropping resistor is not just the cost of replacement or repair, it's downtime.

A fuse could protect an internal 10 ohm resistor from a 2.4amp current.

Some manufacturers use a PTC (positive temperature coefficient) thermistor for AI protection.
 
> 25V / 250 ohms = 100 mA, so a 250 mA fuse is not going to do this job - 50 mA might.

250mA fuse may only got a few Ohms resistance (for example, glass fuse without filler is around 1Ohm).

I'm not sure how the fuse place in the circuit, but I suspect the manufacturer put fuse here to protect the circuit from SHORT Circuit event, but not overload event.
 
Hi,

I have a question regarding this. i am using siemens PLC. In my panel two wire analog inputs are using. Analog input card destruction current limit is given 40 mA in the manual of the card. So how did 100 mA fuse will protect the card.

thanks and regards,
 
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mausahwrtsila

For my case, I've come across 100 mA fuses across a 4-20mA for a Modicon quantum o/p module. The fuses were connected across O/P signals for 3-way valves.
 
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Most of the time the fuses are used for isolation as much as anything, so it really doesn't matter what the rating of the fuse is.
 
The value used is a result of several considerations as you can see from others posts... but in general it is an arbitrary value. The main consideration is protecting other circuits (loops) powered wth the same supply, i.e. so one faulting loop doesn't take out the power supply and thus all the loops.

You want a value high enough that it will not blow on an internal fault as mentioned by Bruce, but yet low enough that a wiring fault will blow the fuse instantly and not take out the power supply.

IMO, using a 250mA fuse is quite possibly a choice made by economics. 250mA is 1/4A and a very common fuse rating compared to others in the mA range.
 
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