Max permitted short circuit current according to ATEX for low voltages

[hm_fa_da]

Hi,

In IEC60079-11 for IS circuit design, there is a table showing max permitted short circuit current for different voltages but the problem is the table starts from 14.8V(for IIA gas group) i.e.

I know there is another figure showing the Voltage/Current ratio for different gas groups ignition, but that figure also shows only +10V ...

1-Does it mean we don't have any short circuit current limitation for lower voltages than 14.8V for IIA ??
2-Or i have to consider the first value for lower voltages ? i.e for IIA safety factor 1.5, max 3300 mA for any voltage lower than 14.8V too ?

I have attached the table, as you see for IIA, the voltage range with value in the table starts from 14.8V.



In my case, the circuit has maximum voltage of 3.9V only.

Thanks

 

[FvM]

Is this a serious question? Obviously the highest current value below has to applied.

 

[hm_fa_da]

Is this a serious question? Obviously the highest current value below has to applied.

Thanks for your reply, but may i know why you say that ?!

Is it a general rule for reading this type of tables ?

Or you say that because you are familiar with the standard ... ?

There is a figure in the standard which shows short circuit current / Voltage curve like this :


(This photo is copied from an LTC application note and is for safety factor 1)

According to this curve (which is figured in the Standard too), as you see in the red circled area, for lower voltages, i.e 10V, if you continue the curve, the value for current will be so much high, or maybe unlimited ?!

As an ignition capability of the spark is dependent on both current and voltage, i thought maybe the lower voltages will cross the bound limit of the graph for current, so a spark made by that voltage range can't ignite at all. then max current will be limited by other limitations like Thermal considerations or power ratings of components or ...

I know it's weird but there is nothing mentioned about this matter in the standard document directly, that's why i asked the question ...

 

[prairiedog]

Do you think you can use 10V at 100A or 1,000A?
If you are off the tables/curves then you use the last entry. This is common sense.

You can get a special spark-test done, if you wanted to use more current at your 3.9V, for example.

 

[d123]

Hi,

Any chance there is some department in the organisation that creates the standard that would be able to advise you, especially if it is a production product? As far as I understand the situation you are wary of using extrapolated "facts" from a table that ends 6V away from your area of operation, so it would be nice to actually know for 3.9V with certainty, I thoroughly understand: I guess has nothing to do with I know to be a fact. All I could suggest, uselessly regarding the actual question, is to try contacting IEC, maybe they even have a super "technical support" section on their website.

 

[prairiedog]

The spark-tables end based on the limits of the spark-test apparatus. They actually make sparks and try to ignite the various gas mixtures at different voltages, currents, inductance, capacitance. This is how the curves were formed. Math will not tell you when things will ignite.

In reality, it is not the 3.9V that is a problem.
The circuit capacitance, wiring inductance, power source, you have to also consider.

 

[hm_fa_da]

Do you think you can use 10V at 100A or 1,000A?
If you are off the tables/curves then you use the last entry. This is common sense.

You can get a special spark-test done, if you wanted to use more current at your 3.9V, for example.

There are other limitations like Thermal considerations which will limit the current too, so it's not only ignition V/A curve then we can't get up to 100A or ..., i just see the curve which is going so much high when getting close to 10V !
However i didn't see anything about this matter in the standard, maybe you are right, the lower voltages should be tested by spark-test ... but i didn't find this note in the standard too, which i think should be mentioned ...

In fact i couldn't accept FVM comment, that's why i asked him the reason of his comment, (however no reply yet :-?) one of my reasons for not accepting it is:

There is another table-curve for limiting capacitance vs voltage, The table starts from 5V only.
according to FVM comment, the cap values for lower than 5V, will be same with the highest value below (for 5V).
There is an online free simple tool in exveritas website, the company is ATEX consultant and NB for certification, In the online tool you can enter the voltage and other factors to see the cap value (simply extracted from the standard table) but there is a note under the page :

"Upgrade to Intrinsically Pro for Inductance Assessment, Mixed Capacitance and Inductance Assessment, Capacitance levels below 5V and much more…."

The last sentence means the cap values for less than 5V is different than the highest value in table !

 

[d123]

Hi,

Yes, but more importantly than the last sentence, perhaps - how much does the upgrade cost and would your employer/client cost that expense?