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ATEX related current limiting calculation question

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hm_fa_da

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Hello,

There is a disagreement between me and a friend on the below problem:

In IEC60079-11 Standard (For ATEX intrinsically safe section), Page 47, 5th paragraph mentions:

"If a current-limiting device is necessary to limit the prospective current to a value not greater
than the rated breaking capacity of the fuse, this device shall be infallible in accordance with
Clause 8 and the rated values shall be at least:
– current rating 1,5 × 1,7 × In;
– voltage rating Um or Ui;
– power rating 1,5 × (1,7 × In)2 × maximum resistance of limiting device."

The simple circuit schematic is as below:
SCH.png

As you see, the maximum input Ui is 3.9V, there is a 3A fuse in series with 1.2 Ohm resistor. for calculating the rating of the resistor, i say the maximum possible short circuit current is limited to I = V/R => Isc = 3.9V/1.2 Ohm = 3.25A.
( I know we should consider resistor tolerance and ... )

So considering safety factor of 1.5, the current rating of resistor should be at least :
1.5 x 3.25A = 4.875A.

But My freind says according to the quoted note from standard (Page 47, 5th paragraph) we should use In of the fuse and the 1.7 factor instead of calculated short circuit current (V=I x R ...),

So the resistor rating is :
1.5 x 1.7 x 3A = 7.65A ...

I think when the Maximum possible current in the circuit is 3.25A, why we should calculate more current value (1.7 x 3A) (safety factor of 1.5 will be used for both values) !
If we don't use any fuse, then the only solution to calculate maximum short circuit current is Vmax / R = 3.25A, so adding a fuse can not increase the current limitation !
another example is using a i.e 100A fuse, then according to my friend opinion the 1.7 x 100A = 170A should be calculated in current rating of the resistor in series with the fuse !

What's your vote ?

Thanks & Regards.
 

FvM

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This looks completely wrong. The resistor has the effect that the fuse will never trip. But the quoted is about guaranteeing a current within the fuse breaking capacity. The breaking capacity of the 3A fuse will be probably infinite with a low voltage source, thus no resistor required.
 

hm_fa_da

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This looks completely wrong. The resistor has the effect that the fuse will never trip. But the quoted is about guaranteeing a current within the fuse breaking capacity. The breaking capacity of the 3A fuse will be probably infinite with a low voltage source, thus no resistor required.

The resistor limits short circuit current to 3.25A for the rest of circuit which is not shown in the schematic. this resistor is not purposed to be used for limitation of breaking capacity current here only.
According to standard, The 3A fuse should be multiplied by factor 1.7 so for calculating flowing current we should assume 5.1A passes the path (if there is no other limiting device such as a resistor).
So here the Fuse has not any effect of safety as an element for ATEX compliance.
As you said it will not trip if we assume it 5.1A ! but in practical usage it will break currents higher than 3A ! of course this fuse has no role for ATEX safety rules (because it is currently limited by the resistor to 3.25A) but it is practically useful to break circuit current and stop battery discharge in case of failure in Circuit (short circuit) although that failure is not dangerous for explosive area.

I say, In this case, for calculating current rating of the resistor we should only use the Isc which is 3.25A. (and safety factor 1.5 ...)
 

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