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We're looking into restricted breathing or encapsulation concept because PCB has a complex circuit that makes it impossible to achieve SMD clearance under 'ic' or increased safety concept.

I have a simple 4-20mA sensor transmitter attached to the MTL7700 series Zener barrier, which connects custom made electronics PCB which contains AFE and 24V Power.

The 24V PSU has a 20uF ceramic X7R capacitance on the output before the Zener barrier (Fuse, 300R and 28V Zener clamp), is there a restriction on output capacitance from the PCB (I recall it was 1.65uF, but can't remember where this number come from in IEC60079-XX). Why this matter because the Zener barrier already limits the energy?

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  • \$\begingroup\$ Why do you need such a large cap? What does SMD Clearance, FM hazard entity approval have to do with Restricted Breathing design? Current limit and energy to charge a large cap will cause current limiting R to overheat from energy size of cap transferred thru R in "instrinsic safe MTL. MTL is to prevent arc from long cable inductance currents. not for your design? Pls explain \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 23 '19 at 16:08
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The purpose of a properly specified and installed intrinsic safety barrier is to make whatever is on the other side not matter, so you don't need to worry about the capacitor if it's on the non-hazardous side of the barrier. The barrier should very reliably limit energy passed into the hazardous area even if the power supply itself fails in some horrible way.

Your custom board is going to be installed in the hazardous area so of course it matters.

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  • \$\begingroup\$ Even it within restricted breathing? \$\endgroup\$ – riscy Dec 23 '19 at 21:34
  • \$\begingroup\$ @riscy I don't know the answer to that. \$\endgroup\$ – Spehro Pefhany Dec 23 '19 at 21:41

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