Intrinsic safety (IEC60079-11) - assessing spark ignition of RLC circuit, but the L is placed before the zener barrier

Question

My IS circuit contains an inductance, capacitance, and resistance. Shown below is the simplified schematic, with uncountable and countable fault applied, resulting in the most onerous condition:

The inductance is from a common mode choke, with the tolerance added. V1 is from an isolating, infallible transformer (non-mains), Rcharge is the ESR.

Since V1 could go to dangerous levels of voltage, a zener barrier + fuse is employed. This will limit the voltage to a safe level.

As you can see, the CMC is located between the isolation transformer and the IS region.

1. When assessing intrinsic safety, it is my understanding that CMC is to be included in the assessment. Is that correct?
2. Then what would be the voltage applied during the assessment? Is it V1max or Vz? V1max - since at fault condition, the CMC would experience the V1max voltage. Or will it be Vz since Vz will be the voltage seen in the EX connection facility (across Cx).

Thanks.

Answer 1

When assessing intrinsic safety, it is my understanding that CMC is to be included in the assessment. Is that correct?

Anything to the left of the fuse may be used to determine what peak current can flow into the Zener diode(s) in order to ascertain that the Zener is adequately power rated for an intrinsically safe barrier. Having said that, it's very, very unlikely that any potential energy contribution from the CMC will be very much at all so, it may be disregarded.

You can of course use a simulator and gather this information your self.

Then what would be the voltage applied during the assessment? Is it V1max or Vz?

It will be V1max plus any slight increase due to the CMC (unlikely).

Or will it be Vz since Vz will be the voltage seen in the EX connection facility (across Cx).

Applying Vz to the safe input side of a Zener barrier tests nothing however, the person assessing the design will be interested in the peak level that Vz rises to when V1max is applied (in order to determine the peak voltage that the capacitors can become charged-up to).

That peak capacitor voltage will determine how much added external circuit capacitance is allowed on external circuits in the hazardous area.

Answer 2

When assessing intrinsic safety, it is my understanding that CMC is to be included in the assessment. Is that correct?

This depends on whether it's part of the intrinsic safety part. I don't see a connector in your diagram. I have the impression that the whole diagram is in the Ex zone? If so, yes, the common mode choke must be evaluated at least for auto-ignition energy. For example, for group IIC, the current in a 750µH choke must not exceed 280mA (see Figure A.4 for group II - EN 60079-11).

Then what would be the voltage applied during the assessment? Is it V1max or Vz?

From my point of view, since V1 is an infallible transformer, the only fault to be considered is an open circuit (because that's its failure mode), so only V1max is to be considered for evaluation.

The voltage Vz will be used to evaluate Cout's auto-ignition energy (see Figure A.3 for Group II - EN 60079-11).