For an IS calculation involving the Stahl 9468/33, do not derive L/R from the barrier’s “Output max. load resistance” unless the manufacturer or certificate explicitly identifies that value as the required resistance for the applicable calculation. The cited L/R formula was reported as absent from newer standards and slated for removal; the available installation information does not provide a replacement equation or the barrier’s Ro. Use the certified Lo and Co method where it satisfies the applicable assessment, or obtain confirmed data or an alternative barrier from the manufacturer.
What should you use when the barrier has no published Ro?
Keep the barrier’s intrinsic-safety parameters distinct from its load specification. “Output max. load resistance” describes a stated output-load condition, shown here as 750 Ω at 20 mA or 700 Ω at 21.8 mA. It is not identified as Ro, so substituting either number into an L/R calculation would assume an equivalence that has not been established.
The source also notes that some assessments can use Lo and Co without using an Lo/Ro ratio. That is not a blanket exemption: follow the applicable certificate conditions, equipment parameters, and governing assessment method for the actual circuit. If the required safety values do not fit the field equipment and cable, ask R. STAHL for confirmed barrier data or select a suitable alternative barrier.
How do the parameter values affect the circuit assessment?
The assessment compares the associated apparatus limits with the field-device and cable parameters. A mistaken or mismatched value can make a cable-length or compatibility conclusion unreliable; it does not establish that the circuit is safe or unsafe by itself.
| Signal or parameter | Source to check | Wrong-value symptom |
|---|---|---|
| Lo and Co | Barrier certificate or manufacturer documentation applicable to the installed barrier | A cable assessment uses limits that do not belong to the barrier or certificate conditions. |
| Ro or Lo/Ro data | Explicit manufacturer or certificate data, or the applicable assessment method | An assumed resistance produces an unsupported L/R result. |
| Output max. load resistance | Barrier documentation; the stated points are 750 Ω at 20 mA and 700 Ω at 21.8 mA | A load rating is mistaken for Ro without documentation linking them. |
| Li and Ci | Field-device certificate or data applicable to the device | Device parameters are omitted or compared against the wrong barrier limits. |
| Cable inductance and capacitance | Cable data for the installed cable and the circuit calculation | Cable length or compatibility is calculated from assumed cable properties. |
Why is the old L/R shortcut not a reliable answer?
The proposed relation, L/R = 4 × Lo / Rs, depends on a resistance value and on the method’s validity for the specific assessment. Here, Rs is missing, and a response says the relation could not be found in newer standards and would be removed from the referenced training material. Do not present it as a current standards-based rule or manufacture an Rs value from the load rating.
A response points to a formula for calculating Lo/Ro in NEK IEC 60079-11:2023, but the formula itself is not reproduced in the supplied technical information. Consult the applicable edition of the standard and confirm the method, conditions, and inputs directly; do not reconstruct the equation from a mention of it.
How should you resolve the missing parameter?
- Identify the exact barrier variant and applicable certification documentation. Record the stated Lo, Co, output-load data, and any conditions or limitations relevant to the circuit.
- Record the field device’s Li and Ci from its applicable documentation, along with the cable’s inductance and capacitance data and the planned cable length.
- Determine which assessment method applies to the installation and certificate. If the method requires Ro or an Lo/Ro relation, obtain an explicitly applicable value or method from the manufacturer or governing documentation.
- Do not substitute 750 Ω or 700 Ω for Ro unless the manufacturer explicitly confirms that use for the calculation. Do not use an unsupported formula to fill the gap.
- If Lo and Co are used instead, complete the assessment under the applicable certificate and method. If the parameter limits do not accommodate the field device and cable, evaluate a manufacturer-approved alternative barrier.
How can you verify the calculation and avoid recurring errors?
Check that every value in the calculation traces to documentation for the actual barrier, field device, or cable. Confirm that units and parameter definitions match, that the selected method is permitted for the circuit, and that the final cable assessment uses the documented cable properties and installed length. Retain the supporting documents and calculation so another engineer can reproduce the result.
The response specifically states that an Lo/Ro ratio can replace Lo for calculating cable length only when field equipment Li is less than 1% of Lo. Treat that as a method-specific condition to verify against the governing standard and documentation; do not extend it to other calculations or use it when the Li comparison fails.
Recurring errors include treating a maximum load resistance as Ro, using the disputed L/R formula as an unquestioned current rule, and assuming that the Lo/Co approach is automatically acceptable in every case. Stop the calculation when a required parameter or method remains unresolved. Escalate to R. STAHL through its official support channel for confirmed data or an alternative barrier when the certificate and device/cable parameters do not yield a valid assessment.
FAQ
What happens if I use 750 Ω as the barrier Ro?
You may be substituting a maximum-load specification for a different parameter. Use 750 Ω only if manufacturer or certificate documentation explicitly validates it as Ro for the applicable calculation.
What happens if field-device Li is not below 1% of Lo?
The stated condition for replacing Lo with an Lo/Ro ratio in cable-length calculation is not met. Use the assessment method and limits applicable to the barrier certificate, and resolve any parameter conflict before accepting the result.
What happens if the Stahl 9468/33 certificate does not provide Ro?
Do not infer Ro from the 750 Ω or 700 Ω output-load figures. Obtain confirmed data or an applicable calculation method from R. STAHL; stop and escalate if the circuit cannot be assessed against the certificate conditions.