Intrinsically Safe Documentation One of the requirements associated with Intrinsically Safe systems is that a document known as the ‘Descriptive System Document’ shall be produced for all systems. BS EN 60079-25:2004 (Electrical apparatus for explosive gas atmospheres – Intrinsically safe systems), section 4 - Descriptive system document states: “A descriptive system document shall be created for all systems. The descriptive system document shall provide an adequate analysis of the level of safety achieved by the system”. As well as the above documentation, loop drawings should also be provided for the equipment that is installed in potentially explosive atmospheres. In potentially explosive atmospheres where intrinsically safe (IS) circuits are installed, the supporting documentation should not only contain the basic instrument loop drawing(s) and apparatus certificates (EC type certificates and Declaration of Conformity) but also a descriptive system certificate which clearly states which items of apparatus may be connected together to form a certified system or circuit loop. It should be remembered that system certificates in certain instances might be generic and cover several identical instrument loops, where the maximum parameters (e.g. cable type and maximum length) are clearly specified. Descriptive systems documents should either be supplied by a ‘third party’ namely the apparatus manufacturer/supplier, the IS barrier manufacturer/supplier, a CENELEC approved test house or via ‘in house’ approval by a competent person(s) to CENELEC/ATEX standards. Marking From BS EN 60079-25:2004, section 12 – Marking. “All apparatus within the system shall be readily identifiable. In the case of ‘simple apparatus’, a traceable plant identification label is acceptable”. The minimum requirements are that the relevant descriptive system document shall be readily traceable. One acceptable technique is a clear instrument loop number, which identifies the loop documentation, which in turn lists the descriptive system document. The requirement is that the descriptive system document contains all the information, which enables a safe system to be created. The installation document should contain the necessary information to enable a particular embodiment of that system to be safely installed in a specific location. Legends Power Source | Uo | Max open circuit voltage V | Io | Max output current mA | Po | Max output power or matched power W | Co* | Max external capacitance µF | Lo* | Max external inductance mH |
It is important to appreciate that the values of Co and Lo as marked on the IS barrier (and certificate) are for the gas group for which the output is certified. If the marking on the barrier is EEx [ia] IIC, then the parameters apply to IIC applications. Certified Apparatus | Ui | Max input voltage V | Ii | Max input current mA | Pi | Max input power W | Ci | Effective capacitance nF | Li | Effective inductance µH |
Safety parameters | Uo ≤ Ui | Yes/No | Io ≤ Ii | Yes/No | Po≤ Pi | Yes/No | Co ≥ Ci + Cc | Yes/No | Lo ≥ Li + Lc | Yes/No | | |
I.S. CIRCUIT PARAMETER CHECK EXAMPLE Company | | Plant | | | | | | | | Date | | | Location | | | | | | | | I.S. Loop no. | | Drawing no. | | | | | | | | Circuit Type | | Circuit length | | | | | | | | | | | | | | Barrier Details | | | | | | | | | | | | Manufacturer | | Model/Type | | | | | | | | Serial No. | | Certificate No. | | | | | | | | ATEX/Certificate Code | | System Certification | | | | | | | | | | | | | | | | | | | | | |
Field Instrument | | | | | | | | | | | Manufacturer | | Model/Type | | | | | | | | Serial No. | | Certificate No. | | | | | | | | ATEX/Certificate Code | | IP Rating | | | | | | | | | | | | | | |
When two items are connected together to form a system, the resultant installation carriers the sub grouping of the worst items of apparatus e.g. EEx ia IIC T4 + [EEx ib] IIB = EEx ib IIB T4
Maximum capacitance or inductance in an IS circuit that can be connected to the terminals without invalidating the intrinsic safety concept.
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