IEC 60079-25:2010 pdf – Explosive atmospheres – Part 25: Intrinsically safe electrical systems.
Where a system contains both lumped capacitances and lumped inductances the interaction of these may increase the risk of ignition capable sparks. This concern is confined to fixed inductance and capacitance and not to the distributed parameters of a cable. Consequently, on those rare occasions when both the lumped inductance (the sum of L1 of the source of power and the field devices) and the lumped capacitance (the sum of C of the source of power and the field devices) are greater than 1 % of the respective output parameters of the source of power L0 and C0 then the permissible output parameters are both to be divided by two. However, the maximum external capacitance C0 derived by using this simple rule shall be limited to a maximum value of I jiF for Group IIB and 600 nF for Group IIC. It should be stressed that this reduction in output parameters is only applicable on very rare occasions since it is unusual for field devices to have both Inductive and capacitive input parameters which are significantly large. Frequently, L and C of a power source are not quoted in the documentation and in these circumstances it can be assumed that they are negligible. There is no suggestion that It is considered necessary to go back and check the safety documentation on existing installations for this most recent requirement. However, new analyses should take this remote possibility into account.
To summarise, it must be checked that either the lumped capacitance or inductance is less than 1 % of the respective output parameters. If it is, then the original calculation is valid. If both parameters are greater than 1 % of the output parameters then C0 and l. of the system should be reduced by a factor of two.
Where a source of power is certified ia or “ib” then the permitted output parameters L0. ç and LdRO are derived using a factor of safety of 1.5 on or l respectively. When such a source of power is used in an ic” circuit, the permitted output parameters may be derived using a unity safety factor. This results in a significant change, which usually removes the necessity to consider cable parameters in detail. Accurate values can be ascertained using the methods and tables in the apparatus standard. An acceptable conservative technique is to multiply the output parameters by two, which normally removes any concern about cable parameters.
g) Check that the level of insulation from earth is acceptable, or that the system earthing requirements are satisfied.
If these criteria are all satisfied, the compatibility of the two pieces of apparatus has been established. A convenient way of recording the analysis Is to create a table. The following example (see Table A.1) uses the values from the typical system’s drawing (see Figure El) and compares the intrinsically safe interface and the temperature transmitter.
This analysis is only applicable when the power sources considered use a linear resistive limited output. It is not applicable to power sources using other forms of current limitation
IEC 60079-14 contains a simplified procedure of determining the maximum system voltages and currents in intrinsically sate circuits with more than one associated apparatus with linear currentivoltage characteristics which gives conservative results, which ensure a safe Installation and may be used as an alternative to this annex.
Where there is more than one source of power and the interconnections are made under controlled conditions so as to provide adequate segregation and mechanical stability in accordance with IEC 60079-11, then the interconnections are considered to fail to open and short circuit but not so as to reverse the connections or to change a series into a parallel connection or a parallel connection into a series one. Interconnections made within a rack or panel constructed In a location with adequate quality control and test facilities are an example of the degree of integrity required.