Cable parameters and all that

The intrinsically safe [IS] system standard [IEC60079-25] discusses in detail how to draw up the system documentation and in particular how to calculate the permitted cable parameters. Unfortunately the standard has to take into account all the possible variations and hence the process looks quite complicated. In the majority of applications the simple precaution of using a single source of power with no significant capacitance or inductance across its output terminals [Ci &Li] and field devices with only small input capacitance and inductance [Ci & Li] removes any problems. The permitted cable parameters [Cc,Lc & Lc/Rc] are then the specified output parameters [Co,Lo & Lo/Ro] of the source of power and there is no difficulty. The majority of certified apparatus has low values of Ci and Li and hence in simple systems this simplification is applicable. For this purpose negligible values of Ci and Li are 5nF and 10mH respectively.

The experimental work done by PTB to support the FISCO standard suggests that adding conventional cable to an IS system makes it safer. It would require extensive further testing [possibly reduced by the use of the PTB spark test simulator] to extend the principle established in the FISCO standard to include all other forms of IS system. Unfortunately there is no finance available to do this work. If anyone can raise the funds, I am sure a means of getting the work done could be found. Until this optimum solution is proven and becomes part of the standard it remains necessary to create system documentation which defines the permitted cable parameters.

Frequently installations do not have a problem because the permitted values are higher than can be created by the cables used. The most common limitation is imposed by the permitted capacitance of the higher voltage circuits. The usual limitation is that the permitted capacitance for 28V circuits in IIC for ‘ia’ and ‘ib’ circuits is 83 nF. The IEC code of practice [IEC 60079-14] suggests practical maximum cable parameters of 200pf/m, 1mH/m and 30 mH/W, which suggests a limit on cable length of 400m. A 400m cable can be considered as having an inductance of 400mH which corresponds to a current of approximately 300mA which is not common in IS circuits. Where high currents do occur then the permitted L/R ratio is usually in excess of the 30 mH/W limit and there is no problem If the cable lengths on an installation are less than 400m and the systems comply with the above restrictions then quite a lot of repetitive documentation can be avoided by including in the safety documentation a statement such as ‘The installation does not use cables longer than 400m and the permitted cable parameters are not greater than 200pF/m, 1mH/m and 30 mH/W and consequently no further consideration of cable parameters is recorded.’

Similarly if an ‘ia’ or ‘ib’ system is used in a Zone 2 location then it can usually become an ‘ic’ system and the permitted inductance increases by a factor of 2.25. The permitted capacitance increases by a variable factor which reduces as voltage increases but at 28V the permitted capacitance becomes 272nF and the cable length problem is effectively removed. A general statement can be utilised, such as ‘Where the installation uses ‘ia’ or ‘ib’ systems and the locations only require ‘ic’ systems the need to consider cable parameters is for all practical purposes removed and consequently no further consideration of cable parameters is recorded.’

An even more obvious case for not considering cable parameters is when a IIC system is used where a IIB gas classification is applicable. The permitted inductance is multiplied by 4 and the capacitance by a large variable factor. At 28V the capacitance moves from 83 to 650nF. The argument for not being concerned about cable parameters becomes very powerful

Conclusion

If possible avoid using sources of power or IS apparatus with significant values of Ci and Li. If you are using cable lengths less than 400m there is no real problem. If you are using ‘ia’ or ‘ib’ systems where an ‘ic’ system is acceptable then there is no significant problem. If you are using a IIC system where a IIB system is acceptable then there is no problem.

If in any analysis of an intrinsically safe system the solution shows that there is a significant cable parameter problem then recheck the calculation. Real problems do occasionally occur but there are always very unusual factors.

Finally if you think you have made a mistake sleep easy because the probability is that the cable made it safer anyway

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Explore posts in the same categories: ATEX Directive, Cable Parameters, Explosion Protection, Hazardous Areas, IEC60079, Intrinsic Safety

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