IS & Ex


How to calculate IS loop approval: Part 3 – impact of Gas Group on loop approval cable lengths

January 2018 IS & Ex

Further to my previous articles: ‘How to calculate an intrinsically safe loop approval’ (http://www.instrumentation.co.za/7571a) and ‘Calculating intrinsically safe loop approvals – Part 2’ (http://www.instrumentation.co.za/7782a), we now consider the impact of Gas Group on cable lengths for IS loops.

In Part 2, I discussed using Exic for Zone 2 applications ‘Intrinsic safety in normal operation’ i.e. with no safety factor required.

To repeat, firstly and importantly, there is a misconception that the Co values are defined by the design of barrier/isolator. The Co value is actually defined in IEC/SANS60079-11: 2012 Table A.2 page 96.

So, any barrier/isolator with 28 V safety description will have Co = 83 nF (Zone 0/1; IIC). In practice, Co will define the maximum allowable cable length for most cases. However, as explained in Part 2, using Exic where applicable is one method to have longer cable lengths.

Gas Group classification impacts Co parameter as per Figure 1.

Figure 1: Co values.
Figure 1: Co values.

Figure 2 shows an Exia loop with Co = 83 nF. The max cable capacitance Cc = 63 nF. With a typical cable capacitance of 95 nF/km, this would equate to maximum cable length of 660 m.

Figure 2: IIC IS loop for Zone 0/1 (Co 83 nF).
Figure 2: IIC IS loop for Zone 0/1 (Co 83 nF).

If the loop was Exic, then Co = 272 nF, so Cc = 252 nF would theoretically allow 2,5 km of cable i.e. no longer a limiting factor. (The limiting factor in this system is likely to be operating voltage at the end of the cable being high enough for the transmitter to work.)

If the Gas Group were IIB then Co = 650 nF, which eliminates capacitance as a limiting factor.

Conclusions

1. For installations requiring long cable runs, classifying the area as Zone 2 offers significant benefit.

2. Alternatively, a IIB (or IIA) classification is another way of allowing longer cables lengths.

For more information contact Gary Friend, Extech Safety Systems, +27 (0)11 791 6000, sales@extech.co.za, www.extech.co.za



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