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 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 (102 of 149).

So, any barrier/isolator with 28V safety description will have Co = 83nF (Zone 0/1; IIC). In practice Co will define the maximum allowable cable length in 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 2 shows an Exia loop with a Co=83nF. The max cable capacitance Cc=63nF. With a typical cable capacitance of 95nF/km, this
would equate to maximum cable of 660m.

If the loop was Exic, the Co=272nF, so Cc=252nF would theoretically allow 2.5kms 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, the Co=650nF 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.

Comments or suggestions welcome.

Gary Friend PrEng
Sales Director

Extech Safety Systems (Pty) Ltd.
TEL : +27 (0) 11 791 6000
FAX : +27 (0) 11 792 8294
CELL : +27 (0) 83 309 8200
http://extech.co.za

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