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’ ( and ‘Calculating intrinsically safe loop approvals – Part 2’ (, 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.


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,,


Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Preconfigured fail-safe circuit from Festo
Festo South Africa IS & Ex
The preconfigured Festo fail-safe circuit is ready to install in the control cabinet as a safe and low-cost alternative when it comes to fail-safe technology.

Flameproof cable glands
Pratley IS & Ex
A new flameproof cable gland range from Pratley allows for soft-bedding cables like PVC to be fitted to flameproof equipment without any concern regarding cable damage.

Eaton Oxalis CCTV cameras available from Extech
Extech Safety Systems News
Extech Safety Systems is pleased to announce that it has been appointed by Eaton to sell the Oxalis CCTV cameras in southern Africa.

Gigabit intrinsically safe Ethernet
Extech Safety Systems IS & Ex
Eaton’s 9470 product range provides cost-effective intrinsically safe (IS) Gigabit Ethernet equipment that can be installed and maintained easily in hazardous areas.

Safety over IO-Link
ifm - South Africa IS & Ex
With the new AL200S PROFIsafe IO-Link module from ifm electronic, it is now possible to operate standard safety sensors and actuators via IO-Link.

IS interface for signal processing
Endress+Hauser South Africa IS & Ex
Endress+Hauser’s RN series interface modules power the foundation of measurement instrument loops and safety instrumented systems.

Fluke’s intrinsically safe tools
Comtest IS & Ex
Fluke offers a growing number of intrinsically safe tools, which dampen the available energy at the output terminals and do not produce any static electricity.

Inventory management for LNG
Endress+Hauser South Africa IS & Ex
Endress+Hauser’s innovative instrumentation and advanced inventory management solutions for LNG storage tanks ensure safe operation of plant and high levels of inventory transparency.

Smart machines improve safety
Rockwell Automation IS & Ex
Manufacturers can harness the power of data to improve their safety compliance and performance using two new smart safety devices from Rockwell Automation.

Troubleshooting process loops in Ex areas
Comtest IS & Ex
Tracking down problems within a process loop can be a difficult challenge in the best of environments. Doing so in an area that has the potential for explosion takes the degree of difficulty to another level.