classic | mobile
 

Search...

SA Instrumentation & Control Buyers' Guide

 

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


Credit(s)
Supplied By: Extech Safety Systems
Tel: +27 11 791 6000
Fax: +27 11 792 8294
Email: sales@extech.co.za
www: www.extech.co.za
Share via email     Share via LinkedIn   Print this page

Further reading:

  • Modular HMI for hazardous areas
    January 2018, Pepperl+Fuchs, IS & Ex
    PC solution extends modular system At the beginning of the year, the VisuNet GXP product family was grown to include a PC solution. The new computing unit is optimally suited for mechanical engineers, ...
  • Radiation protection systems go wireless
    January 2018, Omniflex Remote Monitoring Specialists, IS & Ex
    Omniflex has announced the release of the Teleterm W3 ISA100 Wireless Interface Node. Based on well proven technology and substantiated by the nuclear industry, the Teleterm W3 is the latest device to ...
  • Safety interlocks for automotive production
    January 2018, Deebar, IS & Ex
    The automobile industry is heavily automated and can be hazardous if not safeguarded properly. Ensuring safety whilst delivering productivity is vital to the industry and therefore safeguarding operators ...
  • Ex and marine approval for LSIS equipment
    January 2018, Ana-Digi Systems, IS & Ex
    Due to the dangers prevalent in petrochemical, mining and marine applications there is a need for equipment that is non-volatile and would by extension, not be the cause of explosions and fires. This ...
  • Room integrity testing
    January 2018, Alien Systems & Technologies, IS & Ex
    Alien Systems and Technologies (AST) is the first company in Africa to offer room integrity testing (RIT). The company’s performance in the industry is supported by its highly qualified, specialised team, ...
  • Achieving safety during CIP
    December 2017, Deebar, IS & Ex
    IP69K-rated interlocks and safety switches.
  • Cordex introduces the Toughpix Digitherm pocket-size digital and thermal imaging camera
    December 2017, Extech Safety Systems, Sensors & Transducers
    Compact enough to fit into any work pocket, but rugged enough to take the hits, Toughpix Digitherm is a go-anywhere camera providing the power and resilience to get the job done. Rugged, powerful and ...
  • Low-profile strip lights
    December 2017, RET Automation Controls, IS & Ex
    Banner Engineering has announced the release of the WLS15, a professional grade LED striplight that makes it easy and cost effective to illuminate machines, work spaces and mobile equipment for improved ...
  • Stay safe with SICK Automation
    November 2017, SICK Automation Southern Africa, IS & Ex
    Components evolve into ready to install safety systems.
  • Be safe by monitoring hazardous areas
    November 2017, IS & Ex
    Underground monitoring with the Sperosens SL2010 system The SL2010 telemetry system provides underground data collection and communication from the mine’s fibre-optic backbone to the control centre where ...
  • Remote operating terminals for Ex zones
    November 2017, Extech Safety Systems, IS & Ex
    Introducing a new and unique modular design concept, the MTL Gecma Work Station is a new generation of remote operating terminals for Ex Zone 1/2/21/22. The new modular workstation range addresses the ...
  • AS-i Safety at Work module
    November 2017, ifm Electronic RSA, IS & Ex
    The safe input module allows safety-related components without integrated AS-interface to be connected to the AS-i Safety at Work system, allowing 2-channel inductive sensors with electronic OSSD outputs ...

 
 
         
Contact:
Technews Publishing (Pty) Ltd
1st Floor, Stabilitas House
265 Kent Ave, Randburg, 2194
South Africa
Publications by Technews
Dataweek Electronics & Communications Technology
Electronic Buyers Guide (EBG)

Hi-Tech Security Solutions
Hi-Tech Security Business Directory

Motion Control in Southern Africa
Motion Control Buyers’ Guide (MCBG)

South African Instrumentation & Control
South African Instrumentation & Control Buyers’ Guide (IBG)
Other
Terms & conditions of use, including privacy policy
PAIA Manual





 

         
    classic | mobile

Copyright © Technews Publishing (Pty) Ltd. All rights reserved.