You can also use isolators that are repeaters or signal amplifiers, however as you can see from the above example and diagram, they will require a reliable power supply in the field to insure uninterrupted signal communications, so in this case the “weakest link” is likely to be the field power supply. However, if this is your only option, the risk must be managed in other ways or alternate technologies besides twisted copper pair be used.
The electrical code specifies that IS and non-IS cables must be separated with barriers and minimum spacing. Most installations in which mixed signal types are in the same environment have clear marking to differentiate the IS and non-IS areas within the enclosure. In addition many installations specify a blue mylar sheath on IS cables so technicians working on a cable can immediately see the difference. Designers of barriers and isolators also have spacing limitations that apply to the layout of terminals and connections of the barriers and galvanic Isolators as well.
Key benefits of isolators versus barriers, which are typically installed in the rack room with a good system ground connection the most commonly used option for IS installations, include:
- Fairly complex, statistically lower mean time between failures (MTBF) than barriers – Because the isolator has more components than a barrier, it has a lower MTBF than the barrier option. However the MTBF for barriers and isolators is
- Flexibility in bonding practice – Circuitry is isolated so the grounding requirements for an Isolator are more lenient than the
- Active devices; power and heat – Isolators have electronics, such as transformers and isolators, as part of their design, all of which require power to operate, and as a result generate heat that must be dissipated.
- “Application-specific” – an isolator is defined in terms of function and therefore requires different circuitry and design for analog input, analog output, discrete or frequency signal.
- Wide power supply tolerance – units are available with a range of external power supplies, including DC and a number of AC sources.
On the other hand, isolators are generally more expensive than barriers in part because they have more components.
Intrinsic safety is not the most commonly used explosion prevention technique in North America, however if and when it is required for any of the reasons identified above, or others that may be particular to your plant, it is a globally recognized technique that allows live working of your process at minimal risk or additional expense, while providing complete flexibility on the type of signal cable installed between your field junction boxes and rack room. IS may not be suitable for every hazardous area application, however, it is a viable alternative and flexible enough to be used in a wide range of applications.
About the Author:
Ian Verhappen is an ISA Fellow, Certified Automation Professional and a recognized authority on industrial communications technologies with 25+ years experience in the hydrocarbon industry. Ian can be reached at firstname.lastname@example.org.