To answer the question of whether you need to use an isolated barrier device with your sensor, it’s helpful to first answer the following two questions:
During the years I have worked at Pepperl+Fuchs, I have been asked a lot of questions regarding the implementation of intrinsic safety, or IS for short. As they say, “Knowing is half the battle,” so I decided to share a few of those questions and answers.
Mention the word NAMUR to a valve specialist and the first thing this person will think about are the interfaces used to connect solenoids to actuators and switch boxes or positioners to rotary pneumatic actuators.Mention NAMUR to a controls person and the first thing that will come to mind are intrinsic safety techniques.
Both are correct. A typical automated valve assembly for on/off service consists of a valve (rotary or linear travel), an actuator, position sensors, and a solenoid. An automated valve for control service adds a valve positioner but subtracts the solenoid.
The selection of the components needed in the assembly is dependent upon the service application and the area (environment) where the assembly will be installed.
Such areas, known in the process industry as Divisions or Zones, may be simplified to three categories:
An IS barrier (also called an intrinsic safety barrier) is, quite simply, an electronic safety device that is used to protect a circuit or instrument located in a hazardous location from becoming an ignition source.
A match made for hazardous areas
A very common application we see working with intrinsic safety is one involving the use of a thermocouple. Pepperl+Fuchs has a wide variety of galvanically isolated intrinsic safety barriers that will work with these devices.
When in a hazardous location and working on selecting a barrier, the first question that needs to be answered is, “What type of signal(s) are you looking for on the control side of the barrier?” Once that question is answered, you can begin the process of selecting a barrier.
The first type of galvanically isolated barrier that we offer is a straight, low-voltage repeater: the KFD2-VR2-Ex1.50M. This barrier is powered by a 24 VDC supply and will simply repeat a 0 mV to ±50 mV signal from a thermocouple. There is no configuration required for this barrier.
The second option we have is our KFD2-UT2 series. These barriers will take a thermocouple input and convert the mV signal into a corresponding current or voltage signal, depending on the selection. The UT2 series of temperature converters is programmable via our free-to-download PACTware software and the use of an adapter cable (RS232 or USB options available) that must be purchased as an accessory. We offer these barriers in a few options:
KFD2-UT2-Ex1 --- Single channel, 4 mA … 20 mA output
KFD2-UT2-Ex1-1 --- Single channel, 1 V … 5 V output
KFD2-UT2-Ex2 --- Dual channel, 4 mA … 20 mA outputs
KFD2-UT2-Ex2-1 --- Dual channel, 1 V … 5 V outputs
The third option, the KFD0-TT-Ex1, is also classified as a thermocouple converter, which will take the mV signal input and convert it into a 4 mA ... 20 mA output. This specific barrier requires no outside software or programming cable to set up. All of the adjustments are made with DIP switches and fine adjustment potentiometers on the barrier.
The next barrier we offer is our KFD2-GU-Ex1 model. This barrier provides two Form C relay contacts that will be used as high- and low-temperature trip points that are user programmed. Once again, this barrier will require the use of PACTware and the adapter cable.
The last galvanically isolated barrier we offer for thermocouple applications is our KFD2-GUT-Ex1.D barrier. This is the most advanced barrier in the series and is basically a combination of the features on the “UT2” and “GU” barriers. It will provide two Form C relay contacts along with a 4 mA … 20 mA current output. The barrier can be programmed via a small keypad and LC display on the front or by using the PACTware software.
As with any of our K-System barriers, all of the above barriers have removable terminal blocks. As a separate accessory, we offer a terminal block that has built-in cold junction compensation for use specifically with thermocouples, the K-CJC-BU. This terminal block has an integrated encapsulated Pt100 RTD that is used for cold junction compensation, eliminating the need to maintain a known reference temperature.
...while located in the hazardous (explosive) atmosphere?
While maintenance/live-work on intrinsically safe field-wiring circuits is often confused with maintenance on intrinsically safe equipment, IEC/EN 60079-17 Electrical apparatus for explosive gas atmospheres – Part 17: Inspection and maintenance of electrical installations in hazardous areas (other than mines) provides some information on this topic that can be useful to the reader.
IS barriers protect devices in hazardous areas
An intrinsic safety barrier is used to provide protection to a device mounted in a hazardous location. The basic components that make up most intrinsic safety barriers are a fuse, zener diodes, and a resistor and are shown below in this simple electrical diagram:
Does a field instrument need to be rated IS when it’s used with an intrinsic safety barrier?
If we make the obvious assumption that the area classification in which the field instrument is mounted is rated Class I, Division 1, the answer is YES. The instrument must be intrinsically safe if it’s connected to an intrinsic safety barrier. An