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Isolators
The ins and outsof isolation:a guide to selectingthe right
isolatorAcromag manufactures a variety of Flat Pack iso-lators to
provide a cost-effective solution formany applications. The primary
function of theisolator is to eliminate ground loops that mayexist
between two or more instruments. A clas-sical application isolates
the control room equip-ment (computers, PLC, DCS etc.) from
fielddevices which may have different ground poten-tials. In
addition to breaking up ground loops,the isolators protect control
room equipmentfrom damaging transient spikes and noise gener-ated
in the field. Choosing the proper and mostcost-effective isolator
requires an understandingof the application and consideration of
futureexpansion requirements.
Flat Pack isolators are available in 2-, 3-, and 4-wire
configurations. Isolators are further classi-fied as input, output,
or 3-way (input, output,power) isolators. Input isolation implies
theinput signal has no electrical connection to theoutput and power
signals. Output isolationimplies the output signal has no
connection tothe input and power. And similarly, 3-way isola-tion
refers to a situation where there is no elec-trical path between
the input, output, or power.
This article describes four types of isolators andtypical
PLC/DCS applications in which they arebest suited. Each isolator
produces a 4-20mADC output signal corresponding to the 4-20mADC
input signal.
3-Way IsolatorsFirst, we'll discuss Acromag's 611T, 330i, and
430iisolators which provide 3-way (input, output, andpower)
isolation. They are the safest choice whenretrofitting or expanding
an existing system. Theyprovide complete isolation when
uncertaintiesabout the isolation of instruments in each currentloop
exist or the grounding scheme is unknown.
The 611T and 330i are DC-powered isolatorsthat operate from an
unregulated power supplyof 10 to 36V DC. The 430i is AC-powered
anduses either 115V AC or 230V AC. The inputloop burden of these
isolators is less than 1.5VDC at 20mA or approximately 75 ohms.
These 3-way isolators source current out and donot utilize a
separate output power supply. It isimportant to note that the
outputs of these iso-lators are designed to drive purely resistive
loads.
Figure 1 shows a typical application for a 3-wayisolator. The
primary difference between the611T/330i and 430i models is the
power source.
The isolator can be several or hundreds of feetaway from the
PLC/DCS and obtains power froman external AC or DC source. This
source mayhave a different ground potential than thePLC/DCS or
field device. If this is the case, thepower ground must be isolated
from both theinput and output. In this example, the isolatorinput
is monitoring a grounded field sensor at aremote site 300ft. away.
The ground potential atthe sensor is likely to be different from
thepower source and PLC/DCS. Consequently, inputto output isolation
is also necessary.
Additionally, it is important to note that the611T, 330i, and
430i can isolate 4-20mA controloutputs from the PLC/DCS to field
actuators.The PLC/DCS analog outputs are wired to theisolator input
and the isolator outputs are wiredto the field device/actuator.
Although 3-wayisolation is the safest and most flexible, it is
alsothe most expensive due to the power isolationcircuitry.
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Acromag's compact, DIN rail-mount isolators are available in
AC-, DC-, and loop-powered models.
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4-20mA250(internal)
+
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24V
P
Ground #3
4-20mA
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Field Device
Ground #1
Ground #2+ -
PowerSupply
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+
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IN OUT
611T/330i/430iIsolator300 ft. 50 ft.
PLC / DCSFigure 1: Long lead lengths, multiple grounds, separate
power supply,3-way isolation required.
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Input IsolatorsSuppose we relocate the isolators close to
thePLC/DCS. The output and power can now sharea common grounding
point with the PLC/DCS,and 3-way isolation is no longer necessary.
Butsince the input wiring remains the same (seeFigure 2), input
isolation is still required. A verycost-effective solution is the
Acromag 340i inputisolator which provides 250V AC of
continuousisolation from input to output/power.Eliminating the
power isolation circuitry saves asmuch as $95 per isolated
point.
The 340i is ideal for applications requiringmultiple isolators
at a single location where allisolators share the same power
supply. PLCsand DCSs usually have 24V DC power availableat their
input terminals. This is ideal for the340i which operates over an
unregulated powersupply range of 10 to 36V DC.
Since the 340i is an input isolator, its 4-20mAoutput signal and
DC power share a commonlead. This common lead should be wired
directlyback to the PLC/DCS's internal ground. Avoidthe urge to
daisy-chain the common leads
between multiple isolators and run a single wireback to the
PLC/DCS. Each isolator should haveits own ground path back to the
monitoringdevice to avoid inaccurate and noisy measure-ments. The
340i's output sources current anddoes not use a power supply in
series with theoutput signal. Again, the output of these isola-tors
drives purely resistive loads.
Two-wire Isolators(current repeaters)So far, we have only
discussed isolators thatoutput current to a resistive load. Suppose
yourapplication requires an isolator that has a powersupply in
series with the output signal. This pro-hibits the use of a 611T,
340i, or 430i isolator.In this case, we must use a two-wire
transmitterwith input isolation! Acromag's 651T and 250Tare
two-wire, 4-20mA input transmitters withisolation and can therefore
be considered isola-tors. They are also referred to as a
currentrepeaters.
Some PLC/DCSs are better equipped to handletwo-wire transmitters
than other devices. Figure3 shows a simple connection diagram for
the
651T/250T transmitter. Note that the outputdoes not go to a
purely resistive load. The651T/250T obtains power from the output
loop.Therefore, the transmitter must control currentdraw from the
power supply in the output loopwhich is the 4-20mA signal. Since
output andpower share the same two wires, these modulescan be
placed anywhere between the field sens-ing device and the PLC/DCS
or monitoringdevice. A typical configuration is to place
thetransmitter close to the field device. Since boththe input and
output signal are 4-20mA, locationof the device is not
critical.
The 651T and 250T are also ideal as currentrepeaters. Certain
monitoring applicationsrequire tapping off an existing 4-20mA loop
toadd a measuring or monitoring instrument. Thistransmitter is
ideal for this application because ofthe low (10 ohm) input burden.
If the isolatorfails or loses power, it will not interfere with
theexisting 4-20mA loop. The output of the trans-mitter/isolator in
this application can be wired toa field chart recorder or another
PLC to enableredundant monitoring.
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163
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4-20mA 4-20mAField Device
Ground #1
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IN OUT
340i Isolator
300 ft. 10 ft.
P250(internal)
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24V
P
Ground #2
PLC / DCSFigure 2:Isolator receives power from PLC/DCS.Input
isolation required.
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4-20mA 4-20mAField Device
Ground #1
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IN OUT
651T/250T-C1Transmitter300 ft. 300 ft.
250(internal)
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24V
P
Ground #2
PLC / DCSFigure 3:Input isolator, two-wire transmitter, isolator
location not important.
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Loop-Powered"Output" IsolatorsAll the isolators mentioned
previously have aninput burden of 75 ohm or less. Input burdenis
the load the isolator places on the 4-20mAcurrent loop it is
monitoring. Another type ofisolator is the "loop-powered" isolator.
This unitis useful when adding and isolating a monitoringinstrument
to an existing current loop. In Figure4, field device #2 is added
to the existing loopusing a 671T or 270i isolator. Note that no
addi-tional power source is required for the addedloop or for the
671T/270i. The isolator's outputis powered by the input loop power
and onlydrives passive loads!
A drawback of "loop-powered" isolators is theburden induced to
the initial loop. For example,the 270is input burden is based on
the followingequation where RLOAD represents the output loadon the
270i:
If RLOAD = 0 to 50 ohms:VBURDEN = 5.4V + [.0216(RLOAD +50)]
If RLOAD = 50 to 600 ohms:VBURDEN = 5.4V + [.0216 x RLOAD]
For a 50 ohm load, the input burden is 6.48Vusing the second
equation and 10.8V for a 250ohm load. The sum of the input voltage
burdensmust remain below the loop power supply capa-bility at full
current. The voltage drops at 20mAin Figure 4 are calculated as
follows:
VBURDEN = 12V(Device #1) + 6.48V(270i + Device #2)+ 5V(PLC 250
ohms)
VBURDEN = 23.5V
The PLC power supply could have been used inFigure 4, however if
the 4-20mA input isincreased to 26mA, the voltage burden will
alsoincrease, and the loop will fail due to voltagecompliance
problems. A 28V power supply willprevent this from occurring since
at 26mA thevoltage burden is 25.2V DC. Note that an opencircuit in
the output of the 671T or 270i disruptsthe initial loop operation
due to increased volt-age burden. The 671T/270i can also be used
asan output isolator for PLC/DCS 4-20mA outputs.Again, the voltage
burden cannot exceed thevoltage capability of the PLC/DCS output
device.
Improper isolation techniques and model selec-tion account for a
large percentage of monitor-ing and control problems. Acromag
isolatorsoffer a reliable, cost-effective solution for a
widevariety of applications. If you need technicalassistance,
please call our application engineersfor consultation.
A p p l i c a t i o n N o t e sA p p l i c a t i o n N o t e
s
Tel: 248-624-1541 Fax: 248-624-9234 e-mail: [email protected]
www.acromag.com
164
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4-20mA
250(internal)
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24V
P
Field Device #1 +
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OUT
PLC / DCS
Field Device #2
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671T270i
+ -28V Power Supply
50(internal)
Figure 4:Loop-powered isolator.Output isolation.
