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How to calculate an
IS loop approval
By G Friend, Extech Safety Systems
We all know what can happen when the correct techniques are not used when interfacing into the hazardous area. Using Intrinsic Safety
(Ex i based on IEC/SANS 60079-11 [1] and IEC/SANS 60079-25) [2], the energy in the hazardous area is limited to below the ignition energy of
the gas present, thereby preventing explosions.
F
or an explosion, all three reagents need to be present - gas or
dust, oxygen and source of ignition (spark or heat). IS works
on the principle of removing the source of ignition. This can be
achieved by using a zener barrier or a galvanic isolator.
A zener barrier is a simple device where the voltage is limited by a
zener diode and the current by a resistor. The fuse is there to protect
the zener diode. The key to safety is the intrinsically safe earth. Without it, there is no protection. If the input voltage increases above the
zener diode voltage, the zener conducts and the fuse blows – and the
zener barrier has to be replaced. In addition, the zener barrier has a
volt drop across it under operating conditions so careful calculation
must be done to ensure there is sufficient voltage at the field device.
Current limiting
resistor
Energy-limiting
Hazardous
area
circuit
Hazardous area
connection
Zener barrier
Maximum
output
current
cannot
exceed
VZ/CLR
Certified
component
Safe
area
circuit
Safe area
connection
Certified
transformer
Power
Figure 2
The principle for loop approval is the same for zener barriers and
galvanic isolators, but first let us define Ex i and in which hazardous
zone they can be used.
Fuse protects Zener diode
from becoming overstressed.
FUSE
Output
(Hazardous area)
Voltage-clamping
Zener diode
VZ
Vinput
Input
Ex ia
(Safe area)
Zones 0,1 and 2
Intrinsically safe
with two faults
Maximum
output
voltage
cannot
exceed VZ
Ex ib
Zones 1 and 2
Intrinsically safe
with one fault
Figure 1
Note: Using Zener barriers without an earth is not safe!
"Faults" are those in components upon
which safety of the installation depends.
Galvanic isolator
A galvanic isolator is an active device that energy limits without the
dependence on the IS earth for safety. It also has the advantage of
supplying higher voltage at the hazardous area terminals and allowing
longer cable lengths. Isolators have local LED indication and most
4 - 20 mA isolators transfer Hart communications.
Note.
Intrinsic safety is the only protection concept
that considers failure of the field wiring.
Figure 3
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Ex ic
Intrinsically safe
in normal operation
Zone 2
IS LED – Intrinsic Safety
– Light Emitting Diode
Abbreviations
IS has applications other
than the traditional IS
loops as can be seen
in the IS cameras for
hazardous areas (left).
connected to the hazardous area terminals is 83 nF 4,2 mH. The
transmitter has internal capacitance and inductance, so maximum
cable capacitance Cc = Co-Ci and maximum inductance Lc = Lo-Li.
(Alternatively the cable L/R ratio can be used). The cable specification
typically gives pF/m and µH/m allowing a calculation of maximum
cable length.
Transmitter
EEx ia IIC T4
System
Ui < 30 V
li < 100 mA
Pi < 1,3 W
EEx ia IIC T4
Ci < 20 nF
Li < 10 µH
As you can see from Figure 4, the barrier or isolator has [EEx ia] IIC.
The square brackets indicate that the barrier or isolator (mounted in
safe area) can have connections to the hazardous area. In this case
Ex ia ie zone 0. IIC is the gas group. The transmitter has EEx ia IIC
T4. This means it can be located in zone 0 in gas group IIC. T4 is the
maximum surface temperature of device (135°C).
The barrier or isolator has maximum output parameters for
voltage, current and power - Uo Io Po. These are maximum output
values under fault conditions (known as safety description or entity
parameters). The field device has maximum input parameters Ui Ii
Pi which are the maximum values that can be applied under fault
conditions and still be safe.
Note: For a safe loop all three input parameters must be greater than or
equal to output parameters. (Ui ≥ Uo, Ii ≥ Io, Pi ≥ Po).
Note: Barrier or isolator safety parameters should not be confused with
operational parameters.
For an explosion, gas or dust, oxygen and source of
ignition (spark or heat) need to be present.
To complete the system loop approval, the electrical stored energy
(cabling) needs to be considered. Table A.2 in IEC/SANS 60079-11 [1]
lists the maximum cable capacitance against output voltage. In the
example below the maximum electrical stored energy that can be
Interface
Cable parameters
CC < 0,063 µF
LC < 4,20 mH
L/RC < 55 µH/Ω
[EEx ia] IIC T4
UO < 28 V
lO < 93 mA
PO < 0,65 W
CO < 0,083 µF
LO < 4,20 mH
L/R < 55 µH/Ω
Figure 4
Based on this assessment, a system certificate or loop approval can
be documented.
Note: Inserting a barrier or isolator with a non-certified (active) field device
is not safe.
Some field devices - like thermocouples - are defined as ‘simple apparatus’. A simple loop drawing is still required and an assessment
of power and maximum surface temperature needs to be completed.
What is considered Simple Apparatus?
•
•
•
Passive components eg switches, junction boxes, potentiometers and
simple semiconductor devices
Sources of stored energy within well defined parameters
Sources of generated energy which do not exceed 1,5 V, 0,1 A or 25 mW
IEC/SANS 60079-11:2006, Clause 5.7 says:• "Devices need not be certified or marked"
Examples:
Figure 5
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New isolators for wireless network
antennas allowing antennas outside
enclosures in hazardous areas.
References
Note: Ex nL has been replaced by Ex ic for zone 2 in the standards. This
means Intrinsic Safety can easily be used in zones 0, 1 and 2 and the wiring
can be in same multi-core cable and trunking! Another advantage of Ex ic is
that the safety factor of 1.5 (as shown in Table A.2 of [1]) does not need to
be applied to cable parameters allowing for longer cable runs.
[1] SANS 60079-11: 2007 Ed 3; IEC 60079-11: 2006 Ed 5. Explosive
atmospheres - Part 11: Equipment protection by intrinsic safety ‘i’.
[2] IEC/SANS 60079-25. 2010. Explosive atmospheres - Part 25: Intrinsically safe electrical systems.
Conclusion
Flameproof (Ex d) offers hazardous area protection for zones 1 and
2 and offers protection for higher voltage (110 Vac, 220 Vac) applications and requires mechanical planning and preparation. For 24
V systems, Intrinsic Safety offers a simple and flexible solution for
zone 0, 1 and 2. Intrinsic Safety is the only protection that considers
faults of the field wiring and offers live working without the need for
a gas clearance certificate. It does require some design and planning
to ensure that the system loop analysis is acceptable.
Gary Friend completed his BSc Electrical Engineering at the
University of Witwatersrand in 1990. He worked for Temperature Controls at various stages between 1986 and 1992. In1992
he worked as a design engineer for LTH Electronics in the
United Kingdom, designing process control instrumentation for
water quality monitoring. Gary joined MTL in November 1995
where he program managed the MTL4500 galvanic isolators development. In
2006, he returned to South Africa and he joined Extech Safety Systems. In
1995 he registered with MIET and is a registered Chartered Engineer (UK).
He is registered as a Professional Engineer (SA) with ECSA. Gary sits on the
Johannesburg branch of SAIMC committee, Ex-steering committee and IS
sub-committee of SA Flameproof Association and is Chairman of the Fieldbus
Foundation Southern African Marketing Committee. Enquiries: Tel. 011 791
6000 or email [email protected].
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