APPENDIX III
QUESTIONS
LEARNING UNIT III: INTRINSICALLY SAFE EQUIPMENT
3.1 Calculate the maximum allowable resistance of a barrier under worst
case conditions when the transmitter is transmitting a {I} mA signal. The
supply voltage is given as 24 V with a measuring resistor of 250, which is
placed on the supply leg, and the cable resistance is equal to {RCABLE}.
Please make use of the following values for {I} & {RCABLE}:
I = 12.4mA & RCABLE = 10.0 Ω
I = 6.1mA & RCABLE = 8.1 Ω
I = 10.8mA & RCABLE = 3.4 Ω
I = 20mA & RCABLE = 4.1 Ω
I = 18.1mA & RCABLE = 9.4 Ω
I = 8.2mA & RCABLE = 3.2 Ω
I = 14.4mA & RCABLE = 9.7 Ω
I = 9.7mA & RCABLE = 7.5 Ω
3.2 Calculate the maximum allowable resistance of a barrier under worst
case conditions when the transmitter is transmitting a {I} mA signal. The
supply voltage is given as 24 V with a measuring resistor of 250, which is
placed on the return leg, and the cable resistance is equal to {RCABLE}.
Please make use of the following values for {I} & {RCABLE}:
I = 5.4mA & RCABLE = 22.2
I = 14.1mA & RCABLE = 24.6
I = 9.9mA & RCABLE = 16.2
I = 6.1mA & RCABLE = 11.2
I = 17.4mA & RCABLE = 6.8
I = 17.2mA & RCABLE = 18.3
I = 6.9mA & RCABLE = 8.7
I = 20mA & RCABLE = 10
3.3 List the steps that should be followed using the “Analytical method" for
intrinsically safety.
3.4 Make a sketch of a Positive Single Channel Barrier Circuit that is used as
an intrinsically safe device.
3.5 Make a sketch of a Negative Single Channel Barrier Circuit that is used as
an intrinsically safe device.
3.6 Make a sketch of a Non-Polarised Single Channel Barrier Circuit that is
used as an intrinsically safe device.
3.7 Make a sketch of a Positive Two Channel Barrier Circuit that is used as an
intrinsically safe device.
3.8 Make a sketch of a Negative Two Channel Barrier Circuit that is used as
an intrinsically safe device.
3.9 Make a sketch of a Non-Polarised Two Channel Barrier Circuit that is
used as an intrinsically safe device.
APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
3.10 Name the six conditions that need to be met simultaneously for an
explosion hazard to occur.
3.11 Using the “Analytical method" for intrinsically safety it is essential to
record each combination of “unsafe” condition that may occur under normal
working conditions so that when the analysis is completed, you can verify if the
worst case situation has been analysed. Name four of these conditions that may
occur.
3.12 When a barrier circuit is connected to intrinsically safe apparatus, it is
important to ensure that the maximum (a) ______ and (b) ______ delivered by
the barrier circuit does not exceed the (c) ______ voltage and current levels
for the apparatus. The maximum unprotected capacitance and inductance of
the load are (d) ______ from the maximum values permitted by the barrier
circuit. The remaining inductance and capacitance define the maximum (e)
______ parameters.
3.13 What effect may the improper selection of intrinsically safe barriers in
4-20 mA loops have?
3.14 What is the purpose of using wire wound resistors and redundant zener
diodes inside a barrier circuit?
3.15 Explain why a fuse alone is not a suitable intrinsically safe device?
3.16 Make a sketch of a Barrier Device between a Hazardous Area and a Safe
Area that is used as an intrinsically safe device.
3.17 Name the three types in which barriers for intrinsically safe transmitter
applications can be divided into.
3.18 Barriers for intrinsically safe transmitter applications can be divided into
three types. Construct a detailed table which includes the advantages and
disadvantages of each barrier type.
3.19 List all the advantages/disadvantages of Grounded Safety Barriers.
3.20 List all the advantages/disadvantages of Grounded Repeaters.
3.21 List all the advantages/disadvantages of Ungrounded Repeaters.
3.22 Describe the purpose of a barrier device.
3.23 Name the basic components a barrier device consists of.
3.24 The diodes and supply channel for barrier devices have voltage drops
which must be taken into consideration. Show the correct values by means of a
table if the conversion resistor is placed on the supply leg of the measuring
circuit.
3.25 The diodes and supply channel for barrier devices have voltage drops
which must be taken into consideration. Show the correct values by means of a
table if the conversion resistor is placed on the return leg of the measuring
circuit.
3.26 Name the two evaluations that one has to use to select the proper
barrier.
3.27 Why do transmitters have to be approved as intrinsically safe?
3.28 What type of barrier is use when the placement of the conversion
resistor is placed on the supply leg of the circuit?
3.29 What type of barrier is use when the placement of the conversion
resistor is placed on the return leg of the circuit?
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APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
3.30 List the three main disadvantages of barriers.
3.31 Explain how one can overcome the main disadvantages of barriers.
3.32 Explain how a barrier is rated.
3.33 Describe the operating principles of the barrier circuit shown in Fig. 3.1
below, which is used as an intrinsically safe device.
3 1
4 2
3.34 What should one consider when connecting a barrier circuit to an
intrinsically safe apparatus?
3.35 Name eight fundamental design techniques/methods that can be used to
achieve intrinsic safety at both the device and system level.
3.36 If one considers a typical hazard, one may distinguish between two types
of hazards. Name the two types of hazards.
3.37 Define a first degree hazard.
3.38 Define a second degree hazard.
3.39 Name six typical hazardous materials.
3.40 In order to determine whether a substance should be regarded as a
hazard, certain information about the substance attributes should be
considered. List six of the attributes.
3.41 Define intrinsically safe equipment according to the National Electrical
Code.
3.42 Define intrinsically safe equipment according to the ISA.
3.43 Describe the Mechanical Isolation design technique that is used to
achieve intrinsic safety at both the device and system level.
3.44 Describe the Electrical Isolation design technique that is used to achieve
intrinsic safety at both the device and system level.
3.45 Describe the Current and Voltage Limiting design technique that is used
to achieve intrinsic safety at both the device and system level.
3.46 Describe the Shunt Element design technique that is used to achieve
intrinsic safety at both the device and system level.
3.47 Describe the Analytical Method design technique that is used to achieve
intrinsic safety at both the device and system level.
3.48 A non-hazardous location is also known as a (a)____ location and a
hazardous location or intrinsically safe location is also known as a (b)____
location.
3.49 Describe the Flameproof Equipment design technique that is used to
achieve intrinsic safety at both the device and system level.
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APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
3.50 Of the eleven fundamental design techniques/methods that can be used
to achieve intrinsic safety at both the device and system level, which one is the
most widely used?
3.51 Of the eleven fundamental design techniques/methods that can be used
to achieve intrinsic safety at both the device and system level, which one is the
most useful technique for preventing intrinsically safe and non-intrinsically safe
circuits from contacting each other?
3.52 Describe the Intrinsically Safe Equipment design technique that is used
to achieve intrinsic safety at both the device and system level.
3.53 Describe the Purging design technique that is used to achieve intrinsic
safety at both the device and system level.
3.54 Describe the Increased Safety Methods design technique that is used to
achieve intrinsic safety at both the device and system level.
3.55 Describe the Non-Sparking Apparatus design technique that is used to
achieve intrinsic safety at both the device and system level.
3.56 Describe the Special Methods (Symbol S) design technique that is used to
achieve intrinsic safety at both the device and system level.
3.57 What type of barrier can be used when the conversion resistor is placed
on the supply leg of the circuit?
3.58 What type of barrier can be used when the conversion resistor is placed
on the return leg of the circuit?
3.59 Make a detailed sketch of a barrier circuit between a hazardous and non-
hazardous area whereby the conversion resistor is placed on the supply leg of
the circuit.
3.60 Make a detailed sketch of a barrier circuit between a hazardous and non-
hazardous area whereby the conversion resistor is placed on the return leg of
the circuit.
3.61 Resistance Temperature Detectors (RTDs) are normally insulated to
withstand how many volts?
3.62 What type of barriers can be used with Resistance Temperature
Detectors (RTDs)?
3.63 What type of barriers can be used with Resistance Temperature
Detectors (RTDs) when incorporating three wire circuits?
3.64 What type of barriers can be used with Resistance Temperature
Detectors (RTDs) when incorporating four wire circuits?
3.65 Make a detailed sketch of a barrier circuit configuration whereby a
thermocouple is insulated from earth.
3.66 Please answer the following:
(a) If a thermocouple is earthed, or if its insulation will not pass the 500 V
test, can one connect a barrier device to it?
(b) Make a detailed sketch of the correct intrinsically safe configuration
whereby a thermocouple is earthed.
3.67 What type of barriers can be used with Photocells and AC Sensors and
why?
3.68 An explosion hazard is created when how many conditions are met
simultaneously?
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APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
3.69 Electrical isolation a logical companion to (a)_____.
3.70 When one refers to Intrinsically Safe Equipment, where could one find
details/information on the requirements for flameproof containers?
3.71 What is the most versatile way to avoid excessive quantities of electrical
energy if one refers to intrinsically safe equipment?
3.72 If non-energy storing devices such as thermocouples, RTD’s, photocells,
LED’s and switches, are coupled to barrier circuits. The barrier safety
certificate remains valid. Energy-storing equipment, however, must be
designed to be intrinsically safe. Otherwise a small flow of energy through the
barrier may be stored and released as a sudden burst of energy. Name three of
these instrument/components that usually fall into this category.
3.73 Normally, the power of the transmitter is supplied from the control
system, with a resistor to convert the 4-20 mA signal to a 1-5 V analogue signal.
The resistor can be placed either on the (+) supply lead, or the (-) return lead
of the circuit. What is the value of this measuring resistor in ohms?
3.74 Thermocouples that are insulated from earth can be brought back
through a (a)____.
3.75 Name one example of a shunt clamping device.
3.76 What type of electronic components is used for voltage limiting in
intrinsically safe circuits?
3.77 What type of electronic components is used for current limiting in
intrinsically safe circuits?
3.78 What does the abbreviation “SCR” stand for if one refers to zener-
triggered SCR crowbar circuits?
3.79 Name two advantage of using zener diode voltage limiting barriers.
3.80 Repeaters normally suit most (a)_____ applications, but because of the
(b)____ cost, it is used only when (c)____ methods can’t be used. Repeaters
supply a regulated voltage of (d)____ to the transmitter from a (e)____ _____.
The (f)_____ _____ is then run through the barrier, which repeats it without
any substantial (g)____ _____ _____.
3.81 State whether the following statements are True or False.
(a) The Analytical Method can be used to assess a circuit and a
mechanical layout which already exists, so to analyse the circuit and help
design the mechanical layout.
(b) The Analytical Method can be used to assess a circuit and an electrical
layout which already exists, so to analyse the circuit and help design the
electrical layout.
(c) It is not possible to determine the safety of all circuits through
analysis alone. Inductors, particularly small ones with magnetic cores, may
require testing to verify that they are safe despite high measured
inductance.
(d) Equipment which is mounted in the hazardous environment and
connected to apparatus in non-hazardous locations via barriers circuits
should be intrinsically safe.
(e) Capacitance and inductances of cables connecting barriers to
apparatus in hazardous locations are likely to store energy. Certain
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APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
parameters are therefore specified with respect to the cables used, when a
safety certificate is issued for a barrier circuit.
(f) The measuring resistor can only be placed on the (+) supply lead
of the measuring circuit.
(g) Repeaters normally suit all transmitter applications, but because of
the higher cost, it is used only when cheaper methods can’t be used.
(h) Repeaters supply an unregulated voltage of 15-17 V to the transmitter
from a 24 V source.
(i) The barrier device is mounted in a non-hazardous region, and acts as
an interface between the hazardous and non-hazardous locations.
(j) Because transmitters can’t store energy, they do not have to be
approved as intrinsically safe.
(k) If the thermocouple is earthed, or if its insulation will not pass the 500
V test, safety regulations forbid its connection to a barrier. It is then
necessary to use an isolation interface device.
(l) Thermocouples that are insulated from earth can be brought back
through a shunt diode barrier circuit.
(m) For 3 wire circuits where the bridge is floating, a two-channel barrier
having a low resistance can be used. If the bridge circuit is earthed, a third
barrier channel is needed, which can be half of another two channel barrier.
3.82 State whether the following statements are True or False.
(a) Equipment which is mounted in the hazardous environment and
connected to apparatus in non-hazardous locations via barriers circuits do
not have to be intrinsically safe.
(b) Capacitance and inductances of cables connecting barriers to
apparatus in hazardous locations are likely to release energy. Certain
parameters are therefore specified with respect to the cables used, when a
safety certificate is issued for a barrier circuit.
(c) The measuring resistor can be placed either on the (+) supply lead, or
the (-) return lead of a measuring circuit.
(d) Repeaters normally suit most transmitter applications, but because of
the higher cost, it is used only when cheaper methods can’t be used.
(e) Repeaters supply a regulated voltage of 15-17 V to the transmitter
from a 24 V source.
(f) The barrier device is mounted in a hazardous region, and acts as an
interface between the hazardous and non-hazardous locations.
(g) Because transmitters can store energy, they are considered complex
devices, and must be approved as intrinsically safe.
(h) If the thermocouple is earthed, or if its insulation will not pass the 500
V test, safety regulations forbid its connection to a barrier. It is then
necessary to use a shunt diode barrier circuit.
(i) Thermocouples that are insulated from earth can be brought back
through an isolation interface device.
(j) The isolation interface that is used for thermocouples that are
insulated is suitable for Photocells and AC Sensors. This design handles
signals up to 6 V peaks, which is more than sufficient.
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APPENDIX III LEARNING UNIT III QUESTIONS
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(k) For 3 wire circuits where the bridge is floating, a single-channel
barrier having a low resistance can be used. If the bridge circuit is earthed, a
third barrier channel is needed, which can be half of another two channel
barrier.
(l) Four wire systems can be protected by two ordinary non-polarised
barriers.
(m) Capacitance and inductances of cables connecting barriers to
apparatus in hazardous locations are unlikely to store energy.
3.83 State whether the following statements are True or False.
(a) The star connected barrier that is used for thermocouples that are
insulated is suitable for Photocells and AC Sensors. This design handles
signals up to 6 V peaks, which is more than sufficient.
(b) Four wire systems can be protected by two ordinary polarised barriers.
(c) Current limiting is done by a shunt clamping device in a barrier
circuits.
(d) Voltage limiting is done by a shunt clamping device in a barrier
circuits.
(e) According to the ISA, intrinsically safe equipment is “incapable of
releasing sufficient electrical thermal energy under normal conditions to
cause ignition of a specific hazardous atmosphere mixture”.
(f) An explosion hazard is created when five conditions are met
simultaneously.
(g) Second degree hazards are caused by equipment and conditions that
are inherently dangerous, such as the presence of heat, energy sources, the
presence of toxic or combustible gas mixtures, electric circuits, etc.
(h) Materials which are extremely hot, extremely cold, under high
pressure or present in large quantities are potential hazards.
(i) With the exception of portable or battery-operated systems, an
intrinsically safe system has no connection to a power line.
(j) Using the "Shunt Elements" method for intrinsically safety, the e.m.f.
produced by the inductor is always in a direction that maintains the current
flow in the same direction as before the interruption of the current. The
current resulting from the voltage flows through the protective diode rather
than in the external circuit.
(k) The casing or housing of “Flameproof Equipment” is designed so that it
will withstand, without injury, any explosion of the prescribed flammable gas
or vapour that may occur within it under practical conditions of operation
within the rating of the apparatus, and will prevent the transmission of flame
such as will ignite the prescribed flammable gas or vapour which may be
present in the surrounding atmosphere.
(l) With the exception of some portable devices, all intrinsically safe
circuits require current and voltage limiting to ensure that the energy
released under fault conditions does not exceed safe values.
(m) If energy storing devices such as thermocouples, RTD’s, photocells,
LED’s and switches, are coupled to barrier circuits. The barrier safety
certificate remains valid.
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APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
3.84 State whether the following statements are True or False.
(a) Voltage limiting is done by a fuse and one or more resistors in barrier
circuits.
(b) ISA says “intrinsically safe equipment and wiring are incapable of
releasing sufficient electrical or thermal energy under normal or abnormal
conditions to cause ignition of a specific hazardous atmospheric mixture in
its most ignitable concentration”.
(c) A distinction is usually made between first degree hazard and second
degree hazards.
(d) First degree hazards are the result of second degree hazards and
include explosions, fire release of hazardous materials, exposure effects on
personnel, etc.
(e) With the exception of portable or battery-operated systems, an
intrinsically safe system still has connection to a power line.
(f) In order to minimise the available energy in intrinsically safe circuits,
the voltage of the power supply is kept as low as possible.
(g) The electronics of “Flameproof Equipment” is designed so that it will
withstand, without injury, any explosion of the prescribed flammable gas or
vapour that may occur within it under practical conditions of operation
within the rating of the apparatus, and will prevent the transmission of flame
such as will ignite the prescribed flammable gas or vapour which may be
present in the surrounding atmosphere.
(h) With the exception of some portable devices, nearly all intrinsically
safe circuits require current and voltage limiting to ensure that the energy
released under fault conditions does not exceed safe values.
(i) Redundancy is offered so that if a single fuse or limiting circuit fails,
the second fuse can provide backup voltage limiting.
(j) Various increased safety methods are applied to ensure safe operation
of equipment, such as special terminals that can withstand vibration,
corrosion, etc., thermal insulation materials, etc.
(k) Using the "Shunt Elements" method for intrinsically safety, the
diodes releases energy stored in the inductor if the circuit external to the
protected inductors opens up.
(l) Mechanical Isolation is perhaps one of the most useful techniques for
preventing intrinsically safe and non-intrinsically safe circuits from
contacting each other.
(m) Apparatus that in normal operation and in the absence of electrical or
mechanical failure does not spark or develop temperature high enough to
ignite a prescribed gas or vapour-air mixture is called Non-sparking
apparatus.
3.85 State whether the following statements are True or False.
(a) Current limiting devices/components don’t have to be redundant as
long as they are properly mounted so that there is no possibility of shorting.
(b) Special terminals are the only equipment used in “Increased Safety
Methods”
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APPENDIX III LEARNING UNIT III QUESTIONS
EIPIN3: PROCESS INSTRUMENTATION 3
(c) Mechanical Isolation is perhaps one of the less useful techniques for
preventing intrinsically safe and non-intrinsically safe circuits from
contacting each other.
(d) Apparatus that in normal operation and in the presence of electrical or
mechanical failure does not spark or develop temperature high enough to
ignite a prescribed gas or vapour-air mixture is called Non-sparking
apparatus.
(e) Using the "Shunt Elements" method for intrinsically safety, the diodes
absorb energy stored in the inductor if the circuit external to the protected
inductors opens up.
(f) Using the "Shunt Elements" method for intrinsically safety, the e.m.f.
produced by the inductor is always in the opposite direction that maintains
the current flow in the same direction as before the interruption of the
current. The current resulting from the voltage flows through the protective
diode rather than in the external circuit.
(g) If non-energy storing devices such as thermocouples, RTD’s,
photocells, LED’s and switches, are coupled to barrier circuits. The barrier
safety certificate remains valid.
(h) Capacitance and inductances of cables connecting barriers to
apparatus in hazardous locations are likely to store energy. Certain
parameters are therefore specified with respect to the cables used, when a
safety certificate is issued for a barrier circuit.
(i) Although there are intrinsically safe devices, certification of intrinsic
safety can really only be granted on a system basis. The safety of each
system depends upon the characteristics of all its elements. In effect, the
system is no better or safer than its weakest link.
(j) Current limiting devices/components have to be redundant to prevent
the possibility of shorting.
(k) In order to minimise the available energy in intrinsically safe circuits,
the voltage of the power supply is kept as high as possible.
(l) Redundancy is offered so that if a single diode or limiting circuit fails,
the second device can provide backup voltage limiting.
(m) Current limiting is done by a fuse and one or more resistors in barrier
circuits.
3.86 Add new questions?
