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CHAPTER 9 INTRINSIC SAFETY 53
CHAPTER 9
INTRINSIC SAFETY
Intrinsic safety, Ex-ia, protects low voltage control
circuits from igniting volatile gases or dusts in
hazardous areas by limiting voltage and current thus
preventing ignition. Full details can be found at
www. isbarriers.com or
www.crouse-hinds.com/CrouseHinds/resources/
intrinsically_safe/insafe.cfm
www.isbarriers.com offers easy selection of the correct Ex-ia interface,
engineering drawings, downloadable control drawings and on-line ordering
of intrinsically safe barriers.
9.1 INTRODUCTION
Intrinsic Safety is the method of protection for control and
instrumentation circuits where the nominal voltage is 24 VDC or
less and the current is normally less than 100 mA. The concept
of intrinsic safety is to limit the voltage and current so that there
is never a spark with enough energy to create an explosion.
Intrinsic safety when properly used removes the ignition from
the explosion triangle.
There are three components to an intrinsically safe circuit: the
field device, intrinsically safe barrier and field wiring.
Field devices known as intrinsically safe apparatus are
classified as simple or complex.
Simple apparatus, which do not need to be approved, are non-
energy storing devices such as contacts, thermocouples,
RTDs, LEDs and resistors.
Complex apparatus such as transmitters, solenoids, relays
and transducers may store excess energy and need to be
approved by a third party.
Contacts, transmitters and temperature sensors are the most
commonly used field devices in intrinsically safe applications.
The intrinsically safe barrier limits the current with a resistor
and the voltage with a zener diode.

Intrinsically safe circuits are designed so that they
operate properly under normal conditions, but keep the
energy levels below the ignition curves when a fault condition
occurs.
Control wiring diagrams as required by the NEC are avaialble at
www.isbarriers.com
There are three components to a barrier that limit current and
voltage: a resistor, at least two zener diodes, and a fuse. The
resistor limits the current to a specific value known as the short
circuit current, Isc. The zener diode limits the voltage to a value
referred to as open circuit voltage, Voc. The fuse will blow when
the diode conducts. This interrupts the circuit, which prevents
the diode from burning and allowing excess voltage to reach the
hazardous area. There always are at least two zener diodes in
parallel in each intrinsically safe barrier. If one diode should fail,
the other will operate providing complete protection.
A simple analogy is a restriction in a water pipe with an
overpressure shutoff valve. The restriction prevents too much
water from flowing through the point, just like the resistor in the
barrier limits current. If too much pressure builds up behind the
restriction, the overpressure shutoff valve turns off all the flow in
the pipe. This is similar to what the zener diode and fuse do with
excess voltage. If the input voltage exceeds the allowable limit,
the diode shorts the input voltage to ground and the fuse blows,
shutting off electrical power to the hazardous area.
9.2 DETERMINING SAFE ENERGY LEVELS
Voltage and current limitations are ascertained by ignition
curves, as seen in Fig. 4. A circuit with a combination of 30 V
and 150 mA would fall on the ignition level of gases in Group A.
This combination of voltage and current could create a spark
large enough to ignite the mixture of gases and oxygen (with a
safety factor of 1.5). Intrinsically safe applications always stay
below these curves where the operating level of energy is about
1 watt or less. There also are capacitance and inductance
curves which must be examined in intrinsically safe circuits.
Cooper Crouse-Hinds Ex Digest
Intrinsic Safety 54
The energy from the spark created by the combination of voltage and current
must be below the ignition curves to be intrinsically safe.
Consider the ignition curves to demonstrate a point about
thermocouples. A thermocouple is classified as a simple device.
It will not create or store enough energy to ignite any mixture of
volatile gases. If the energy level of a typical thermocouple
circuit were plotted on the ignition curve in (Fig 2.), it would
not be close to the ignition levels of the most volatile gases in
Group A. Is the thermocouple which is installed in a hazardous
area (Fig 3) intrinsically safe? The answer is no, because a fault
could occur on the recorder which could cause excess energy
to reach the hazardous area, as seen in (Fig 4). To make sure
that the circuit remains intrinsically safe, a barrier to limit the
energy must be inserted (Fig 5).
The unprotected thermocouple is not considered safe in the hazardous area.
Ignition is possible from a fault occurring on the non-hazardous side.
An intrinsically safe barrier will limit the excess voltage and current from
reaching the hazardous side.
9.3 INTRINSICALLY SAFE BARRIERS
There are 3 types of barriers most commonly used:
1. Zener barriers passive devices which required grounding
for safety
2. Isolation barriers do not require grounding and contain
additional electronics for isolation and signal conditioning, or
3. Ex-ia I/O combines I/O with intrinsic safety into one
package.
9.3.1 GROUNDED ZENER BARRIERS
Grounded barriers, also referred to as zener barriers, are
passive devices which contain zener diodes to limit excess
voltage, resistors to limit current and fuses. These are the basic
building blocks which are contained in all other intrinsically safe
barriers. There is always a voltage drop across grounded
barriers because of the resistors so some selection is required
as well as a ground connection. This selection has been greatly
simplified in recent years as manufacturers make them more
application specific. Grounded barriers are also very versatile
and can be applied in many other applications. If your
application has less than 20 outputs or inputs and grounding is
not a consideration, this may be the best solution.
Cooper Crouse-Hinds Ex Digest
Intrinsic Safety
Figure 2
Ignition Curves - Resistance
Figure 3
Thermocouple Installed in a Hazardous Area
Figure 4
Thermocouple with Fault
Figure 5
Thermocouple with Barrier 55
Cooper Crouse-Hinds has a complete selection of grounded barriers.
Refer to www.isbarriers.com
DIN Rail Grounded Barriers
Advantages Other
Considerations
Lowest initial cost per unit
Requires ground
Very small < 1/2" wide
Barrier resistance
can influence
circuit function
Very precise signal response
Small power requirements
Versatile for other circuits
9.3.2 ISOLATION BARRIERS
DIN rail isolated barriers, also referred to as transformer
isolated or galvanically isolated barriers, are zener barriers with
additional electronics to isolate and condition the signals.
Adding the isolation has the advantage that an intrinsically safe
ground connection is not required. The signal conditioning of
isolated barriers simplifies the selection process as each
isolated barrier is manufactured for specific functions such as
switching, temperature measurements or 4-20 mA readings.
These isolated units are ideal for digital inputs or for OEMs
where grounding may cause problems at the local installation.
Crouse-Hinds DIN rail isolation barriers do not require a ground for
protection. Refer to www.isbarriers.com
Din Rail Isolated Barriers
Advantages Other
Considerations
Does not require IS ground
Higher cost than
grounded barriers
Loop layout & barrier
Larger width
selection is easier
1" wide
Integrated signal
Larger power
conditioning requirements
9.3.3 REMOTE I/O PRODUCTS
Until recently there were limited improvements made in this
industry. The latest generation of products now reduce the total
installed cost by combining the intrinsically safe barriers with the
I/O, eliminating extra hardware. These new systems called
intrinsically safe remote I/O can be mounted almost anywhere
in hazardous or ordinary locations reducing the wiring and
terminations.
These systems were initially designed for the German chemical
industry, which wanted to reduce installation costs and no
longer had enough space in control rooms to house termination
panels. Their reasoning was to extend the 2 wire
communication lines out as far away as possible in the process
area to minimize field wiring to the sensors and extra
termination cabinets.
Signals to and from the hazardous area are made intrinsically safe, processed
by the remote I/O electronics, and transmitted to a memory module through a
communication link. These signals are updated every 5 milliseconds and
stored for the main control system. The intrinsically safe remote I/O system is
connected to the controller by a simple 2 wire or fiber optic link to relay
information back and forth.
These systems are ideal for users, who want to eliminate wiring
from the control system to the I/O and can communicate via a
bus system such as Modbus, Profibus, or Fieldbus.
Cooper Crouse-Hinds Ex Digest
Intrinsic Safety 56
Intrinsically Safe Remote I/O Systems
Advantages Other
Considerations
Lowest installed cost
Some expertise