INTRINSIC SAFETYPRESENTED BY

Hadrian Jones

WHY DO WE NEED

INTRINSIC SAFETY?

Class 1, Div 1 or 2 Location

Non-Hazardous

Location

The Explosion Triangle

I.S. LIMIT

GAS AIR

HEAT or SPARK

CONTROL SYSTEM

DEFINING INTRINSIC SAFETY!

INTRINSIC:

SAFETY:

BUILT-IN, INHERENT, A NATURAL PART OF

THE STATE OF BEING SAFE

Definition per Code... A system, compromising

apparatus and interconnecting wiring, in

which any spark or thermal effect in any part of the system

intended for use in hazardous areas is incapable of causing

ignition.

WHERE DID

INTRINSIC SAFETY

ORIGINATE?

Spark ! Bare Wires

BellBatteries

Winch

Mining Bare Wire Signaling System c. 1910

Limit Voltage Limit Current Limit Stored Electrical

Energy

Basic Objectives of Intrinsically Safe Designs

Plant and Installations are classified according to:

The nature of the Hazardous Atmosphere

Class 1………….. GassesClass 2………….. Dusts, PowdersClass 3………….. Fibers & Flyings

GAS CLASSIFICATION

Division 1Division 1

Hazardous atmosphere is likely to be present in normal operation

Division 2Division 2

Hazardous atmosphere is unlikely to be present in normal operation

Area Classification in the Americas

The Probability that the Hazardous Atmosphere Will be Present

NFPA 497 Table 2-1NFPA 497 Table 2-1

Representative

(Test) GAS

NEC 505

Zones 0,1 & 2

NEC 500

Divisions 1 & 2Spark ignition

Acetylene

Hydrogen

Ethylene

Propane

Group IIC

Group IIC

Group IIB

Group IIA

Group A

Group B

Group C

Group D

Ease of ignition

from spark energy

MTL,Inc.MTL,Inc.

GAS CLASSIFICATIONS

Spark Ignition CharacteristicSpark Ignition Characteristic

LFL(Lower Flammable Limit)

UFL (Upper Flammable Limit)

Minimum Ignition Energy (MIE)

Ignition EnergyIgnition Energy(milli Joules)(milli Joules)

Volume concentration (%)Volume concentration (%)

1.0

0.1

0.01

Hydrogen-air(1 atmos.)

Flammable Range

Propane-air (1 atmos.)

Ethylene-air(1 atmos.)

0 10 20 30 40 50 60 70 80 90 100

180 microjoules

60 microjoules

20 microjoules

Group B

Group C

Group D

20mA

10mA

50mA

100mA

200mA

500mA

1A

1 2 5 10 20 50

Matched power rating 1.3W for T4Hydrogenresistive curvewith 1.5 safetyfactor

Capacitiverestriction0.1µF

UseableArea

500µH (260mA)Inductive restriction

Open circuit voltage V

Short circuitcurrent

Practical Limitations of an IS circuit

Area Classification Example

FlammableSubstance

Zone 1

Zone 0

Zone 2 / Div 2

Containment wall

Tank vents

Liquid surface

KeyArea Classificationfor a fixed-roof tank

Div 1

T amb = 40 °C

Apparatus is marked with eitherT-rating or maximum surface

temperature

User has responsibility to ensure thatthe T-rating is below the Spontaneous

Ignition Temperature ( SIT )

T Class ºC

T1T2T2AT2BT2CT2DT3T3AT3BT3CT4T4AT5T6

450300280260230215200180165160135120100 85

“T” RATINGS

Gas/equipment compatibility

Ammonia 630 Methane 595 Hydrogen 560

Propane 470 Ethylene 425

Butane 365

Gas IgnitionTemperature

700

600

500

400

300

200

100

T1

T2

T3

T6

T4 T5

ApparatusTemperatureClassification

Cyclohexane 259

Carbon Disulfide 100

ºC

T4 for apparatus is usually OKExceptions are: Carbon Disulfide and Ethyl Nitrate,

both requiring T5

Ambient may be raised by process temperature (for instance, a solenoid valve mounted on a hot steam line)

(

NOTES TO TEMPERATURE CLASSIFICATION

METHODS OF PROTECTION IN

HAZARDOUS AREAS

AMERICAS IEC/Europe

Class 1Division 1

Class 1Division 2

Recognized in principleby some users, apparatussuitable for Zone 0 usually specified

Types of Protection:-explosion-proof-purging-intrinsic safety-oil immersion

Types of Protection:all types suitablefor Div 1 non-incendive

ZONE 0

ZONE 1

ZONE 2

Intrinsically safe Ex ia orspecifically approved for Zone 0

Types of Protection:(d) flameproof(p) pressurized(i) intrinsic safety

ia & ib(e) increased safety(q) powder filled(m) encapsulationTypes of Protection:all types suitable for Zone 0 & Zone 1(n) Type N(o) oil immersion

NON-INCENDIVE

PURGING

EXPLOSION PROOFING

INTRINSIC SAFETY

NON INCENDIVE__(DIVISION 2 ONLY)ANSI/ISA S12.12 is the Standard

Falls Into two categories:

NON ARCING/NON SPARKING

Requires mechanical protection, hermetically sealed contacts.24VDC or 120VAC may be used

NON INCENDIVE(Energy Limited)

Similar to intrinsic safety including entity parameters,relaxed ignition curves,approved Div. 2 field devices, but

less well definedDesigned to eliminate hot surfaces or incendive sparks

under normal operating conditions

Purging: a complex technique, but quite often the only solution

Pressure Switch

mains inlet

air inlet

initialpurge

X, Y, Z Purging

Purging & Pressurization

AdvantagesAdvantages DisadvantagesDisadvantages

Sometimes the only solution

Can protect large volumes, panels and control rooms

Can have large margin of error before danger results

Clean air is not free; has to filtered, pumped, etc. Control system can be complex, includes other forms of pro- tection as well Live working is not

permissible Bulky

Explosion-proof Enclosures/Conduits

Gap

Flame Path Length

Explosion Proof Enclosures

Explosion-proofing: Pros & Cons

Permits sparking apparatus in hazardous areasPROS

Older design of enclosures were difficult to weatherproof

No live working is permitted without gas clearance certification

Boxes tend to be substantial Special fittings are necessary Installation errors or faults are dangerous

C

O

N

S

Intrinsic Safety…

the Better Way to

Prevent Explosions

The The “Part s ” of a “Part s ” of a ZenerZenerproduction ZENERs have more production ZENERs have more

partspartsdual-redundant Zener diodesdual-redundant Zener diodesadditional resistanceadditional resistanceoften multiple fusesoften multiple fuses

+24V+24V

0V

Control Room

300Ω 20Ω

28V28V

MandatoryMandatoryinternal fuseinternal fuse

ReplaceableReplaceablefuse (option)fuse (option)

50mA

FT

HAZ

Polarities--or + & -Polarities--or + & -

positivepositive

negativenegative

Non-Polarized!Non-Polarized!

““a.c.” or bipolar (dual-channel)a.c.” or bipolar (dual-channel)

Classified Area Control Bldg

Controller,

ROC, RTU

DCS, PLC,ROC, RTU or

SCADA

I.S Barrier or Isolator

Intrinsically Safe SYSTEMS

Certified I.S. Device

Classified Area Control Bldg

Controller,

ROC, RTU

DCS, PLC,ROC, RTU or

SCADA

I.S Barrier or Isolator

Intrinsically Safe SYSTEMS

“Simple Apparatus”

Simple Apparatus

"Devices in which, according to the manufacturer'sspecifications, none of the values 1.2 V, 0.1A, 20µJ or25mW is exceeded, need not be certified or marked "

Simple Apparatus Definition:

Hazardous Location apparatus can be either:

Certified, energy -storing (e.g instruments) or

Uncertified, "non-voltage producing, non-energy storing

Simple Apparatus"

Examples:

Equi pment Rat i ngEqui pment Rat i ng

• Type of protectionType of protection e.g. Intrinsic Safetye.g. Intrinsic Safety

• Temperature ClassificationTemperature Classification

• Hazardous location classificationHazardous location classification

847efgdjeijruhnsjei4uuitim doir 7354378 ujmfnrieir ur785 e375463 834

Compat i bl e Compat i bl e Part ners ?Part ners ?

safety parameters - is it a safe safety parameters - is it a safe combination?combination?

operational parameters - will the operational parameters - will the system actually work?system actually work?

FIELDFIELD

INSTRUMENTINSTRUMENT SAFETYSAFETY

BARRIERBARRIER

SAFETY VALUES versus

OPERATIONAL VALUES!!

SAFETY CHARACTERISTICS

28V 93mA 300R is a common barrier safety description

where V = 28 volts Current Limiting Resistor (CLR) = 300Rz

Barriers are usually described in terms of their safety parameters:

HazardousArea

Connection

SafeArea

Connection

4 1

2

3

∩∪

∩∪

< 1Ω

5

6

V+

Sig

0v

28V 300R93mA

The Entity ConceptThe Entity Concept

250v max.

Vmax…...….> > …. ..…..Voc

Imax….........> > ....…….….Isc

Ci +Ccable...< <...Ca(allowed)

Li+Lcable....< <...La(allowed)

I.S. Field Device I.S. Barrier

Rosemount? Transmitter

Vmax = 30V

Imax = 300mA Ci = 20nF

Li = 10 µH

MTL7787+Barrier

Voc = 28V

ISC = 93.0mA

Ca = .011µF

LA = 4.2mH

Is It Safe??

>><<

**If cable is less than 1,000 feet, DISREGARD

C & L of Cable!!**Over 1,000 feet,

ADDCi & C Cable < CALi & L Cable < LA

**If cable is less than 1,000 feet, DISREGARD

C & L of Cable!!**Over 1,000 feet,

ADDCi & C Cable < CALi & L Cable < LA

Operational Characteristics

342R supply

V out V in

** End-to-end resistance is greater than the safety resistance

**Maximum permissible safe area voltage is less than thesafety (i.e hazardous area output) voltage

V wkg=26.0

V max=26.9

4 1

2

3

∩∪

∩∪

< 1Ω

5

6

43 +0.9V return

250R (5V)

V +

0VHazardous area Safe area

Control System

1-5V

2 WIRE Transmitter with MTL 7787+

The Next Question That Must be Asked Then Is...

WILL IT WORK?IS Transmitter

12V (MIN) 24 V DC(NOM)

4

5

6

1

2

3

MTL7187+

.9V33R 10R

300R 42R

43R-0.9V

342R

Loop voltage drops at 20mA are:Barrier (6.66+.9+.52) = 8.08Transmitter = 12.0DCS or PLC = 5.0

Lines = ?? _______________

TOTAL = 25.08V

Nom. working voltage = 24.0V

DOES THE MTL7787+ WORK HERE????

Answer: No, the System Does Not Work!!!!

Solutions: “Tweak” 24V power supply to 26V?

Use an “ACTIVE” ZEN--MTL7706!

Use a GIB--(Isolator)--MTL4000 or 5000 Series!

Going t o Gr oundGoing t o Gr ound

ANSI/ISA RP12.06.01-2003 STATES:

THE BARRIER-GROUNDING TERMINAL MUST BE CONNECTED TO THE GROUNDING

ELECTRODE…USING AN INSULATED CONDUCTOR NO SMALLER THAN 12AWG.

ALL GROUNDING PATH CONNECTIONS SHOULD BE SECURE, PERMANENT, VISIBLE AND ACCESSABLE. THE GROUNDING PATH

RESISTANCE FROM THE FARTHEST BARRIER TO THE GROUNDING ELECTRODE SHOULD NOT

EXCEED 1 OHM.

O O

Field mountedinstrument

Plant bond

X

Controlsystem

L

E

N

Plant Grounding Scheme

Hazardous AreaNon-Hazardous Area

Barrier

E

Plant Power Ground Grid

One (1) ohm

Instrumentsystem‘0v’ rail

PRIMARY 12 AWG GROUND

CONNECTION (LESS THAN 1 OHM!)

Earthing and Bond

HAZARDOUS AREA WIRING

HAZARDOUS ARA WIRING

SAFE AREA WIRING

SAFE AREA WIRING

Dual Conductor I.S. Ground

SECONDARY GROUND CONNECTION

Instrumentsystem‘0v’ rail

Resistance meterchecks completeloop

Earthing and Bond

HAZARDOUS AREA WIRING

HAZARDOUS ARA WIRING

SAFE AREA WIRING

SAFE AREA WIRING

I.S. GROUND TESTING

But, what happens if I simply don’t have a “good” ground at the location??

Use the “other” I.S. approach:

“GIBS, TIBS, ISOLATORS, RELAYS.”

But how do THEY work?? Am I still intrinsically safe?? Are the rules still the same??

Load

Power supply20-35 V dc

4-20 mA 17.5 V

300R

28 V

Isolation

Hazardous area Safe area

Typicallyup to 800R

Galvanic Isolator for 2-wire transmitters

SIMPLE AND RELIABLE FAIRLY COMPLEX, LOWER MTBF

HIGH INTEGRITY GROUND FLOATING, ISOLATED

INEXPENSIVE MORE EXPENSIVE

GENERIC APPLICATIONS APPLICATION SPECIFIC

ZENERS ISOLATORS

Advantages of Intrinsic Safety

Simple Apparatus: Permits the use of normal industrial devices if they are non-energy storing.

Safest Technique: Only method permitted in Zone “0” in Europe.

Fault Tolerant: Can have two failures in system and remain safe!

Live Maintenance: Can work on system with live power on.

No Explosion Proof Fittings or Conduit: Wiring is electrically--not mechanically--protected.

Personnel Safety: Lowest currents and voltages.

This concludes the INTRINSIC SAFETY portion of the seminar.