8. 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.
13. Plant and Installations are
classified according to:
The nature of the Hazardous Atmosphere
Class 1…………..Gasses
Class 2…………..Dusts, Powders
Class 3…………..Fibers & Flyings
GAS CLASSIFICATION
14. 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
15. NFPA 497 Table 2-1NFPA 497 Table 2-1
Representative
(Test) GAS
NEC 505
Zones 0,1 & 2
NEC 500
Divisions 1 & 2
Spark 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
16. SparkIgnitionCharacteristicSparkIgnitionCharacteristic
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
17. 20mA
10mA
50mA
100mA
200mA
500mA
1A
1 2 5 10 20 50
Matched power rating 1.3W for T4
Hydrogen
resistive curve
with 1.5 safety
factor
Capacitive
restriction
0.1µF
Useable
Area
500µH (260mA)
Inductive restriction
Open circuit voltage V
Short circuit
current
Practical Limitations of an IS circuit
19. T amb = 40 °C
Apparatus is marked with either
T-rating or maximum surface
temperature
User has responsibility to ensure that
the T-rating is below the Spontaneous
Ignition Temperature ( SIT )
T Class ºC
T1
T2
T2A
T2B
T2C
T2D
T3
T3A
T3B
T3C
T4
T4A
T5
T6
450
300
280
260
230
215
200
180
165
160
135
120
100
85
“T” RATINGS
20. Gas/equipment compatibility
Ammonia 630
Methane 595
Hydrogen 560
Propane 470
Ethylene 425
Butane 365
Gas Ignition
Temperature
700
600
500
400
300
200
100
T1
T2
T3
T6
T4
T5
Apparatus
Temperature
Classification
Cyclohexane 259
Carbon Disulfide 100
ºC
21. T4 for apparatus is usually OK
Exceptions 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
23. AMERICAS IEC/Europe
Class 1
Division 1
Class 1
Division 2
Recognized in principle
by some users, apparatus
suitable for Zone 0
usually specified
Types of Protection:
-explosion-proof
-purging
-intrinsic safety
-oil immersion
Types of Protection:
all types suitable
for Div 1
non-incendive
ZONE 0
ZONE 1
ZONE 2
Intrinsically safe Ex ia or
specifically approved
for Zone 0
Types of Protection:
(d) flameproof
(p) pressurized
(i) intrinsic safety
ia & ib
(e) increased safety
(q) powder filled
(m) encapsulation
Types of Protection:
all types suitable
for Zone 0 & Zone 1
(n) Type N
(o) oil immersion
25. 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 defined
Designed to eliminate hot surfaces or incendive sparks
under normal operating conditions
26. Purging: a complex technique, but quite often
the only solution
Pressure
Switch
mains
inlet
air inlet
initial
purge
X, Y, Z Purging
27. 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
30. 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
32. TheThe “Part s ” of a“Part s ” of a
ZenerZenerproduction ZENERs have moreproduction ZENERs have more
partsparts
dual-redundant Zener diodesdual-redundant Zener diodes
additional resistanceadditional resistance
often multiple fusesoften multiple fuses
+24V+24V
0V
Control
Room
300Ω 20Ω
28V28V
MandatoryMandatory
internal fuseinternal fuse
ReplaceableReplaceable
fuse (option)fuse (option)
50mA
FT
HAZ
37. Simple
Apparatus
"Devices in which, according to the manufacturer's
specifications, none of the values 1.2 V, 0.1A, 20µJ or
25mW 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:
38. 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
847efgdjeijruh
nsjei4uuitim doir 7354378 uj
mfnrieir ur785 e375463 834
39. Compat i bl eCompat i bl e
Part ners ?Part ners ?
safety parameters - is it a safesafety parameters - is it a safe
combination?combination?
operational parameters - will theoperational parameters - will the
system actually work?system actually work?
FIELDFIELD
INSTRUMENTINSTRUMENT SAFETYSAFETY
BARRIERBARRIER
41. 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:
Hazardous
Area
Connection
Safe
Area
Connection
4 1
2
3
∩∪
∩∪
< 1Ω
5
6
V+
Sig
0v
28V 300R
93mA
43. 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,
ADD
Ci & C Cable < CA
Li & L Cable < LA
**If cable is less
than 1,000 feet,
DISREGARD
C & L of Cable!!
**Over 1,000 feet,
ADD
Ci & C Cable < CA
Li & L Cable < LA
44. 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 the
safety (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
45. 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+
.9V
33R 10R
300R 42R
43R-0.9V
342R
46. Loop voltage drops at 20mA are:
Barrier (6.66+.9+.52) = 8.08
Transmitter = 12.0
DCS 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!
48. 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.
50. Instrument
system
‘0v’ rail
PRIMARY 12 AWG GROUND
CONNECTION (LESS THAN 1
OHM!)
Earthing and Bond
H
A
Z
A
R
D
O
U
S
A
R
E
A
W
I
R
I
N
G
H
A
Z
A
R
D
O
U
S
A
R
A
W
I
R
I
N
G
S
A
F
E
A
R
E
A
W
I
R
I
N
G
S
A
F
E
A
R
E
A
W
I
R
I
N
G
Dual Conductor I.S.
Ground
SECONDARY GROUND
CONNECTION
51. Instrument
system
‘0v’ rail
Resistance meter
checks complete
loop
Earthing and Bond
H
A
Z
A
R
D
O
U
S
A
R
E
A
W
I
R
I
N
G
H
A
Z
A
R
D
O
U
S
A
R
A
W
I
R
I
N
G
S
A
F
E
A
R
E
A
W
I
R
I
N
G
S
A
F
E
A
R
E
A
W
I
R
I
N
G
I.S. GROUND TESTING
52. But, what happens if I simply
don’t have a “good” ground at
the location??
53. 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??
54. Load
Power supply
20-35 V dc
4-20 mA 17.5 V
300R
28 V
Isolation
Hazardous area Safe area
Typically
up to 800R
Galvanic Isolator for 2-wire
transmitters
55. SIMPLE AND RELIABLE FAIRLY COMPLEX,
LOWER MTBF
HIGH INTEGRITY GROUND FLOATING, ISOLATED
INEXPENSIVE MORE EXPENSIVE
GENERIC APPLICATIONS APPLICATION SPECIFIC
ZENERS ISOLATORS
56. 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.
Ask question: How many major fires or explosions such as this occur in the U.S. per year on average. Answer 18. How many in Europe. Answer 1. Why? Point out the use of I.S. and that it is the only acceptable technique for "explosion proofing" in Zone 0 in Europe.
Ditto.
What it takes to have an explosion and that I.S. limits the amount of energy getting into the Hazloc such that it can not create an explosion.
Self explanatory. Ask the attendees to help define before giving them the answer.
The "official" definition...
History.
On October 13, 1914, 413 miners were killed in the Senghyyd Colliery in Wales as a result of an explosion determined to be caused by the mine signalling system shown on the next slide.
Define the circuit shown in the slide and how on October 13, the miner shorted the bell wires but happened to be standing in a pool of methane gas, which ignited.
Subsequent investigation by the British Home Office at Eskmeals determined the cause as the combined stored electrical energy in the bell coil added to the battery energy. They then determined that 3 values had to be limited: Volts (24-30V) which is what we use today. Current..approximately 30-50mA Stored energy...ask audience to determine where this is on next repeat slide.
Energy comes from: Stored: Bell coil Battery Any inductance or capacitance in wire (negligible).
Restatement of objectives.
Getting down to basics.
Self explanatory.
Self explanatory.
Self explanatory.
Self explanatory with the comment on hydrogen MIE of 20 microjoules being the same amount of energy standard for simple apparatus.
While this graph is a good guide to practical limits, beware that it can be misinterpreted! We can not, for instance, have 30V and 125mA (3.75W) but we can have 6V@300mA in Gp D, or 12V@180mA, etc.
Self explanatory.
“ T” Ratings determine the maximum surface temperature of the field instrument…and consequently its suitability for the hazloc environment in which it is placed. The Customer must determine what their SIT for the hazloc area is and then if the device safely operates there.
Self explanatory.
Self explanatory.
Self explanatory.
These are the primary methods of “explosion proofing” and are detailed in the next slides.
Important notes: This is the least understood technique. It is Div 2 only!!! It is 2 distinct levels: Non arcing/non sparking which is like “explosion proof lite”. Must have thin wall conduit or armored cable, seal offs at instrument and hazloc transition point. Field instruments must have corresponding rating. Can not work live. 2. Energy limited Non-Incendive. This is like “intrinsic safety lite” but instead of a 1.5 safety factor it is 1.1. Field instrument must have non-incendive rating including entity parameters. Live working permitted. Still Div 2 only.
X Level purging: Suitable for Division 1, it reduces the level hazard to the contained equipment to non-hazardous. It is the most complex, having initial purge valves, timers to ensure at least 4 volumes of air are changed, pressure switches to ensure air flow, and often safety barriers connected to control systems, prior to initiating power on. Y Level: Similar to X level only it reduces the equipment inside to Div 2 safety level only. Hence, equipment must be certified for acceptability in a Div 2 area. Z Level: A Div 2 only type of purge and may consist simply of an air source, pressure switch, and rely on manual timing for cycling of air changes.
Self explanatory.
Ask audience to explain how this works? Look for answer to include: Allows internal explosion to take place from surrounding gas(es) and spark(s) but.. 2. Allows the resulting hot flame to escape via a precisely defined gap (or thread chase) such that it is sufficiently cooled so as not to PROPAGATE the flame from the inside of the box to the outside.
Walk through the sequence of explosion while detailing that… 1. The gap must be precisely maintained at less than .7mm (.0030 ”). The gap must NOT be filled with waterproofing such as silicon seal, etc., or a “bomb” is created. All the bolts must be put back in the cover after maintenance. The threaded cover on the condolets must be torqued to the proper specification.
Self explanatory.
Shunt zener diodes are made up of: Dual redundant chains of zeners such that if one chain should fail, the likelihood of the second likewise failing are so infinitely small as not to be considered. Current limiting resistor, in this case 300R, in addition to resistors which enable testing of the diode chains, in this case 20R, plus the fuse(s).
Diodes can be arranged in either a positive to ground, or a negative to ground, arrangement. A positive arrangement allows for a positive voltage up to the limitation of the Vmax of the diode to be passed. A negative arrangement allows a negative voltage to be passed.
Back to back zeners allow for a non-polarized, or AC voltage or sine wave to be passed.
Explain: In an I.S. system, the control panel must not have over 250V present due to the limitation of the 250V isolation rating of the barrier/isolator. The wiring between the control panel and the barrier can be ordinary general purpose wiring, i.e., shielded TSP. The barrier/isolator may be located in a GP or Div 2 location, if so rated. The terminals and wiring on the hazloc side of the barrier/isolator is low energy, IS wiring, and must be segregated from all other types of wiring by 50mm or 2 ” or approved partition. See ISA RP12.6 for details. It should also be light blue in color (IEC recognized wire color for IS) or in wire trays, ducts, etc., that is marked at 15’ intervals with permanent labels designating the wiring as IS. Hazardous area device must be IS. Explain example of Customer wanting to “make” a non IS transmitter IS by adding barrier and the stored energy problems associated.
Exception to the rule of IS transmitter is simple apparatus as defined in the next slide.
Simple apparatus can be a part of an IS loop without further certification. Examples are switches, RTD, TC, LED, etc.
Self explanatory.
Two (2) questions MUST be asked when determining whether a circuit is IS. Is it safe (entity parameters) 2. Will it work? Operational parameters.
Ensure that the audience understands the difference between safety description on the left side (this is an instantaneous, fault value) and operational or working description on the right. Ensure that the audience understands a 2 channel barrier and it ’s dual listing above.
Stress that the safety description of the MTL7787+ of 28V/93mA/300R means that this is a instantaneous fault energy value that only occurs at the point of rupturing of the safety fuse…and it means that 28V at 93mA is being drawn through a 300R resistor. It does not mean that one can design a circuit requiring 28V or 93mA!
Entity parameters are values of Vmax or Voc, Imax or Isc, Ci or Ca, Li or La, which are assigned by a 3 rd party certifying authority such as FM, UL, CSA, etc. The Vmax or voltage maximum, Imax or current max, Ci or inherent, unprotected capacitance, and Li or inherent, unprotected inductance, are values assigned to the field device by FM, etc. The Voc, or open circuit voltage, Isc, or short circuit current, Ca, or allowed capacitance, and La, or allowed inductance, are figures assigned to the IS interface by FM, etc. The Vmax and Imax of the field device must be greater than (>) the Voc and Isc of the ISB respectively. The Ci and Li of the field device, when added to the C, or capacitance of the cable, and the Li of the field device, when added to the L, or capacitance of the cable, both must be less than (<) the Ca and La of the ISB respectively. NOTE: If the cable is under 1,000 ’, it is safe to ignore the C and L figures of the cable. If over 1,000’, then the C and L of the cable must be calculated from manufacturer’s specification and the value added to the Ci and Li of the field instrument.
Show the audience where to find the entity parameters of an MTL7787+ in the MTL catalog. Then walk the audience through an entity parameter matching of a typical Rosemount transmitter with an MTL7787+ barrier to “prove” entity parameter match for safety.
Explain the elements of a typical 4-20mA current loop using an MTL7700 series barrier.
Walk the audience through where the volt drops are in a typical 4-20mA loop including the 330R supply side of the barrier, 12V for the transmitter, 0.9V =
Talk through the math with the audience. Obviously it doesn ’t work. Rarely will a control system allow for tweaking a power supply up 2V. Offer active barrier like 7706 or ISOLATOR!
Introduce critical grounding with reference to safety barriers…move to next slide.
Self explanatory.
Define the IS ground as being a less than 1 ohm path to true power ground. Further define the power ground as being the plant power ground grid, building steel or grounding mat that is tied to the AC distribution neutral/ground.
Propose a second 12AWG IS ground wire for two reasons: Safety in redundancy. 2. The ability to due scheduled ground resistance checks per the next slide.
The IS ground can be safely tested by removing one of the two IS grounds, inserting an ohmmeter between the in the loop as shown…and as long as the reading is less than 2 ohms (1 ohm to and 1 ohm from ground), the safety grounding system is good.
Self explanatory.
Self explanatory.
There are no IS grounds required using an isolator! The galvanic components within the isolator are designed and certified by 3 rd party agencies to be incapable of transferring fault energy in excess of the safety limits. Further energy limiting circuits are designed into the output circuit…in this case, 28V/93mA/300R. Isolators are generally separately powered, supply the voltage/current to both the field instrument as well as the control loop as shown. Note: The safe area load, as in this case, must be passive, i.e., non-powered…or there will be “bucking currents” and the system will not work!
