Earthing and Bonding in hazardous areas

1. Earthing and Bonding in hazardous areas

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2. Earthing and bonding

are necessary to :
Reduce personnel shock risk
Operate protective devices (fuses & circuit breakers)
Lightning protection
Electrostatic Discharge control
Minimise interference & provide signal reference
Standards around the world covering earthing have similar aims and
often result in similar installations, though sometimes by different
means .
IS earthing requirements are a logical extension of natural earthing
practices, with the added requirement of keeping fault voltages and
currents out of the hazardous area.
Earthing and bonding
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3. Definitions - Terminology

Earthing:
where fault currents are caused to flow via a
dedicated return path (backed up by
connection to physical ground) in such a way
as to operate a protective device in an
appropriately short time
Bonding:
where voltage differences between parts of
plant, handrails etc., are eliminated by physical
cross - connection
What about Boats, Planes, & Platforms ?
Definitions - Terminology
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4. Human sensitivity

25V for 5mA shock
5-10V
Body threshold
500 ohms
500
ohms
500 ohms
Threshold of sensation
Disturbing shocks
Freezing current (women)
(men)
Ventricular fibrillation
(Defibrillators)
Cardiac arrest
500 ohms
1 mA
5 mA
6 - 25 mA
9 - 30 mA
1-5A
10 A
Data at 60 Hz
Human sensitivity
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5. Power Fault Return Path

Close circuit protection required.
Return path low resistance for rapid operation.
Protective
devices
Generator
Motor
FAULT
Distribution
system
Fault Current
Return path
Power Fault Return Path
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6. Bonding System

Bonding to prevent local potential differences
Distribution
system
Protective
devices
Generator
Motor
FAULT
Bonding
Conductors
Bonding System
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7. Overall (combined) Protection

Bonding to prevent local potential differences
Distribution
system
Protective
devices
Generator
Motor
FAULT
Earth Return path
Through TWO parallel routes!
Overall (combined) Protection
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8. Grounding and Lightning

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9. Power distribution systems

Power distribution uses high voltages to minimise losses.
Local supply transformers reduce voltage for plant use.
Typical supply transformer provides 440V 3ph, 240V 1ph.
Transformer neutral star-point is bonded to a substantial
earth mat as required by Electricity Supply Regulations and
utility company specifications.
Supply neutral and protective earth conductors are usually
bonded back to the neutral star point/main electrical system
earth point.
Power distribution systems
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10. Power distribution system

Protective
conductor
NB : Distribution system is
protected by out of balance
current detection across
phases; so return path
resistance is not critical
Local site
transformer
Generation
Distribution
Fault
path
Power distribution system
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11. Lightning protection and Earthing

Lightning protection is an example of when a true connection
to terrestrial earth is required
Lightning is a current flowing between cloud and ground charge
centres and is usually of enormous magnitude for a short time
Should this current flow horizontally in even a well bonded plant,
very large potential differences can be generated
Vertical structures on plant should be strapped to a good earth mat
Hazardous area equipment bonding becomes even more important
Lightning protection and Earthing
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12. Charge accumulation within cloud

+
Return strike
along ionised
path, usually
occurring
2 or 3 times
+
+
+
Down
draughts of
cold air
_
Updraughts of
warm air
+ +
_ _ _
_ _
Negative charged
cloud base
Surface rain
Charge accumulation within cloud
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13. 8/20µs short circuit current pulse

Incoming Surge
SPD
DC Power
Common earthing system
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14. Common earthing system

Incoming Surge
SPD
DC Power
To
distribution
earth
Preferred earthing system
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15. Preferred earthing system

SPD
SPD
IS Barrier
TX
Hazardous Area
Safe Area
Installation of SPDs’ with IS Barriers
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16. Installation of SPDs’ with IS Barriers

Safe Area
Hazardous Area
SPD
SPD
IS Barrier
TX
Preferred solution SPDs’ with IS Barriers
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17. Preferred solution SPDs’ with IS Barriers

SPD
SPD
Galvanic Isolator
TX
Hazardous Area Safe Area
SPDs’ with Galvanic Isolation
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18. SPDs’ with Galvanic Isolation

Static electricity
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19. Static electricity

Static generated by charge separation
occurring as a result of intermittent contact
between non-conducting
materials +
+
-
-
+ + +
-
-
+
+
-
-
Possible areas of risk:
Vehicle tanker loading and unloading
Drum filling
Flour mills & other organic powder production
Static Electricity
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20. Static Electricity

Static risks can be controlled by:
Effective bonding
Static risk avoidance
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21. Static risk avoidance

Gantry
FLAMMABLE
LIQUID
Bond
Antistatic Bond
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22. Antistatic Bond

Static risks can be controlled by:
Effective bonding
Conductive product additives
Reduced pumping rates
Locally induced ionisation
Use of 'static combs'
Maintaining high relative humidity
Static risk avoidance
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23. Static risk avoidance

Noise is any unwanted electrical signal overlaying or altering
desired signal .
Coupling can be capacitive, inductive or via the earthing system .
Twisted-pair cables are normally used in instrumentation
to reduce magnetic pick-up by reversing mutual
inductance at every twist .
Screens/shields play a large part in eliminating both inductive
and capacitive pick-up ;
1. screen surrounding cores constitutes a virtual "Faraday cage"
and reduces the magnetic field around the cores .
2. Screen/earth offers a much lower impedance than screen/core
capacitance thus diverting noise current .
Interference avoidance
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24. Interference avoidance

Parasitic
Capacitance
Armoured Cable
Motor
Distribution
Transformer
Secondary
“Dirty Earth” Currents
Source of typical plant structural currents
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25. Source of typical plant structural currents

Transformer
Motor
0 volts
Source of typical plant structural currents
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26. Source of typical plant structural currents

Parasitic
capacitance
At 50Hz
i = V.2 fC = 250µA
i
3000pF ?
DC Power
supply
circuitry
L
Mains
supply
i
Instrument
amplifier
N
E
i
Noise induced via supply transformer capacitance
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27. Noise induced via supply transformer capacitance

Parasitic
capacitance
At 50Hz
i = V.2 fC = 250µA
i
DC Power
supply
circuitry
3000pF ?
L
Mains
supply
i
Instrument
amplifier
N
E
Noise induced via supply transformer capacitance
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28. Noise induced via supply transformer capacitance

300pF
250V 50Hz
DC Power
supply
circuitry
L2
L1
Cable screen
i
300pF = 10M ohms @ 50Hz
i = 25µA
Field pickup reduction via screened cable
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29. Field pickup reduction via screened cable

Shunt diode safety barriers
and Isolators!
Earthing
Intrinsic Safety considerations
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30. Intrinsic Safety considerations

Field mounted
instrument
Instrument
system
L
X1
Isolated
internal
components
Plant bond
N
E
Neutral
Mains Earth
Instr. Panel Earth
Ex i Earth
X
Plant Bond
Safe area mains fault (to Common)
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31. Safe area mains fault (to Common)

Field mounted
instrument
Instrument
system
L
X1
Isolated
internal
components
Plant bond
N
E
Neutral
Mains Earth
Instr. Panel Earth
Ex i Earth
X
Plant Bond
Safe area mains fault (to Signal Line)
E

32. Safe area mains fault (to Signal Line)

Instrument
system
L
X1
N
E
Neutral
Mains Earth
Instr. Panel Earth
Ex i Earth
X11
X
Plant bond
Isolated
internal
components
Plant Bond
Field mounted
instrument
Hazardous Area mains fault
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33. Hazardous Area mains fault

O
Instrument
system
Barrier
Busbar
O
O
O
O
O
O
Field mounted
instrument
O
O
O
O
O
L
N
E
Junction
box
System
0V
Rail
Busbar
bond
Isolated
internal
components
Structure
bond
Plant bond
Typical Installation Practice
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34. Typical Installation Practice

L1
N
L2
L3
Protective earth
Field
instrument
MTL 728+
Shunt-diode safety barrier
Instrument
System
IS earth
Plant structural bond
Separate Earth Paths
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35. Separate Earth Paths

Safe Area
wiring
S
A
F
E
A
R
E
A
Resistance meter
checks complete
loop
W
I
R
I
N
G
H
A
Z
A
R
D
O
U
S
H
A
Z
A
R
D
O
U
S
A
R
E
A
A
R
E
A
W
I
R
I
N
G
W
I
R
I
N
G
Hazardous
Area wiring
S
A
F
E
A
R
E
A
W
I
R
I
N
G
Instrument
system
0v rail
Normal connection
when not making measurements
Installing the I S Earth
E

36. Installing the I S Earth

Field mounted
instrument
Instrument
system
L
N
E
X1
Isolated
internal
components
Neutral
Mains Earth
Instr. Panel Earth
X
Plant Bond
Plant bond
Ex i Earth
REARTH
?
Separate Earth Rod
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37. Separate Earth Rod

Barrier earthing practice varies, but the impedances through the structure
are small so local earthing to the structure can be acceptable
MTL 728+
Shunt-diode safety barrier
Instrument
System
Gen
Must be Greater
than 2 Metres
Barrier earthing on offshore structures
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38. Barrier earthing on offshore structures

The earthing system and its
connection to Control
Systems
Earthing
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39. Earthing

Neutral star
Reference
point
Computer 0 V
X
Y
Option A
Barrier busbar to computer link may carry field equipment return
current; hence must be of low impedance. Resistance XY of less
than 0.1 ohm is necessary to prevent adjacent circuit interaction.
Ground connections (A)
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40. Ground connections (A)

If the computer installation already has its 0V grounded, then this
solution becomes the preferred technique
X
Y
Neutral star
Reference
point
Computer 0V
Option B
Is equally acceptable as option A; the same requirements exist,
e.g. resistance XY of less than 0.1 ohm to prevent adjacent
circuit interaction. XY is now part of the protective network and
the operational currents can cause difficulties when monitoring
the impedance of this link. The barrier busbar will be elevated
above zero by a few millivolts due to the standing current, but
this is rarely a problem.
Ground connections (B)
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41. Ground connections (B)

Neutral star
Reference
point
Y
Computer 0V
X
Instrument 0V Busbar
Option C
This is the practical reality of many control room installations,
it is somewhere between Option A and Option B
Ground connections (C)
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42. Ground connections (C)

Neutral star
Reference
point
Y
X
Computer 0V
Option D
Possibly acceptable but not preferred method since impedance
of XY bond is now probably higher and is not well defined.
Ground connections (D)
E

43. Ground connections (D)

Neutral star
Reference
point
X
Y
Computer 0V
Option E
This is a common development as a result of Option D not working
very well and being cross-bonded to make it work. It is acceptable
from a safety point of viewpoint but is an ill-defined situation, leading
to potential earth loop problems and under some transient fault
conditions produces some complex system malfunctions. Many such
installations work perfectly adequately but there are possible causes
for concern.
Ground connections (E)
E

44. Ground connections (E)

• Options A,B, and C are all acceptable from both an operational and
safety viewpoint
• The order of preference would be:
• A, then C, then B
– but there is no significant difference between the three systems
• Option D and E are acceptable from a safety viewpoint
• The system D should always be converted to E for operational
reasons, but neither circuit is preferable. Conversion to A or B by the
removal of one ground connection is the preferred solution
Note;
The bonding conductors are shown as single conductors. Consideration
should be given to duplicating them for testing purposes and to increase
reliability.
The preferred solution:
E