Transcript PPT

GROUNDING SYSTEM DESIGN
FOR ISOLATED LOCATIONS
AND PLANT SYSTEMS
Marcus O. Durham
Robert A. Durham
THEWAY Corp / U of Tulsa
Central & Southwest Srvcs, WTU
GROUNDING
Introduction
 Common
feature
 Technology well defined
not always effective
application is art
PRESENT TECHNOLOGY
Introduction
 NEC
 Green
 Emerald
 Lightning
 Previous
35 pp requirements -- not design
RP industrial -- not electronics
RP power quality sensitive equip
shield & shunt discharges - not devices
requirements & design procedures
PRESENT TECHNOLOGY
Introduction
 NEC
 Green
 Emerald
 Lightning
 Previous
35 pp requirements -- not design
RP industrial -- not electronics
RP power quality sensitive equip
shield & shunt discharges - not devices
requirements & design procedures
PROJECT
10 Case Studies of Ground Problems
 Situation
present problem
 Environment influences effectiveness of system
 Analysis
investigates alternatives & unique
 Summary
brief response to situation
little things mean a lot -- pay attention to detail
REMOTE SENSOR ERRATIC
Case 1
Situation very low readings prior to lightning
Environment
petrochemical plant, Alabama Gulf Coast
pipeline station 1/2 mile
soil = sand , > 50,000 ohm-cm
isoceraunic 80 TSDY , intense
combination = one of most difficult
REMOTE SENSOR ERRATIC
Case 1
Situation very low readings prior to lightning
Environment
petrochemical plant, Alabama Gulf Coast
pipeline station 1/2 mile
soil = sand , > 50,000 ohm-cm
isoceraunic 80 TSDY , intense
combination = one of most difficult
REMOTE SENSOR ERRATIC
Cause
V
diff common (-) to remote gnd
signal current fluctuation
 Cloud crossing creates potential
varies w / location under storm
 Strike = saturate ground system
elevated potential to outside
REMOTE SENSOR ERRATIC
Effect
 Inside
= equipotential
 Egress = difference
 Trigger protectors short time
 Dump excessive transient I on lines
 Time - delay on response
 Isolate remote ground , metallic bonding won’t
REMOTE SENSOR ERRATIC
No Such Thing As Common Ground
AWG 2/0 bond
Lo R = 0.0003 / ft
L = 0.5 uH / ft
XL = 3 ohm / ft
Hi Z to lighting
f = 1 Mhz
(4 order, 10000X)
I surge > 3000 A
V drop = 9000 v / ft
EE
UNUSED POINTS INDUCE ERRORS
Case 2
Situation unused analog damaged during storm
Environment
pipeline station S . Texas cap rock
soil = rock 15 , 000 to 100 , 000 ohm-cm
isoceraunic 38 TSDY
UNUSED POINTS INDUCE ERRORS
Cause
 Extra
input cards
 Multi - conductor cable
 Surges induced on every wire
 Analog in more susceptible to spurious
 Digital less prone but surges coupled
 Never let float
UNUSED POINTS INDUCE ERRORS
Connections
Load on unused
Join intermediate terminals
Open field ends
Care to isolate shield from other ground
UNUSED POINTS INDUCE ERRORS
Termination
Digital
In
+
_
Digital
Out
+
_
LOAD
Analog
In
+
_
Analog
Out
+
_
250 W
GND
FAILURE DURING STORM
Case 3
Situation sensors commonly failed w/o direct hit
Environment
power generation, SW Oklahoma
pipeline gas yard 200 yds
soil = river bottom, 3000 ohm-cm, excellent
isoceraunic 55 TSDY
FAILURE DURING STORM
What Is It?
 Remote
xmitters in yard
 Existing unshielded cable - entry lightning energy
 Effective grounds in plant & yard
 No strike
lose DCS input
 Close strike to towers, tanks
severe differential voltage
lose transmitter also
FAILURE DURING STORM
Fix
 Isolate
unshielded cable
 Common mode
DCS
 Differential mode
xmitter
 In - line
both
 MOV 36 V, 160 J
 Woops, commercials ground protector
MOV
DATA FAILURE BETWEEN BLDGS
Case 4
Situation perimeter comm lines , terminals 5 X
Environment
petrochemical plant, Alabama Gulf Coast
pipeline station 1/2 mile
soil = sand , > 50,000 ohm-cm
isoceraunic 80 TSDY , intense
combination = one of most difficult
DATA FAILURE BETWEEN BLDGS
Tap Protection
 Different
ground potential
 Isolate signal & protection
 Prefer fiber
 Twisted - pair, shield 1 end
 2 Com, 1 Dif MOV
DATA FAILURE BETWEEN BLDGS
AC & Remote Protection
 Gas
tube
 MOV or Transzorb
 Inductor
comm type
frequency / baud
length
V & I
PROTECTION
Local , AC & Remote
xmitter
+
_
+
_
s
xmitter
+
_
s
Isolated Signal
Ground
+
_
PROTECTION BLOWS FUSES
Case 5
Situation protectors operate , fuses blow
Environment
petrochemical plant, Alabama Gulf Coast
pipeline station 1/2 mile
soil = sand , > 50,000 ohm-cm
isoceraunic 80 TSDY , intense
combination = one of most difficult
PROTECTION BLOWS FUSES
Follow Through Current
 Gas
tubes
 Zener
 MOV
 Shorts
long
short
15 sec
transmitter
 90 fuses per storm
PROTECTION BLOWS FUSES
Non - Incendive Resistor
 Small
 Large
 Power
4
minimize loop compliance
limit I
continuous
- 20 ma, 24 VDC ~ 600 ohm
 In positive lead
PROTECTION BLOWS FUSES
Resistance
+24V fuse
limiting
indicator
protector
wire
X
P
-24V
(gnd)
wire
xmitter
Total = Sensor + Wire + Add
= 250 + 17 /kft + 330 > 580
PROTECTION BLOWS FUSES
Loop Compliance
Resistor = 24V / 50 ma > 480 ohm
I loop = ( 600 / Total R) * 20 ma < 20
P = (0.020) 2 * 330 = 0.13 W
PROTECTION BLOWS FUSES
Semiconductor Failure
 Generally
shorts
 Positive lead
 Negative lead
blow fuse
ground loop
I
S
D
I
S
CURRENT IN ELECTRODES
Case 6
Situation I = 14 A in both ground beds
Environment
petrochemical plant, Alabama Gulf Coast
pipeline station 1/2 mile
soil = sand , > 50,000 ohm-cm
isoceraunic 80 TSDY , intense
combination = one of most
CURRENT IN ELECTRODES
Non - Isolation
 Marshalling
panel
 Chassis
 Each
 1400
digital inputs
pos, neg, gnd
short to shield
small circulating I
large I
CURRENT IN ELECTRODES
Shields
 Shorted
together
 Do not crop to jacket
 Insulate individual ends
 Do not touch metal
 Terminate on shield strip
 Single point ground
CURRENT IN ELECTRODES
Single Point
Supply
PLC
24+
24-
24+
24s
+
_
24+
24s
+
_
120V
Load
Source
H
N
G
Xducer
H
N
G
FAN MOTOR ON TOP OF BOILER
Case 7
Situation Fails when lightning hits surrounding
Environment
power generation, W Texas
soil = shallow top soil on rock, ~ 100,000 ohm-cm
isoceraunic 43TSDY
FAN MOTOR ON TOP OF BOILER
Equipment
 4160
V motor
 70 ft above earth
 5 kv unshielded triplex
 In tray & conduit
 Acts as antenna
 2 grounding conductors
FAN MOTOR ON TOP OF BOILER
Gnd Conductor Z ( AWG 4 )
L = 0.279 uHy / ft
L = 56 uHy / 100 ft
L = 0.5 uHy / ft typical
XL = 220 ohm @ 1 MHz
XL >> R
FAN MOTOR ON TOP OF BOILER
Fix
 Lightning
arrestors on terminals
 MOV
 Bond
all metal
 Shield cable
STRUCTURE ATTRACTS LIGHTNING
Case 8
Situation Damages xmitters & boards
Frequent hits
Environment
research facility NE Oklahoma
soil = loam, 6500 ohm-cm
isoceraunic 55 TSDY
STRUCTURE ATTRACTS LIGHTNING
Dissipate Energy
 Not
tallest structure
 More conductive than others
 Bond around base pins
 Bond framework
 1 / 0 AWG
 Non - corrosive terminals
 Gound guy wires
STRUCTURE ATTRACTS LIGHTNING
Ground Network
 Rat
- race ring
 If > 50 ft, criss - cross
 Bond to existing grid
 Radials 1 / 0 AWG
 5 ft rods, 20 ft spacing
HIGH RESISTIVITY SOIL
Case 9
Situation Single motor from distribution system
Environment
pump SE New Mexico
soil = 6500 ohm-cm on rock at 4 ft
isoceraunic 47 TSDY
HIGH RESISTIVITY SOIL
5 / 8 “ x 8 ‘ Rod
 Dwight’s
formula
R = [ p / 191.5 b] [ ln ( 48 b / a ) - 1 ]
R = 177 ohm , 1 rod
R = 76 ohm, 3 rods
> 25 ohm
>> 5 ohm
HIGH RESISTIVITY SOIL
Concrete
 Retain
moisture
 Alkalinity
free ions
 Resistivity
3000 ohm - cm
 1 rod
10.6 ohm
 3 rod
4.6 ohm
ACME
CONCRETE
SHOCK ON GROUNDED PANEL
Case 10
Situation Grounded panel permitted shock
Environment
pumping panel, NE Oklahoma
soil = loam, 6500 ohm-cm
isoceraunic 55 TSDY
SHOCK ON GROUNDED PANEL
System
 Power
panel outdoors
 Overhead distribution, 277 / 480 Y gnd
 5 / 8 “ x 8 ‘ rod
 Occasional shocks
 Inspected returned to service
SHOCK ON GROUNDED PANEL
Problem
 Secondary
pinch ~ 50 ohm
 Circulating I = 1 amp
 Two ground paths
 Gnd rod ~ 25 ohm
 Body ~ 40,000 to 1,000 ohm
SHOCK ON GROUNDED PANEL
277/480 v
120 v
SHOCK ON GROUNDED PANEL
Body Current
0.6 ma
24.4 ma
341 ma
40 K
1K
14 A
Install lo Z ground
SHOCK ON GROUNDED PANEL
GFCI
 Personnel
6 ma
 Equipment
30 ma
 Not required for this
CURRENT IMPACT
MA
1
8
15
20
50
100
Physiology
sensation
painful shock
can’t let go
possible damage
ventricular fibrillation
probable death
REVIEW
Grounding systems
Various environments
Diverse problems
Common solutions
Little things mean a lot
SUMMARY
1.
2.
3.
4.
Environment - thunderstorms vs hits
Earth resistivity - multiple in concrete
Equipotential network
Single point connection
power neutral
signal common
shield
SUMMARY
5.
6.
7.
8.
Bond metal
Protection devices
Bond to chassis vs isolate remote
Terminate unused
short in
load out
bond wires