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