Maintenance Technology September 2017 - 24
PLANT SAFETY MATTERS
tion and conditions:
■ Composition: Clay, sand, or rocks
in the soil increase resistance. Moist,
loamy soil decreases it. For example,
soil containing a high percentage of
clay can be an insulator. Sandy soil is
an insulator when dry, and loamy soil
can be a slightly better insulator.
■ Condition: Wet and dry cycles can
cause air gaps to develop in the soil,
compromising contact with buried
electrodes. Seismic activity, watertable cycles, or equipment vibration
can cause conductor breaks or
sand migration through soil,
resulting in collection around
buried objects and changed
contact resistance.
Maintenance 'musts'
As Hostettler and Thomas pointed
out, in light of rapidly aging infrastructure, industrial operations and
other entities, i.e., municipalities,
need to put increased emphasis
on the proper design and management of grounding systems,
including adequate testing,
analysis, and maintenance. Specifically, they must:
■ perform baseline earthing
tests, calculate the total earth
resistance for each system, and
analyze the results determining
risk exposure
■ deal with identified risks
by retesting, establishing
a corrected baseline, and
developing a compliance
program that incorporates testing
and mitigation of future risks.
Those two tasks can be broken into
several activities:
Testing the grounding hub. Most
industries test equipment grounds
annually. Each grounding hub must
be tested to verify an adequately low
resistance ground and low differentials.
The top of the ground post is the hub,
which may also be a ground plate/strip
in a building. These will be connected to
a buried rod, plate, loop, or grid made
of steel. At specific places, a connection point is brought to the surface
for equipment ground attachment. If
individual ground rods are used, they
24 |
MAINTENANCE TECHNOLOGY
must be made of approved material
and sufficiently long to meet regulations. They must be linked into a loop
by connecting cables or tested more
frequently to verify a sufficiently low
resistance differential.
Any resistance differential between
adjacent grounding points must be
investigated and corrected because
it sets up a fault-induced difference
between adjacent assets on the surface.
That situation can cause fatalities.
In-ground components, cables, and
Effective Ground-Fault
Current Paths
An effective a ground-fault current
path, i.e., a low-resistance route
to clear a ground fault, must:
■ provide a path for fault current
that is lower in resistance than people, to
save lives
■ reduce voltage stress on electrical
insulation, providing longer insulation
life for components such as motors and
transformers
■ help reduce fire potential
■ stabilize voltage and help reduce damage
from lightning and fault currents
■ reduce threats to solid state/digital systems
(which are extremely sensitive to voltage
transients, variations, and electrical noise)
to improve their service life and reliability.
connections can suffer corrosion
and physical damage, negating any
protection.
Measuring conditions. Ground
resistivity can be measured with testers
made specifically for this purpose.
Properly using the testing equipment
requires specific knowledge and
training.
For in-house testing, computer
programs can record, track, and analyze
system conditions. For testing not done
in-house, contracted electrical vendors
can perform tests, analyze results, and
advise on required risk mitigation.
Relevant analyses and reviews should
be conducted by qualified electrical
engineers. All grounding systems
should be tested at installation for resistance and the differential between such
systems and adjacent systems. Results
must fall within National Electric Code
(NEC) parameters.
Note that the NEC limits are for
total resistance to a proven ground.
Since segments of the grounding
circuits are tested separately, totals are
calculated for each path and must meet
the minimum. Calculations can be
complicated, so electrical engineering
personnel must be involved in
analysis.
Differences exist in ground
conductor types, e.g., copper
bar, stranded copper, copper
clad, and braids. Qualified
engineers must determine
the appropriate application.
Keep in mind that changes
to electrical loads could require
a grounding-system upgrade.
Conductor size, connection type,
length, and rod dimensions should
be recorded for every system part.
Good records can reduce the need
to re-check size for each implemented change.
Most companies have an
equipment-grounds testing
program. After performing initial
baseline measurement, testing
frequency should consider soil pH,
soil makeup, wet/dry cycling, and
historical corrosion rates.
A 5- to 10-year comprehensive
test schedule might be reasonable for
in-ground components. If individual
ground rods are used near equipment
and not connected to a loop, more
frequent testing should be conducted
to verify that dangerous differentials
have not developed between adjacent
equipment. Corrosion monitoring
might be needed.
Tests should be performed after any
extreme ground-fault occurrence-a
lightning strike, fire, or explosion. If
the protection did its job, verify that
protection is undamaged.
Proving compliance. While an
OSHA inspector might ask to see
inspection records for equipment
SEPTEMBER 2017
Table of Contents for the Digital Edition of Maintenance Technology September 2017
Editorial
Uptime
On The Floor
Bionics Will Drive Future Factories
Filling The Skills Gap
Virtual Simulation and Plant Safety
Update Grounding Knowledge
Don't Overlook STF Hazards
Alternative LOTO Measures
SAP: Tips and Tricks
Compressed Air Filters
IR Inspections
Crack The Lube-Fault Code
DCIM Drives Efficiencies
Products
Ad Index
Showcase
Final Thought
Maintenance Technology September 2017 - Ad
Maintenance Technology September 2017 - Cover1
Maintenance Technology September 2017 - Cover2
Maintenance Technology September 2017 - 1
Maintenance Technology September 2017 - 2
Maintenance Technology September 2017 - 3
Maintenance Technology September 2017 - Editorial
Maintenance Technology September 2017 - 5
Maintenance Technology September 2017 - Uptime
Maintenance Technology September 2017 - 7
Maintenance Technology September 2017 - On The Floor
Maintenance Technology September 2017 - 9
Maintenance Technology September 2017 - Bionics Will Drive Future Factories
Maintenance Technology September 2017 - 11
Maintenance Technology September 2017 - 12
Maintenance Technology September 2017 - 13
Maintenance Technology September 2017 - 14
Maintenance Technology September 2017 - 15
Maintenance Technology September 2017 - Filling The Skills Gap
Maintenance Technology September 2017 - 17
Maintenance Technology September 2017 - 18
Maintenance Technology September 2017 - 19
Maintenance Technology September 2017 - Virtual Simulation and Plant Safety
Maintenance Technology September 2017 - 21
Maintenance Technology September 2017 - 22
Maintenance Technology September 2017 - Update Grounding Knowledge
Maintenance Technology September 2017 - 24
Maintenance Technology September 2017 - 25
Maintenance Technology September 2017 - Don't Overlook STF Hazards
Maintenance Technology September 2017 - 27
Maintenance Technology September 2017 - 28
Maintenance Technology September 2017 - Alternative LOTO Measures
Maintenance Technology September 2017 - 30
Maintenance Technology September 2017 - 31
Maintenance Technology September 2017 - 32
Maintenance Technology September 2017 - SAP: Tips and Tricks
Maintenance Technology September 2017 - Compressed Air Filters
Maintenance Technology September 2017 - 35
Maintenance Technology September 2017 - IR Inspections
Maintenance Technology September 2017 - 37
Maintenance Technology September 2017 - Crack The Lube-Fault Code
Maintenance Technology September 2017 - 39
Maintenance Technology September 2017 - 40
Maintenance Technology September 2017 - DCIM Drives Efficiencies
Maintenance Technology September 2017 - 42
Maintenance Technology September 2017 - Products
Maintenance Technology September 2017 - 44
Maintenance Technology September 2017 - 45
Maintenance Technology September 2017 - Ad Index
Maintenance Technology September 2017 - Showcase
Maintenance Technology September 2017 - Final Thought
Maintenance Technology September 2017 - Cover3
Maintenance Technology September 2017 - Cover4
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