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Chapter 1
PLC Electrical Safety
Programmable Logic Controllers • PLC Safety •
Electrical Properties • Grounding • Force and
Disable Safety Considerations • Electrical Noise
Suppression • Static Electric Charges • Electrical
Safety • Personal Protective Equipment •
Lockout/Tagout • Inspecting a PLC System •
Chapter 1 — PLC Electrical Safety
PLC system
programming is viewed
by various devices that
connect to micro-sized
PLCs (8 to 10 inputs and
outputs) or to large-sized
PLCs that control
thousands of inputs and
outputs.
Chapter 1 — PLC Electrical Safety
Improperly installed and/or maintained PLCs can
overheat, leading to fire or explosion.
Chapter 1 — PLC Electrical Safety
An advantage of a PLC
controlling a process is
that a PLC can be
programmed and
reprogrammed as
process conditions
change.
Chapter 1 — PLC Electrical Safety
An electric shock results anytime a body becomes
part of an electrical circuit.
Chapter 1 — PLC Electrical Safety
Possible effects of
electrical shock
include the heart
and lungs ceasing
to function, and/or
severe burns
where the
electricity (current)
enters and exits
the body.
Chapter 1 — PLC Electrical Safety
Current is the amount of
electrons flowing through
an electrical circuit and is
measured in amperes.
Chapter 1 — PLC Electrical Safety
Voltage is the amount of
electromotive force in a
circuit and is measured in
volts.
Chapter 1 — PLC Electrical Safety
Resistance is the
opposition to the flow of
electrons and is
measured in ohms.
Chapter 1 — PLC Electrical Safety
Grounding provides a direct path for unwanted fault
current to travel to earth without causing harm to
technicians or equipment.
Chapter 1 — PLC Electrical Safety
Building grounding,
equipment
grounding, and
electronic
equipment
grounding are used
to create a safe
working
environment for
technicians.
Chapter 1 — PLC Electrical Safety
Building grounding ensures that there is a low
impedance (low resistance) grounding path for fault
current (electrical short or lightning) to earth ground.
Chapter 1 — PLC Electrical Safety
Equipment grounding prevents
electrical shock when a person
comes in contact with electrical
equipment or exposed metal of
machinery.
Chapter 1 — PLC Electrical Safety
Electronic equipment
grounding is used to
provide a quality
ground for electronic
systems to enable
better communication
(less noise) with PLCs,
process control
equipment, and other
facility operations.
Chapter 1 — PLC Electrical Safety
Ground resistance
measurements are
taken on grounding
conductors used with
service entrances,
transformers, utility
transmission, and
communication
(control circuit)
grounds.
Chapter 1 — PLC Electrical Safety
PLC force and
disable commands
are used during
system start-up and
for troubleshooting.
Chapter 1 — PLC Electrical Safety
Electrical noise enters a
PLC system through
input devices, output
components, and power
supply lines.
Chapter 1 — PLC Electrical Safety
To prevent false
signals from
entering a PLC,
input and output
lines must cross at
right angles (90°)
and not run parallel
to each other.
Chapter 1 — PLC Electrical Safety
A shielded cable uses an outer
conductive jacket (shield) to
block electromagnetic
interference from the inner,
signal-carrying conductors.
Chapter 1 — PLC Electrical Safety
Snubber circuits
are used to
suppress voltage
spikes in PLCs.
Chapter 1 — PLC Electrical Safety
Depending on the PLC
application, an
enclosure with a cooling
unit can be required.
Chapter 1 — PLC Electrical Safety
Article 500 of the NEC®
classifies hazardous
locations according to the
properties and quantities
of the hazardous material
that may be present.
Chapter 1 — PLC Electrical Safety
Electrical safety
rules aid in the
prevention of
injuries from
electrical energy
sources.
Chapter 1 — PLC Electrical Safety
PLC safety begins
with a sufficient
number of
emergency stops and
a master control relay
that removes power
to the inputs and
outputs of the PLC
and stops all motion
of the machine(s) or
process.
Chapter 1 — PLC Electrical Safety
Personal protective
equipment includes items
that protect a technician
from electrical and other
hazards.
Chapter 1 — PLC Electrical Safety
Arc-flash protective
clothing made of
Nomex®, Basofil®,
and/or Kevlar® fibers
must be used when
working with live highvoltage electrical
circuits.
Chapter 1 — PLC Electrical Safety
The National Fire Protection
Association (NFPA) specifies
boundary distances that vary
depending on voltage.
Chapter 1 — PLC Electrical Safety
Protective helmets are
identified by class of
protection. For example,
Class E protective
helmets protect against
high-voltage shock and
burns.
Chapter 1 — PLC Electrical Safety
Eye protection must be worn to
prevent eye or face injuries caused
by contact arcing, radiant energy,
or flying particles.
Chapter 1 — PLC Electrical Safety
Ear protection is worn
to prevent technician
hearing loss caused
by electrical systems,
machinery, power
tools, and HVAC
equipment.
Chapter 1 — PLC Electrical Safety
Rubber insulating
gloves have colorcoded labels that
represent voltage
ratings for specific
applications.
Chapter 1 — PLC Electrical Safety
Rubber insulating gloves must be air tested before
each use and when there is cause to suspect
damage.
Chapter 1 — PLC Electrical Safety
Insulated rubbersoled shoes are
typically worn during
electrical work to aid
in the prevention of
electrical shock.
Chapter 1 — PLC Electrical Safety
Lifting an object with the legs
reduces the possibility of a back
injury.
Chapter 1 — PLC Electrical Safety
When carried on the
shoulder by one
person, objects such
as conduit must be
transported with the
front end down.
Chapter 1 — PLC Electrical Safety
Rubber insulating matting aids in protecting a
technician from electrical shock when working on
energized electrical circuits.
Chapter 1 — PLC Electrical Safety
Lockout/tagout kits
contain reusable
danger tags, tag
ties, multiple lockout
hasps, magnetic
signs, and
information on
lockout/tagout
procedures.
Chapter 1 — PLC Electrical Safety
Lockout devices resist
chemicals, cracking,
abrasion, and
temperature changes
and are available in
colors to match
American National
Standards Institute
(ANSI) pipe colors.
Lockout devices are
sized to fit standard
industry equipment.
Chapter 1 — PLC Electrical Safety
A proper
inspection of a
PLC ensures
safe control of
an electrical
system.
Chapter 1 — PLC Electrical Safety
Proper grounding is
important in PLC
applications. Improper
grounding can lead to
interference (noise)
being induced into
PLCs, which can
cause output devices
to be falsely turned
ON and put personnel
and equipment at risk.