ircon Modline 4, Installation And Operation Manual

Page: 82 / 88

Appendix
A–3
3.3.2 Polarity and Phase Sequence
Input power and control signals should be applied
with polarity and phase sequence as specified by
the manufacturer. Solid state devices can be dam-
aged by the application of reverse polarity or incor-
rect phase sequence.
3.4 PLANNING ELECTRICAL NOISE -
REJECTION
The low energy levels of solid state controls may
cause them to be vulnerable to electrical noise.
This should be considered in the planning stages.
3.4.1 Assessing Electrical Environment
Sources of noise are those pieces of equipment
that have large, fast changing voltages or currents
when they are energized or de-energized, such as
motor starters, welding equipment, SCR type,
adjustable speed devices, and other inductive
devices. These devices, as well as the more com-
mon control relays and their associated wiring, all
have the capability of inducing serious current and
voltage transients on their respective power lines. It
is these transients which nearby solid state controls
must withstand and for which noise immunity
should be provided.
An examination of the proposed installation site of
the solid state control should identify equipment
that could contaminate power lines. All power lines
that will be tapped by the proposed solid state con-
trol should be examined for the presence, severity,
and frequency of noise occurrences. If found, sys-
tem plans should provide for the control of such
noise.
3.4.2 Selecting Devices to Provide Noise
Immunity
Installation planning is not complete without exami-
nation of the noise immunity characteristics of the
system devices under consideration. Results of
tests to determine relative immunity to electrical
noise may be required from the manufacturer. Two
such standardized tests are the ANSI (C37.90a-
1974) Surge Withstand Capability Test and the
NEMA (ICS 1-1983) noise test referred to as The
Showering Arc Test. These are applied where
direct connection of solid state control to other elec-
tromechanical control circuits is intended. Circuits
involving analog regulating systems or high speed
logic are generally more sensitive to electrical
noise; therefore, isolation and separation of these
circuits is more critical.
Further information on electrical noise and evalua-
tion of the severity of noise may be found in
ANSI/IEEE Publication No. 518-1982.
Where severe power line transients are anticipated
or noted, appropriate filters such as commercially
available line filter, isolation transformers, or volt-
age limiting varistors, should be considered.
All inductive components associated with the sys-
tem should be examined for the need for noise sup-
pression.
3.4.3 Design of Wiring for Maximum Protection
Once the installation site and power conductors
have been examined, the system wiring plans that
will provide noise suppression should be consid-
ered.
Conducted noise enters solid state control at the
points where the control is connected to input lines,
output lines, and power supply wires.
Input circuits are the circuits most vulnerable to
noise. Noise may be introduced capacitatively
through wire to wire proximity or magnetically from
nearby lines carrying large currents. In most instal-
lations, signal lines and power lines should be sep-
arate. Further, signal lines should be appropriately
routed and shielded according to the manufactur-
er’s recommendations.
When planning system layout, care must be given
to appropriate grounding practice. Because design
differences may call for different grounding, the
control manufacturer’s recommendations should be
followed.
3.5 COUNTERING THE EFFECTS OF OFF-
STATE CURRENT
3.5.1 Off-State Current
Solid state components, such as triacs, transistors,
and thyristors, inherently have in the off-state a
small current flow called “off-state current”.
Off-state current may also be contributed by
devices used to protect these components, such
as RC snubbers.
3.5.2 Off-State Current Precautions
Off-state currents in a device in the off-state may
present a hazard of electrical shock and the device
should be disconnected from the power source
before working on the circuit or load.