1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA , and in the event of failure in the machine, process, or the monitoring system, the monitoring circuits should initiate a safe shut -down sequence.3.1.4 Overcurrent ProtectionTo protect triacs and transistors from shorted l
2、oads, a closely matched short circuit protective device (SCPO) is often incorporated. These SCPDs should be replaced only with devices recommended by the manufacturer.3.1.5 Overvoltage ProtectionTo protect triacs, SCRs, and transistors from over voltages, it may be advisable to consider incorporatin
3、g peak voltage clamping devices such as varistors, zener diodes, or snubber networks in circuits incorporating these devices.3.2 CIRCUIT ISOLATION REQUIREMENTS3.2.1 Separating VoltagesSolid state logic uses low-level voltage (e.g., less than 32 volts dc) circuits. In contrast, the inputs and outputs
4、 are often high-level voltages (e.g., 120 volts ac). Proper design of the interface protects against an unwanted in teraction between the low-level and high-level circuits; such an interaction can result in a failure of the low voltage circuitry. This is potentially dangerous. An input and out put c
5、ircuitry incorporating effective isolation techniques (which may include limiting impedance or Class 2 sup plied circuitry) should be selected. 1999 National Electrical Manufacturers AssociationICS 1.1-1984 (R1988, R1993, R1998, R2003, R2009, R2015)Page 43.2.2 Isolation TechniquesThe most important
6、function of isolation components is to separate high-level circuits from low-level circuits in order to protect against the transfer of a fault from one level to the other.Isolation transformers, pulse transformers, reed relays, or optical couplers are typical means to transmit low-level logic signa
7、ls to power devices in the high-level circuit. Isolation impedance means also are used to transmit logic signals to power devices.3.3 SPECIAL APPLICATION CONSIDERATIONS3.3.1 Converting Ladder DiagramsConverting a ladder diagram originally designed for electromechanical systems to one using solid sta
8、te control must account for the differences between elec tromechanical and solid state devices. Simply replacing each contact in the ladder diagram with a corresponding solid state “contact” will not always produce the desired logic functions or fault detection and response. For ex ample, in electro
9、mechanical systems, a relay having a mechanically linked normally open (NO) and normally closed (NC) contact can be wired to check itself. Solid state components do not have a mutually exclusive NO-NC arrangement. However, external circuitry can be employed to sample the input and “contact” state an
10、d compare to determine if the system is functioning properly.3.3.2 Polarity and Phase SequenceInput power and control signals should be applied with polarity and phase sequence as specified by the manufac turer. Solid state devices can be damaged by the applica tion of reverse polarity or incorrect
11、phase sequence.3.4 PLANNING ELECTRICAL NOISE REJECTIONThe 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 EnvironmentSources of noise are those pieces of equipment that have lar
12、ge, 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
13、 inducing serious current and voltage tran sients 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 equip
14、ment that could contaminate power lines. All power lines that will be tap ped by the proposed solid state control should be examined for the presence, severity, and frequency of noise occur rences. If found, system plans should provide for the con trol of such noise.3.4.2 Selecting Devices to Provid
15、e Noise ImmunityInstallation planning is not complete without examina tion of the noise immunity characteristics of the system devices under consideration. Results of tests to determine relative immunity to electrical noise may be requested from the manufacturer. Two such standardized tests are the
16、ANSI C37.90.1-2012 Surge Withstand Capability Test and the NEMA ICS 1-2000 (R2005, R2008, R2013) noise test referred to as The Showering Arc Test. These are applied where direct connection of solid state control to other electromechanical control circuits is intended. Circuits involving analog regul
17、ating systems or high speed logic are generally more sensitive to electrical noise; therefore, isolation and separation of these circuits is more critical.Where severe power line transients are anticipated or noted, appropriate filters such as commercially available line filter, isolation transforme
18、rs, or voltage limiting varistors, should be considered. 1999 National Electrical Manufacturers AssociationICS 1.1-1984 (R1988, R1993, R1998, R2003, R2009, R2015)Page 5All inductive components associated with the system should be examined for the need for noise suppression.3.4.3 Design of Wiring for
19、 Maximum ProtectionOnce the installation site and power conductors have been examined, the system wiring plans that will provide noise suppression should be considered.Conducted noise enters solid state control at the points where the control is connected to input lines, output lines, and power supp
20、ly 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 car rying large currents. In most installations, signal lines and power lines should be separate. Further, signal lines should b
21、e appropriately routed and shielded according to the manufacturers recommendations.When planning system layout, care must be given to appropriate grounding practice. Because design dif ferences may call for different grounding, the control manufacturers recommendations should be followed.3.5 COUNTER
22、ING THE EFFECTS OF OFF-STATE CURRENT3.5.1 Off-State CurrentSolid 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
23、 as RC snubbers.3.5.2 Off-State Current PrecautionsOff-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.Precautions should be taken to prevent the off-state cur rent
24、of an output device which is in the off-state from energizing an input device.3.6 AVOIDING ADVERSE ENVIRONMENTAL CONDITIONS3.6.1 TemperatureSolid state devices should only be operated within the temperature ranges specified by the manufacturer. Because such devices generate heat, care should be take
25、n to see that the ambient temperature at the device does not exceed the temperature range specified by the manufacturer.The main source of heat in a solid state system is the energy dissipated in the power devices. Since the life of the equipment can be increased by reducing operating temperature, i
26、t is important to observe the manufacturers “maximum/minimum ambient temperature” guidelines, where ambient refers to the temperature of the air pro viding the cooling. The solid state equipment must be allowed to stabilize to within the manufacturers recommended operating temperature range before e
27、nergizing control functions.When evaluating a system design, other sources of heat in the enclosure which might raise the ambient temperature should not be overlooked. For example, power supplies, transformers, radiated heat, sunlight, fur naces, and incandescent lamps should be evaluated.In instanc
28、es where a system will have to exist in a very hot ambient environment, special cooling methods may have to be employed. Techniques that are employed in clude cooling fans (with adequate filtering), vortex coolers, heat exchanges, and air conditioned rooms. 1999 National Electrical Manufacturers Ass
29、ociationICS 1.1-1984 (R1988, R1993, R1998, R2003, R2009, R2015)Page 6Over-temperature sensors are recommended for systems where special cooling is employed. Use of air condition ing should include means for prevention of condensing moisture.3.6.2 ContaminantsMoisture, corrosive gases and liquids, an
30、d conduc tive dust can all have adverse effects on a system that is not adequately protected against atmospheric contaminants.If these contaminants are allowed to collect on printed circuit boards, bridging between the conductors may result in malfunction of the circuit. This could lead to noisy, er
31、ratic control operation or, at worst, a permanent malfunction. A thick coating of dust could also prevent adequate cooling on the board or heat sink, causing malfunction. A dust coating on heat sinks reduces their thermal efficiency.Preventive measures include a specially conditioned room or a prope
32、rly specified enclosure for the system.3.6.3 Shock and VibrationExcessive shock or vibration may cause damage to solid state equipment. Special mounting provisions may be re quired to minimize damage.3.7 THE NEED FOR EDUCATION-KNOWLEDGE LEADS TO SAFETYPlanning for an effective solid state circuit re
33、quires enough knowledge to make basic decisions that will render the system safe as well as effective.Everyone who works with a solid state control should be educated in its capabilities and limitations. This in cludes in-plant installers, operators, service personnel, and system designers. 1999 Nat
34、ional Electrical Manufacturers AssociationICS 1.1-1984 (R1988, R1993, R1998, R2003, R2009, R2015)Page 7Section 4 INSTALLATION GUIDELINES4.1 INSTALLATION AND WIRING PRACTICE4.1.1 Proper installation and field wiring practices are of prime importance to the application of solid state controls. Proper
35、wiring practice will minimize the influence of electrical noise, which may cause malfunction of equipment.Users and installers should familiarize themselves with and follow installation and wiring instructions in addition to requirements of all applicable codes, laws, and stan dards. The manufacture
36、r of the device or component in question should be consulted whenever conditions arise that are not covered by the manufacturers instructions.4.1.2 Electrical noise is a very important consideration in any installation of solid state control. While wiring practices may vary from situation to situati
37、on, the following are basic to minimizing electrical noise: a) Sufficient physical separation should be maintained between electrical noise sources and sensitive equipment to assure that the noise will not cause malfunctioning or unintended actuation of the control.b) Physical separation should be m
38、aintained between sensitive signal wires and electrical powerand control conductors. This separation can be accomplished by conduits, wiring trays, or as otherwise recommended by the manufacturer.c) Twisted-pair wiring should be used in critical signal circuits and noise producing circuits to minimi
39、ze magnetic interference.d) Shielded wire should be used to reduce the magnitude of the noise coupled into the low-level signal circuit by electrostatic or magnetic coupling.e) Provisions of the 2014 National Electrical Code * with respect to grounding should be followed. Additional grounding precau
40、tions may be required to minimize electrical noise. These precautions generally deal with ground loop currents arising from multiple ground paths. The manufacturers recommendations should be followed.4.2 ENCLOSURES (COOLING AND VENTILATINGa) Suitable enclosures and control of the maximum operating t
41、emperature, both of which are environmental variables, may be needed to prevent malfunction of solid state control.b) The manufacturers recommendations should be followed for the selection of enclosures, ventilation, air filtering (if required), and ambient temperature. These recommendations may var
42、y from installation to installation, even within the same facility. 4.3 SPECIAL HANDLING OF ELECTROSTATIC SENSITIVE DEVICESSome devices may be damaged by electrostatic charges. These devices are identified and should be handled in the special manner specified by the manufacturer.NOTEPlastic wrapping
43、 materials used to ship these devices may be conductive and should not be used as in sulating material. * Available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. 1999 National Electrical Manufacturers AssociationICS 1.1-1984 (R1988, R1993, R1998, R2003, R2009, R2015
44、)Page 84.4 COMPATIBILITY OF DEVICES WITH APPLIED VOLTAGES AND FREQUENCIESPrior to energization, users and installers should verify that the applied voltage and frequency agree with the rated voltage and frequency specified by the manufacturer.NOTEIncorrect voltage or frequency may cause a malfunctio
45、n of or damage to the control.4.5 TESTING PRECAUTIONSWhen testing solid state control, the procedures and recommendations set forth by the manufacturer should be followed.When applicable, instrumentation and test equipment should be electrically equivalent to that recommended by the manufacturer for
46、 the test procedure. A low-impedance voltage tester should not be used.High-voltage insulation tests and dielectric tests should never be used to test solid state devices. If high voltage insulation of field wiring is required, solid state devices should be disconnected. Ohmmeters should only be use
47、d when and as recommended by the equipment manufacturer.Testing equipment should be grounded; if it is not, special precautions should be taken.4.6 STARTUP PROCEDURESChecks and tests prior to startup and startup procedures recommended by the manufacturer should be followed. 1999 National Electrical
48、Manufacturers AssociationICS 1.1-1984 (R1988, R1993, R1998, R2003, R2009, R2015)Page 9Section 5 PREVENTIVE MAINTENANCE AND REPAIR GUIDELINES5.1 GENERALA well-planned and executed maintenance program is essential to the satisfactory operation of solid state electrical equipment. The kind and frequenc
49、y of the maintenance operation will vary with the kind and complexity of the equipment as well as with the nature of the operating conditions. Maintenance recommendations of the manufacturer or appropriate product standards should be followed.Useful reference publications for setting up a maintenance program are NFPA 70B-2013, Maintenance of Electrical Equipment, and NFPA 70E-2015, Electrical Safety Requirements for Employee Workplaces.5.2 PREVENTIVE MAINTENANCEThe following factors should be considered when formulating a maintenance program:a
