Why are NEMA symbols such as push buttons, float switches, or limit switches not used in programmable logic controller schematics?
The correct answer and explanation is :
Answer:
NEMA symbols, such as those for push buttons, float switches, or limit switches, are not typically used in PLC schematics because PLC schematics follow IEC (International Electrotechnical Commission) standards rather than NEMA (National Electrical Manufacturers Association) standards. IEC symbols are used in PLC schematics because they provide standardized graphical representations of control elements that align with international automation and electrical control standards.
Explanation:
- Standardization and Global Use
PLCs are used worldwide, and IEC 60617 symbols are the global standard for electrical and automation schematics. While NEMA symbols are common in the United States, IEC symbols ensure uniformity in PLC designs across different countries and manufacturers. This helps engineers, electricians, and programmers read and interpret PLC diagrams easily, regardless of their geographical location. - Logical Representation vs. Physical Representation
- NEMA symbols focus on the physical appearance of devices (e.g., push buttons, relays, and switches).
- IEC symbols and PLC schematics prioritize the logical representation of signals and control functions rather than their physical form. Since PLCs process digital and analog signals, their schematics abstract away the physical details and use simplified logical symbols that represent functions, rather than hardware.
- Ladder Logic & Electrical Schematic Differences
Ladder logic diagrams (used in PLCs) use contacts and coils instead of physical switch symbols. For example:
- A limit switch in a PLC ladder logic diagram appears as a normally open (NO) or normally closed (NC) contact, rather than a NEMA-style symbol.
- This logical representation makes it easier to program and debug control sequences in industrial automation.
- Clarity and Simplification
IEC-based PLC diagrams allow a cleaner and more functional representation of circuits, improving troubleshooting and readability for engineers and technicians.
NEMA symbols and PLC IEC-based schematics:
NEMA symbols and IEC PLC representations for comparison.
NEMA symbols and IEC PLC symbols.