Understanding Electrical Permissiveness: What Does It Really Mean?

In electrical engineering, the term permissive refers to a condition or set of conditions that must be met for a process to start or continue. It is often used in conjunction with interlock, which is a device or software that checks whether the permissive conditions are satisfied. For example, before starting an electric device, an interlock system might check the permissive condition that it must not be raining and disable the power switch if this condition is not met. Permissives can exist in software, hardware, or a combination of both. In the context of electrical systems, permissives are essential for safety, ensuring that certain criteria are fulfilled before allowing a device or process to operate.

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Permissive vs interlock

In electrical engineering, the term "permissive" is used to describe a condition within a logic sequence that must be satisfied before the sequence is allowed to proceed. In other words, it is a set of pre-defined criteria that must be met for a process to start. For example, "Starting an Electric Device" may require the permissive condition that "it must not be raining".

An "interlock", on the other hand, is a physical device, equipment, or software routine that enforces the permissive condition. It is used to prove the physical state of a required condition and to furnish that proof to the primary safety control circuit. Interlocks can be used to prevent an operation from beginning or changing function until some condition or set of conditions (i.e., the permissives) are met. For example, a Water Detector (Interlock) can be used to check if it is raining and disable the Power Switch when the permissive condition of "not raining" is not satisfied.

Permissives and interlocks are often used together to ensure safe and proper operation of electrical systems. Interlocks can exist purely in software or hardware, or a combination of both, depending on the application. Similarly, permissives can be implemented in software or hardware, or both, and are often used to prevent harmful conditions from occurring.

It is important to note that the terms "permissive" and "interlock" are not interchangeable and have distinct meanings and functions in electrical engineering. The programming and treatment of permissives and interlocks also differ, and they should not be confused with each other.

In summary, a permissive is a condition or set of criteria that must be met for a process to start, while an interlock is a device or routine that enforces that condition and enables the process to continue. Permissives and interlocks work together to ensure safe and proper operation of electrical systems, with interlocks often requiring permissives to be followed as well.

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Permissive in PLC burner control systems

In electrical engineering, a "permissive" is a condition that must be satisfied before a process can start. A "permissive" can become false after a process starts without affecting the process. An "interlock" is similar but must remain true throughout the process. Both "permissives" and "interlocks" can exist in software or hardware, but "interlocks" can also exist purely in hardware.

Programmable Logic Controllers (PLCs) are used in burner control systems to monitor and control functions, ensuring safety and optimal performance. PLCs can control the temperature by modulating the gas valve, starting and stopping the blower(s), and monitoring safety mechanisms. Burner management systems, such as the PSSuniversal PLC, can control multiple burners in an application, irrespective of their number and physical arrangement.

In the context of PLC burner control systems, a "permissive" could refer to a condition that must be met before the burner is allowed to start. For example, a permissive could be that the "burner battery must be at least 90% charged". A corresponding "interlock" would be a mechanism that checks this condition, such as a battery level indicator, which would prevent the burner from starting if the battery level is too low.

Another example could be a "permissive" condition that the "burner must be properly ventilated before starting". The corresponding "interlock" would be a sensor that measures the ventilation or oxygen levels and prevents the burner from starting if the levels are unsafe.

It is important to note that the specific "permissives" and "interlocks" in a PLC burner control system may vary depending on the application and safety requirements.

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Permissive Over-Reaching Transfer Trip (POTT)

In electrical engineering, a "permissive" is a condition that must be checked by an "interlock" before a process can start or proceed. For example, "Starting the Electric Device" requires the permissive condition "It must not be raining". A Water Detector (the interlock) checks this condition and enables or disables the Power Switch accordingly.

A Permissive Over-Reaching Transfer Trip (POTT) is a communication-assisted trip scheme. It is a type of permissive scheme that tells other relays protecting a line that they can trip faster if they also detect a fault in the correct direction. In a standard impedance protection scheme, Relay-2 would trip after a 20-40 cycle Zone-2 time delay. However, if Relay-2 receives a POTT signal from Relay-1, it can trip faster if it also detects a Zone-2 pickup.

The "P" in POTT stands for "Permissive", and the scheme relies on receiving a "permission to trip" signal from both the remote-end relay and the local relay to initiate a breaker trip. This is in contrast to an interlock, which must remain true throughout the process. If a fault occurs on the line, it may short out the permissive trip signal, and without this signal, the relay operation is inhibited.

End-to-end tests should be performed on either side of each protective zone to ensure the protected line is isolated quickly when a POTT scheme is applied. Additional tests are required for POTT schemes to prove that the Zone-3 timer is appropriate for the application by simulating a phase reversal from both directions.

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Permissive rules in the NEC

The National Electrical Code (NEC), sponsored by the National Fire Protection Association, is the most widely adopted code in the world. It is also, perhaps, the most widely misinterpreted code. The NEC is adopted by federal, state, and local governments, as well as by private industry. While the NFPA regards the NEC as purely advisory, it becomes enforceable as law when adopted by an agency with the authority to enforce its rules.

The NEC is written based on the requirements found in the "NEC Style Manual". Mandatory rules are indicated by terms such as “shall”, “shall not”, and “shall not be”, while permissive rules use phrases such as "shall be permitted" or "shall not be required". Permissive rules indicate allowed actions without obligation.

• Article 90 serves as an introduction to the NEC and provides a scope to specify the electrical installations that are covered and those that are not.
• Article 100 contains definitions that can help clarify vague requirements in other parts of the NEC. For instance, the definition of overload states that a fault, such as a short circuit or ground fault, is not an overload.
• Article 90.4 grants authorities having jurisdiction the power to enforce requirements, approve plans, and waive specific requirements or permit alternative methods if equivalent objectives can be achieved while maintaining effective safety standards.
• Article 90.5 distinguishes between mandatory and permissive rules, with the latter indicating allowed actions without obligation.
• Article 90.94 provides more than one way to specify units of measurement.

Permissive in software and hardware

In electrical engineering, a "permissive" is a condition that must be satisfied before a process can start or move on to the next phase. This is often implemented in hardware, software, or a combination of both. For example, before starting an electric device, a water detector (interlock) checks the condition that "it must not be raining" (permissive) and enables the power switch when this condition is met.

In software and hardware, the concept of "permissives" is crucial for ensuring the safe and proper functioning of systems. A permissive in this context refers to a specific condition or set of conditions that must be true or satisfied before a certain action or process is allowed to occur. These conditions are typically defined by system designers or programmers to ensure the safe and reliable operation of hardware and software systems.

In hardware, permissives can be implemented using physical devices, equipment, or sensors that monitor and control the operation of machines or devices. For example, in a manufacturing plant, a machine may have a sensor that detects the presence of an object before initiating a specific action. This sensor acts as a permissive, ensuring that the required conditions are met before the machine proceeds with its task. Similarly, in a home automation system, a motion sensor may act as a permissive, triggering certain actions, such as turning on lights or activating security systems, only when motion is detected.

In software, permissives are often used to control the flow of a program or to ensure that certain conditions are met before proceeding to the next step. For example, in a video game, a character may be allowed to advance to the next level only if they have completed certain tasks or acquired specific items. Here, the completion of these tasks or the acquisition of the necessary items acts as a permissive condition. In a more general sense, software permissives can include user permissions, where certain actions or data are only accessible to users with the appropriate clearance or authorization.

Additionally, permissives play a crucial role in ensuring data integrity and security. In database management systems, for instance, permissives can be employed to enforce data constraints or validate user inputs. This helps maintain the accuracy and consistency of data by ensuring that any new data entered into the system adheres to predefined rules or conditions. Similarly, in cybersecurity, permissives can be utilized to implement access control mechanisms, where users must satisfy certain conditions, such as providing valid credentials or meeting specific security protocols, before being granted access to sensitive information or resources.

In both hardware and software, the concept of permissives is closely tied to the idea of interlocks. An interlock is a mechanism that physically or logically prevents a process from proceeding until one or more permissives are met. This ensures that the system operates safely and reliably, as any deviation from the required conditions will trigger the interlock, halting the process until the issue is resolved.

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