Okay, this is a common scenario with tricky details. Let's break down your questions and the best path forward, keeping safety as the absolute priority. I'm focusing on US standards (NEC) and common best practices. Remember, I am an AI and this is advice based on general knowledge. A qualified, licensed electrical engineer familiar with hazardous location installations must review and approve your design before any work is performed. Consult local authorities having jurisdiction (AHJ) for definitive guidance.

Analysis of Your Original Solution (ATEX Zone 0 Sensors)



Junction Box Rating (NEMA 7 vs. Other)

This is the critical question. Yes, you very likely need a NEMA 7 (Explosionproof) junction box. Here's why, even though the sensors are Zone 0 rated:

Fault Conditions

The key phrase in hazardous location work is "under fault conditions." While the sensor itself should not create an arc under normal operation, what happens if the wiring to the sensor develops a fault within the junction box? A loose connection, damaged insulation, or water ingress could create a spark.

Enclosure Design

NEMA 7 boxes are designed to contain an internal explosion and prevent it from propagating to the surrounding atmosphere. They are much more than just "weatherproof."

Code Compliance

While the sensors are Zone 0 rated, the entire installation needs to be compliant with the NEC Article 500-504. Typically, in Class I, Div 1 locations, all enclosures must be rated for that environment unless they are specifically proven and approved to be intrinsically safe.

Potential for Damage

A junction box is not as robust as the prox sensor. It is easier to damage during installation or operation.

Seal-offs



Required

Yes, you absolutely need seal-offs. NEC 501.15 outlines where seals are required in Class I, Div 1 locations.

Purpose

Seal-offs prevent the passage of flammable gases or vapors through the conduit system. If an explosion occurs in one part of the system, the seal-off prevents it from spreading.

Sensor Conduit

The flexible conduit supplied with the sensor almost certainly will not be suitable as is. You will need to transition from the sensor's flexible conduit to a rigid metal conduit immediately after the sensor. This transition must also be in a Div 1 rated enclosure (typically part of the sensor's design) where you can also install a seal-off as close as possible to the sensor, but no more than 18 inches.

Seal Location

There needs to be a seal-off installed within 18 inches of the enclosure in the unclassified area.

Analysis of Your Alternate Solution (General Area Sensors with Switch Amplifiers)

This is a potentially viable and often preferred approach, but only if executed correctly. This is where the concept of Intrinsic Safety (IS) comes in.

Intrinsic Safety (IS) Principle

Intrinsic Safety limits the electrical energy in the hazardous area to a level that cannot cause ignition, even under fault conditions.

Switch Amplifiers (IS Barriers)

Pepperl+Fuchs and similar manufacturers make IS barriers that are specifically designed to interface with sensors in hazardous areas. These barriers limit voltage and current to safe levels.

Here's how the alternative would work (with crucial caveats)


General Area Sensors

You use standard, non-hazardous location rated sensors.2.

Cordsets

You run the sensor's M12 cordsets (or other suitable cable) into a standard junction box.3.

Junction Box

The junction box needs to be properly rated for the environment, though probably not explosion-proof. NEMA 4 would be a good start.4.

Cable Run to PLC Panel

You run a shielded cable from the junction box to the IS barriers in the PLC panel.5.

IS Barriers

The IS barriers are installed in the safe area. They provide the power to the sensors and limit the energy back to the sensors.6.

Wiring

Wiring from the IS barriers to the PLC inputs is standard, safe-area wiring.

Important Considerations for the Alternate Solution



IS Barrier Selection

You must select IS barriers that are specifically designed for use with your sensors. The barrier manufacturer will provide detailed instructions on wiring, cable types, grounding, and distance limitations.

Grounding

Proper grounding of the IS system is critical. Follow the manufacturer's instructions.

Cable Type

The cable run from the junction box to the IS barrier must be a specified type (typically shielded twisted pair) and meet the IS barrier manufacturer's requirements. Distance limitations may apply.

Documentation

The entire IS system needs to be documented, showing that the sensor, cable, IS barrier, and wiring configuration are approved for use together.

Field Verification

Once installed, the IS system should be field-verified by a qualified IS technician to ensure it is operating correctly.

Entity Approval

The IS system must have "Entity Approval" that allows interconnection of the sensor, the IS barrier and the field cable.

Intrinsically Safe Certification

You will need to have an expert analyze the design and installation and issue an Intrinsically Safe Certification.

Addressing your specific questions about the alternative



Junction Box for Sensor Cordsets

You can use a regular, general-purpose (but likely NEMA 4 rated) junction box,

provided you are following the IS principles outlined above, using approved IS barriers, cables, and documented configurations. RMC with Seal-Off

A single RMC with a seal-off back to the PLC panel from the general-purpose junction box is likely still required, but this depends on the total wiring in the j-box and the potential fault current. Consult an expert.

Cord Grips

Yes, you can use cord grips to bring the sensor cordsets into the general-purpose junction box.

Which Solution is Best?

The IS solution (alternative) is generally preferred because: Simplified Wiring in Hazardous Area

You don't have to deal with explosion-proof conduit, seal-offs, and NEMA 7 enclosures in the Class I, Div 1 area.

Easier Maintenance

Troubleshooting and replacing sensors is much easier.

Cost Savings

Often, the overall cost is lower, despite the cost of the IS barriers.

Recommendations


Engage a Qualified Electrical Engineer

This is essential. You must* have an engineer experienced in hazardous location design and IS principles review your design, select appropriate components, and provide detailed drawings and specifications.2.

Consider Intrinsic Safety (IS)

Seriously evaluate the alternative solution using IS barriers. It is often the better approach.3.

Contact Pepperl+Fuchs (or similar vendor)

Work directly with the IS barrier manufacturer. They can help you select the correct barriers, cables, and provide detailed documentation.4.

Documentation, Documentation, Documentation

Document every component, wiring configuration, grounding scheme, and approval certificate.5.

Training

Ensure your installation and maintenance personnel are properly trained in hazardous location wiring practices and IS principles.

In summary: Don't cut corners on safety. The potential consequences are far too great. Get expert help, follow all codes and standards, and document everything meticulously. Flag for review