You're right, NFPA 79, Section 7.2.8 can be a source of confusion, especially when dealing with numerous field devices connected via terminal blocks. Let's break down the requirements and address your question

Understanding NFPA 79, 7.2.8
The Core Requirement

NFPA 79, Section 7.2.8 mandates that each branch circuit supplying power to a load (field device) must have overcurrent protection (OCPD). This is based on the general principle of protecting wiring and equipment from overcurrents that could lead to fire or damage.

The Key Phrase: "Branch Circuit" This is where the interpretation comes in. What exactly constitutes a "branch circuit" in the context of a control panel supplying multiple field devices? Considerations



Single Power Source

It's generally not practical or necessary to have individual OCPDs for every single push button, sensor, or indicator light, if they are all connected to a single branch circuit. A single branch circuit can feed multiple devices, as long as the OCPD is sized appropriately for the smallest device and the feeder wiring is sized appropriately for the load.

Grouping Functionally Similar Devices

It's common practice to group similar devices (e.g., all the push buttons on an operator station, or all the proximity sensors on a conveyor) onto a single branch circuit protected by a single OCPD.

Multiple Branch Circuits

When you have multiple devices with varying current requirements or devices that perform different functions that would make them a "branch" of their own, you need to consider separate branch circuits and corresponding OCPDs. This is commonly done, e.g., for a safety scanner, a safety relay, a motor, etc.

Your Scenario: Terminal Blocks and Field Devices

In your scenario, where terminal blocks are used to distribute power to multiple field devices, the following applies:1. Determine Branch Circuits

Analyze how the field devices are being powered. Is each device truly connected to its own independent branch circuit? Or are multiple devices fed from a single branch circuit via the terminal blocks?2.

OCPD Sizing



Single Branch Circuit, Multiple Devices

If multiple devices are on the same branch circuit, the OCPD must be sized: To protect the smallest wire gauge in that branch circuit. To protect the lowest current device in the branch circuit. Based on the total current draw of all devices on the branch circuit, ensuring the OCPD isn't overloaded.3.

Example: Common Practice Imagine you have a single 24VDC power supply within your control panel. You use terminal blocks to distribute this 24VDC to: Ten push buttons (each drawing a negligible amount of current) Five proximity sensors (each drawing 50mA) One indicator light (drawing 20mA)In this case, you would likely have one branch circuit, protected by one OCPD. You would size the OCPD based on: The wire gauge used to connect the power supply to the terminal blocks and the terminal blocks to the devices. The total current draw (10\0 + 5\50mA + 20mA = 270mA). A 1 amp fuse or circuit breaker would likely be sufficient, as long as the wiring is also appropriately sized.

When Individual OCPDs ARE Required/Recommended

There are situations where individual OCPDs for each field device are required or strongly recommended: Specific Device Requirements

The manufacturer's documentation for a specific device may require individual overcurrent protection. Safety-related devices (e.g., safety light curtains, safety relays) often have this requirement.

Fault Isolation

If a fault in one device could disrupt the operation of the entire system, it's good practice to provide individual OCPDs for each device. This allows you to isolate the fault without taking down the entire branch circuit.

Complex Systems

In large, complex systems, individual OCPDs can simplify troubleshooting and maintenance.

Remote I/O

Many remote I/O blocks and devices have their own individual fusing or overcurrent protection.

In Summary

You don't necessarily need an individual OCPD for every single field device in your control panel. The key is to properly define your branch circuits and size your OCPDs appropriately for each branch circuit, considering the wire size, the current draw of all devices on that branch, and any specific device requirements. Flag for review