You've identified a valid concern. Placing the inverters within the array on a metal roof, especially a standing seam, introduces a safety risk regarding potential fault conditions and the energizing of the roof. Let's break down your worries and explore possible solutions

Your Concerns Are Valid:
Fault Potential

Inverters are complex electronic devices and, despite their reliability, can fail. A ground fault within the inverter could energize the metal housing and, consequently, the standing seam roof.

Uncontrolled PV Production

Without module-level rapid shutdown devices (MLRSDs) or other approved methods to achieve rapid shutdown, the PV modules will continue to produce DC voltage even after the AC disconnect is opened. This means the energized metal roof connected to a faulted inverter remains a hazard.

Bonding and Grounding Limitations

While proper bonding and grounding are crucial for safety, they primarily provide a low-impedance path to trip a breaker in the event of a ground fault. They

do not prevent the initial energization of the metal roof before the fault is cleared. They also don't address the continued DC voltage presence. Firefighter Safety

In a fire situation, firefighters risk electrocution if the roof is energized and there is no rapid shutdown system to de-energize the PV system.

Maintenance and Service

Servicing the inverters will be challenging and require careful safety protocols, especially if the system cannot be easily shut down.

Why MCIs (Module Combiner Inverters) Helped (Hypothetically)

MCIs, in your original thought process, likely implied integrated MLRSD within the inverter or required for the inverter to function. In that case, opening the AC disconnect would trigger the MLRSDs to shut down the DC voltage at the module level, mitigating the risk of an energized roof.

What You're Missing (or Need to Consider)


NEC Rapid Shutdown Requirements (Section 690.12)

The National Electrical Code (NEC) mandates rapid shutdown for PV systems on or in buildings. You must comply with the latest version of the NEC adopted by your local jurisdiction. The details vary depending on the NEC version, but generally, it requires the ability to reduce the voltage of the PV conductors within the array boundary to 80 volts within 30 seconds of rapid shutdown initiation. Without MLRSDs, this typically involves moving the inverters closer to the point of interconnection and further from the array.2.

Metal Roof Considerations

The metal roof itself introduces specific challenges. You're right to be concerned about its potential to become energized.3.

Alternatives to MLRSDs (But Not Always Practical)



String Inverters at a Safe Distance

If you moved the inverters to ground level or a location away from the array, you could eliminate the proximity hazard. However, this would require longer DC home run wires, potentially larger conductors, and added conduit costs. You'd still need a rapid shutdown means, likely a disconnect switch located near the array.

String Inverters with String-Level Rapid Shutdown Devices

Some string inverters are compatible with string-level rapid shutdown devices. These devices shut down the DC voltage at the string level instead of the module level. This is typically more cost-effective than MLRSDs but might be less flexible for monitoring and future expansion.4.

Approved Rapid Shutdown Methods

Understand what methods are "approved" by the AHJ (Authority Having Jurisdiction) in your area for meeting the rapid shutdown requirements. MLRSDs are typically the easiest path to compliance, but other solutions may be accepted.5.

AHJ Interpretation

The interpretation and enforcement of the NEC can vary by AHJ. Discussing your plans with the local inspector before you begin the installation is highly recommended.6.

Module-Level Monitoring (Optional, but Beneficial)

MLRSDs often provide module-level monitoring, allowing you to track the performance of each module and quickly identify any issues. This can improve system efficiency and simplify troubleshooting.

Solutions and Recommendations


Embrace MLRSDs

The simplest and often most cost-effective solution is to use MLRSDs (module-level rapid shutdown devices) from a reputable manufacturer (e.g., Tigo, APsystems, Enphase with 690.12 compliance). These devices are installed at each module and, when triggered, shut down the DC voltage at the module level. This drastically reduces the risk of an energized roof and satisfies NEC rapid shutdown requirements. They might add cost, but they significantly improve safety and offer other benefits.
2.

Relocate Inverters (If Possible)

If MLRSDs are not an option, consider relocating the inverters to a safer location, such as ground level. This eliminates the direct proximity hazard to the metal roof. However, the increased DC wiring runs and associated costs must be factored in.3.

Consult with a Licensed Electrical Engineer

Given the complexity of the project and the safety considerations, it's prudent to consult with a licensed electrical engineer experienced in PV system design. They can perform a thorough analysis of your site and recommend the best solution to meet NEC requirements and ensure safety.4.

Communicate with the AHJ

As mentioned before, discuss your plans with the local AHJ before* beginning the installation. They can clarify any specific requirements or concerns they may have.5.

Detailed Grounding and Bonding Plan

Develop a comprehensive grounding and bonding plan that includes bonding the standing seam roof to the system grounding electrode system. This should be done in accordance with NEC Article 250. This is mandatory, but doesn't solve the energizing potential.
6.

Consider a Hybrid Approach

Maybe your cost analysis can lead to some MLRSDs in problem areas and the inverters at ground level to save costs where the DC home runs are manageable.

In summary, your concerns are valid. The most effective solution is likely to use MLRSDs in conjunction with your inverters, or relocate the inverters. Consulting with a qualified electrical engineer and the AHJ is highly recommended to ensure the safety and compliance of your PV system. Flag for review