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Generic•Generic RSD

Analysis by Marcus & Team

Generic RSD: Rapid Shutdown Relay Failure

Troubleshoot a clicking but non-engaging rapid shutdown switch relay. Diagnose relay failure, ensure NEC compliance, and maintain solar system safety.

Frequently Asked Questions

•What is the expected lifespan of a generic RSD unit in a residential solar system?

The typical lifespan of a generic RSD unit is 5-10 years, depending on the manufacturer and environmental conditions. Regular inspection and testing can help identify potential failures before they occur.

•How can I verify that the RSD unit is properly communicating with the MLPEs?

Check the manufacturer's documentation for the specific communication protocol used. Use a multimeter to verify the signal voltage on the communication wires. If available, consult the inverter monitoring system for communication status reports.

•What are the NEC requirements for rapid shutdown in residential solar systems?

NEC 690 mandates rapid shutdown of PV systems to reduce voltage hazards for first responders. The requirements specify voltage limits (typically <30V within 30 seconds) outside the array boundary. RSD systems must comply with these requirements to ensure safety.

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Marcus leads the technical research department at Storage Faults. Together with a team of systems analysts, he specializes in translating complex solar storage error codes into actionable diagnostic steps for the US market.

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Observed Symptoms:

The rapid shutdown (RSD) switch emits an audible clicking sound upon activation, but the photovoltaic (PV) string voltage does not drop below 30V within 30 seconds, as required by NEC 2017/2020. Inverter continues to operate normally and PV modules still produce power.

Why This Happens:

Failed Relay Coil: The coil inside the RSD relay is damaged, preventing it from creating a magnetic field to close the relay contacts.
Corroded Relay Contacts: Oxidation or corrosion on the relay contacts prevents proper electrical contact.
Low Control Signal Voltage: Insufficient voltage from the RSD switch to energize the relay coil. Check voltage at the RSD output terminals with a multimeter.
Wiring Issues: Loose or damaged wiring between the RSD switch, the relay, and the MLPE communication channel.
MLPE Communication Failure: Inability for the RSD to communicate with the MLPE devices after a successful relay closure.
Incompatible RSD/MLPE Configuration: Mismatched communication protocols, or voltage between the RSD, and MLPE devices.

Critical Safety Precautions:

Working with PV systems involves high DC voltages, which can cause electric shock and arc flash hazards. Always de-energize the system before performing any maintenance or repairs. Only qualified electricians with proper personal protective equipment (PPE) should perform diagnostics and repairs on RSDs. Follow NEC guidelines for safe work practices.

Step-by-Step Fix:

1

De-energize the PV system by disconnecting the AC breaker to the inverter. Wait for the inverter to completely shut down.
2

Locate the RSD unit and visually inspect for any signs of physical damage or corrosion. Document any abnormalities.
3

Use a calibrated multimeter to check the voltage at the RSD output terminals when the switch is in the “on” and “off” positions. Compare the readings to the manufacturer’s specifications. The voltage should drop to zero when switched off.
4

Verify continuity across the relay contacts using a multimeter in continuity mode. The circuit should be open when the RSD is off and closed when it is on.
5

If the voltage or continuity is incorrect, replace the RSD unit with a compatible model.
6

After replacement, re-energize the PV system and test the RSD functionality. Use a clamp meter to confirm that string current drops to zero when the RSD is activated.
7

Document all findings and actions taken, including voltage and current measurements before and after the repair. NEC 690 documentation is required.

Technical Recommendation:

Some RSD units have a manual override or test function. Consult the manufacturer's manual to locate this function and use it to verify relay operation. Check the DC voltage on the RSD input and output with the system running, and again with the RSD activated. The output voltage should drop to zero. Use an IR thermometer to check for thermal abnormalities on the RSD unit. High temperatures may indicate a failing relay.

How It Works:

Residential solar systems use RSDs to reduce voltage hazards during emergencies. The RSD communicates with the inverter and module-level power electronics (MLPEs). Upon activation, the RSD sends a signal (often via power line communication or a dedicated control wire) to the MLPEs to shut down power production at the module level. A failed relay prevents the MLPE from receiving this shutdown signal, thereby bypassing the system’s rapid shutdown function. System is designed for NEC 690 compliance.

Preventative Care:

Perform routine visual inspections of the RSD unit for any signs of corrosion or damage.
Annually test the RSD functionality by activating the switch and verifying that PV string voltage drops to safe levels (<30V within 30 seconds, as per NEC requirements).
Maintain proper wire terminations and connections to prevent loose wiring. Check for corrosion with an antioxidant paste.
Replace the RSD unit proactively according to the manufacturer's recommended lifespan, typically every 5-10 years.