FAQ
TL;DR: "The SPD cost less than 5%," and either AC protection sequence works; choose quality SPD and the right RCD. This FAQ shows how to place breaker, SPD, and RCD, and what RCD type to use. [Elektroda, Jan_Werbinski, post #18915512]
Why it matters: Homeowners and installers need a clear, safe way to wire PV inverter AC protections and choose correct RCD/SPD.
Quick Facts
- AC sequence (breaker/SPD → RCD vs RCD → breaker) shows no functional difference. [Elektroda, Jan_Werbinski, post #18910881]
- Type B RCD is preferred for transformerless inverters; “A will not turn off the direct current.” [Elektroda, czesiu, post #18913386]
- Fronius Symo guidance: use Type A RCD; follow the inverter maker’s instructions. [Elektroda, toolpusher, post #18913474]
- Use T1 spark gap SPD when LPS exists or separation is hard; T2 varistor is enough for T2 duty. [Elektroda, Jan_Werbinski, post #18912985]
- SPD spend is often <5% of system cost; inexpensive protection for electronics. [Elektroda, Jan_Werbinski, post #18915512]
Which order is correct for AC protections after the inverter?
Both layouts you showed are acceptable. “No difference.” Focus instead on correct device selection and quality. Prioritize a robust SPD strategy and proper coordination. Many installers favor spark-gap T1 for higher surge energy. Brands like Dehn or Phoenix Contact are common choices. Use the layout that simplifies safe maintenance and inspection. [Elektroda, Jan_Werbinski, post #18910881]
Do I need an RCD on the AC side of a PV inverter?
Often yes, especially for transformerless inverters. Type B detects smooth DC components from faulted inverters. Transformer-based inverters may not need an RCD. Some transformerless units with DC earth-fault testing may also not require one. Check your inverter documentation. Placing the RCD at the main board can protect the feeder cable. This improves fault detection on that run. [Elektroda, czesiu, post #18913386]
Type A or Type B RCD for transformerless PV inverters?
Use Type B unless the inverter maker specifies otherwise. Type A can miss smooth DC from a faulted transformerless unit. “A will not turn off the direct current.” Type B addresses this failure mode and improves safety. Confirm the inverter’s built-in DC fault monitoring features. Always size current and poles to match the feeder. Label the RCD clearly for PV use. [Elektroda, czesiu, post #18913386]
Does Fronius Symo require Type B or Type A RCD?
Fronius guidance points to Type A for Symo. Follow the manufacturer’s instruction for final selection. Match RCD sensitivity and current rating to that guidance. Ensure correct coordination with upstream protection. Document the setting on your single-line diagram. “Fronius recommends type A.” [Elektroda, toolpusher, post #18913474]
Where should I place the RCD: at the inverter or in the main switchboard?
Place it in the main switchboard feeding the inverter circuit. That way, the RCD also monitors the feeder cable. It can disconnect a damaged cable during export or idle conditions. Mounting at the inverter leaves the cable energized on faults upstream. Choose enclosure space and heat dissipation carefully. Test the RCD function after commissioning. Keep test records with the installation file. [Elektroda, czesiu, post #18913386]
When should I use a T1 spark-gap SPD on the AC side?
Use T1 spark-gap SPD when a building has LPS or separation is difficult. T1 manages higher lightning surge energy than varistors. For T2 duty, a varistor device is sufficient. Coordinate with DC-side protection to avoid overlap issues. Verify earthing and bonding before final selection. Keep leads short to reduce let-through voltage. [Elektroda, Jan_Werbinski, post #18912985]
Are varistor-only SPDs safe on the AC side?
Cheap varistor SPDs are a known fire concern. Use reputable devices and consider spark-gap T1 units. “It’s the SPD that is the fire risk. Pay more for Dehn or Phoenix Contact.” Budget devices may degrade faster under repeated surges. Verify certification and replace status indicators when tripped. Size backup fusing per the SPD datasheet. [Elektroda, Jan_Werbinski, post #18910881]
Is skipping AC SPD reasonable with underground supply and no overhead lines?
Risk depends on exposure and the value of connected electronics. One user with TN-C supply reported 30 years without electronics damage. However, that is anecdotal and does not remove surge risk. Consider the inverter, TVs, servers, and appliances at stake. Surge energy can enter through many paths, not just overhead lines. Perform a risk review before deciding to omit SPD. [Elektroda, toolpusher, post #18911228]
Do SPDs actually reduce real-world damage?
Yes, users report fewer device failures after installing SPDs. “They certainly paid off” even when using varistor units. This reflects reduced surge stress on electronics. Use coordinated stages and keep leads short for best effect. Inspect and replace end-of-life modules promptly. Log surge events if the SPD offers counters. [Elektroda, Jan_Werbinski, post #18915326]
What RCD sensitivity should I choose on the inverter’s AC side?
Follow the inverter manufacturer’s advice first. Installers often choose at least 100 mA when an RCD is used. Select the correct RCD type (A or B) per the inverter design. Ensure compatibility with any inrush or leakage characteristics. Test with the inverter both producing and idle. Record trip times and currents for compliance. [Elektroda, toolpusher, post #18910719]
Is a 300 mA RCD common in PV AC protection?
Some installations use 300 mA devices on the PV feeder. That choice reflects project-specific coordination and nuisance trip considerations. Always follow the inverter manufacturer’s RCD guidance. Consider Type B where DC components are possible. Verify discrimination with upstream protections. Document rationale on the single-line. [Elektroda, Matuesz_99, post #18913216]
How do I place the RCD so it also protects the inverter feeder?
- Install the RCD in the main switchboard upstream of the inverter circuit.
- Use Type B for transformerless inverters without DC-fault detection; Type A otherwise per OEM.
- Test leakage detection with the inverter on and off; log results for records. [Elektroda, czesiu, post #18913386]
Are premium SPD brands and spark-gap T1 worth the spend?
Yes, they are often a small share of system cost and improve resilience. “The SPD cost less than 5%.” Premium T1 spark-gap devices handle severe surges better. They also mitigate thermal runaway versus cheap varistors. View them as insurance for high-value electronics. Keep spare modules for quick replacement. [Elektroda, Jan_Werbinski, post #18915512]
