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g/solar • Posted by u/solar_steve •

Rapid shutdown compliance keeps failing inspection on one specific inverter combo Commissioning

Residential, string inverter with module-level rapid shutdown devices. Third inspection failure on this combination and the AHJ is now looking at all my permits sideways.

Failure mode: inspector initiates rapid shutdown, measures at the combiner, and reads voltage that takes about 45 seconds to drop below 30V. Code requires it within 30 seconds.

The RSD devices are listed, the inverter is listed, both are on the manufacturer's compatibility matrix.

Everything is installed per the instructions. Why is it slow?

💬 6 replies

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u/battery_backup_brad •
✓ Accepted Solution

Ask what the ambient temperature was at each failed inspection, and I will bet all three were cold mornings.

RSD devices shut off fast. What takes 45 seconds is bleeding down the DC bus capacitance in the inverter, and the bleed resistor network is temperature dependent. Manufacturers publish discharge times at 25°C. At 5°C on an early morning inspection the same hardware can take 40 to 60% longer.

Two real fixes:

1. Schedule inspections for the afternoon. Sounds like gaming it. It is not — the listing is at standard temperature and you are entitled to be tested near it. Every inverter on the market has the same behavior.

2. Check whether your inverter has a fast-discharge option. Several manufacturers added an active discharge circuit in firmware specifically because of this, and it is often not enabled by default on older stock. A firmware update on the inverter may fix it outright.

Call the manufacturer's tech line with your serial and ask about active discharge and the firmware revision. On two brands I work with, this exact problem was a known issue with a firmware answer.

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✓ Solution
u/solar_steve OP •

All three inspections were between 8 and 9 in the morning in the last month, and it has been unseasonably cool here.

Called the manufacturer. Firmware on my stock is two revisions back and the release notes on the current one mention improved DC bus discharge. Updating the one that failed last week and requesting a re-inspection for early afternoon.

I have been assuming a listed combination cannot fail and therefore the problem had to be my installation. It did not occur to me that the timing spec had a temperature dependency at all.

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u/roof_mount_ray •

Worth telling the AHJ what you found, in writing, once you confirm it. You have burned some credibility with three failures and "here is the mechanism and here is the firmware note from the manufacturer" gets a lot of it back.

Inspectors deal with contractors who argue. A contractor who comes back with a root cause is memorable in the other direction.

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u/string_inverter_sarah •

What is the string length and how many RSD devices per string? Discharge time scales with the capacitance you are trying to bleed down, and a long string with a lot of devices takes longer than the same devices on a shorter string.

Also, where exactly is the inspector measuring? Inside the array boundary and at the inverter are different requirements with different limits.

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u/string_inverter_sarah •

Adding this to my commissioning checklist — verify inverter firmware revision before the inspection request goes in. I have never once checked it on a new unit out of the box because I assumed new meant current. Warehouse stock sits for a year.

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u/ev_charger_eli •

The "new stock is not current firmware" thing bites in EVSE too. I have pulled chargers out of a box with firmware from two years prior and a known charging fault fixed eighteen months ago.

Update before you commission. Every time, every product.

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