MPPT RS 450/200 error #27

Interesting one — I've been down a similar rabbit hole with my RS 450/100 in the motorhome, though on a smaller array than you're running.

The #27 error (short circuit detected) on the RS series is worth investigating carefully before assuming it's a genuine fault. A few things worth checking:

String voltage at the controller input

With 6 panels in series on 650W modules, you'll want to verify your open-circuit Voc doesn't creep too close to the 450V ceiling on a cold morning. UK winters can drop ambient temps enough to push Voc noticeably higher than the STC rated figure. Worth doing the temperature-corrected calculation properly.

Intermittent wiring issues

In my experience, intermittent #27 errors on Victron MPPT RS units are often down to a dodgy MC4 connection rather than an actual short. Any marginal connection can cause the controller to see an unexpected voltage spike or drop and throw the error defensively. I'd go through every MC4 joint on each string methodically — I use a proper MC4 assembly tool (the Amphenol one) and I've found loose crimp connections more than once.

String isolation testing

If you have a decent insulation tester, do a string-by-string check to rule out a panel with degraded insulation to frame. A single damp junction box can cause exactly this kind of intermittent behaviour, especially if you're parked somewhere exposed.

Has anyone else on here dealt with #27 errors specifically on the larger RS units? Curious whether Victron Connect logs are giving you anything useful in the history tab — that's often where I start before touching anything physical.

Had #27 pop up on my garden office Victron setup last summer — turned out one of my MC4 connectors had corroded internally and was creating a partial short under load, looked absolutely fine from the outside though 🔍

@VoltBarry corrosion is a classic one, especially in damp climates — boats teach you that lesson fast.

Worth checking whether the #27 is triggering under specific conditions though. On my setup I noticed it only fired up when irradiance spiked after cloud edge events — the RS series can be very twitchy about sudden current surges if your array wiring has any marginal connections that are borderline fine under normal load.

Have you run the VictronConnect history to see what time of day it's hitting? If it's consistently around midday when your panels peak, I'd look hard at:

• String balance (mismatched panels or partial shading)
• Any dodgy MC4 joins under thermal stress
• Earth fault possibilities — the RS has separate PV+ and PV- ground fault detection

The 450/200 running a large array has sod all tolerance for marginal wiring.

Good shout from @VoltBarry and @ExBrickie on the connectors, but with the RS 450/200 specifically I'd also look hard at your combiner box if you're running multiple strings.

On my van build I had intermittent #27 events that turned out to be a failed bypass diode in one of the panels causing a reverse-current path that the RS misread as a short. Swapped the panel and cleared immediately.

The RS series is notably more aggressive than older BlueSolar MPPTs at detecting these fault conditions — which is actually a feature, not a bug, given the higher voltage/current ratings it's handling.

Two things worth doing:

• Isolate each string individually and measure open-circuit voltage against expected spec
• Check your array Voc doesn't exceed 450V under cold conditions — the RS will fault protectively if it does

What's your string configuration?

@Lefty28 makes a fair point — with the RS 450/200 you're pushing serious wattage and the firmware history on that unit is patchy. Worth checking you're on the latest release via VictronConnect before pulling everything apart physically.

On my shepherd's hut array I had a phantom #27 that turned out to be a voltage mismatch between string configurations rather than any physical fault. The RS series is quite aggressive about protecting itself if the strings aren't balanced properly.

Double-check your Voc calculations against actual measured open-circuit voltage on each string before reconnection — especially right now with the cold weather pushing Voc higher than your STC nameplate figures suggest. Easy to accidentally creep over the input threshold during a British winter morning.