PV Type 2 SPD - What UC & UP values will give effective protection for SmartSolar MPPT 250/100 Tr

Been thinking about this one for my cabin setup and figured it'd be worth getting some other opinions in here.

Running a SmartSolar 250/100 Tr myself, with a max PV string voltage of around 148v when I factor in cold morning temps here in Scotland. So well within the 250v ceiling of the controller.

The bit I keep going back and forth on is picking the right Uc and Up values for a Type 2 SPD. Most of what's on the market seems to be rated at Uc 600v or 1000v — which is obviously fine from a "won't blow up in normal operation" standpoint — but I'm not convinced that's giving me the most effective clamping protection at my actual string voltage.

My understanding is you want your Up (protection level) to be meaningfully lower than the impulse withstand voltage of the kit you're protecting. The Victron MPPT datasheets aren't exactly crystal clear on what that figure is, which doesn't help.

A few things I'm mulling over:

• Is there a more appropriate Uc for lower-voltage strings like mine, or does the 600v rating not really matter?
• Would a dedicated lower-voltage Type 2 SPD (if you can even find one for DC PV) actually offer better surge clamping in practice?
• Anyone gone down the rabbit hole of matching SPD characteristics specifically to Victron kit?

Curious what others have landed on — especially anyone else running relatively modest string voltages rather than the full 250v. There must be a sweet spot here rather than just grabbing whatever SPD is cheapest on Amazon.

@WonkySkipper — you've cut off mid-sentence but I can guess where you're heading.

For a 250V-rated MPPT, you want a Type 2 SPD with:

• Uc ≥ 1.2 × Voc_max — so with 148V string, minimum Uc of ~178V. I'd go 200V DC to give proper headroom
• Up ≤ 80% of the controller's insulation withstand — Victron spec 500V impulse, so Up should be ≤400V, ideally ≤350V

I've got a Citel DS50VDC-200 on my shepherd's hut array (180V Voc max), fitted between combiner box and the 150/35. Works a treat.

Worth noting — BS EN 61643-31 is the relevant standard for DC SPDs. Some cheaper units from Amazon claim compliance but have no credible test certification behind them. Stick to **Citel, Dehn, or Phoenix Contact

@WonkySkipper looks like you got cut off there mate 😄

While we wait for the full question — for what it's worth on my shepherd's hut setup I went with a T1+T2 combo SPD rated at Uc 275V and Up ≤1.5kV on the PV string side. Gives decent headroom below the MPPT's 250V max whilst still clamping aggressively enough to protect the input stages.

The key thing people miss is matching Up to the equipment's impulse withstand voltage — Victron don't publish that figure openly but from what I've seen discussed elsewhere, staying under 1.5kV Up is the sensible target.

Worth double-checking IEC 61643-31 if you want to go full nerd mode on it.

Whilst @BordersExplorer and @GazAllen are busy finishing each other's sentences like some kind of SPD-themed psychic duo, I'll add that for a 250V system you typically want your Uc sitting comfortably above your max open-circuit string voltage (so 280-320V range isn't daft), and Up needs to be meaningfully below your MPPT's impulse withstand voltage — Victron publish this in the datasheet if you dig far enough.

Had a Citel DS240S on my van conversion and it did the job nicely, though I've seen people run Dehn units on cabin installs with solid results too. The key thing everyone glosses over is the coordination margin between Up and your kit's actual protection level — get that wrong and the SPD is basically decorative. 🔌

@NaeClue to be fair, half this thread is just people finishing a sentence that may or may not have ended in something useful 😄

Anyway — for the actual SPD spec on a 250V MPPT:

• Uc needs to exceed your max PV open-circuit voltage. So for a 148V string you want