Câblage ABB CM-IWN.5

Anyone else been looking at the ABB CM-IWN.5 insulation monitoring relay for their off-grid setup?

Been thinking about this after seeing a similar discussion elsewhere. I've got a Victron SmartSolar 250/70 running a 3S2P array (east/west split) and I've been wondering about adding proper insulation monitoring — especially as my backup system powers some critical loads and I want belt-and-braces protection.

The ABB unit seems decent for detecting insulation faults to earth on an isolated DC system, which is exactly what we've got with most off-grid PV setups here. No grid reference means standard RCDs don't really cut it for detecting that kind of fault.

Few things I'm not clear on though:

• Wiring it into a 3S2P array — do you connect the monitoring terminals across the full string voltage or just one string?
• Anyone know if it plays nicely with Victron kit, or does the inverter's internal capacitance cause nuisance tripping?
• What threshold are people setting? The default 50kΩ seems conservative for a damp UK install 😅

I've seen some installs where folk just skip this entirely and rely on fusing, but for anything with people around I reckon proper insulation monitoring is worth the faff.

Not many people seem to talk about this side of off-grid safety on here — feels like it gets overlooked compared to fuse sizing and cable runs.

Anyone actually fitted one of these or something similar (Bender do a range too)? Curious how you wired it up.

@BorderOffGrid the CM-IWN.5 is worth serious consideration for any ungrounded IT-system setup — which is exactly what most Victron off-grid inverters create by default.

Key points worth noting:

• Measurement range: 1kΩ to 10MΩ, suitable for typical DC bus monitoring
• Response threshold: set below 50kΩ for meaningful fault warning before things get dangerous
• Wiring: L+ and L- both feed into the relay's measuring circuit — it's monitoring both poles simultaneously against PE
• Power supply: needs separate 24V or 110-240V AC depending on variant — don't feed it from the bus you're monitoring

On my narrowboat I run a similar insulation monitoring approach (different brand, but same principle). Caught a degraded cable before it became a bilge fire risk, which rather proved the point.

Make sure your PE bonding is actually solid before trusting the readings. Floating earth defeats the whole exercise.

@BorderOffGrid great shout on the CM-IWN.5 — I've been running one on my cabin setup for about eight months now and it's genuinely transformed how I think about fault monitoring.

Key thing people miss: the CM-IWN.5 requires a measurement voltage reference point, so you'll need to connect the KE terminal correctly relative to your DC bus. On an ungrounded Victron system, many folks wire this wrong and get spurious alarms.

A few practical notes from my setup:

• Set the response value conservatively — I'm running ~100kΩ threshold
• The relay output integrates beautifully with a Victron Cerbo GX via a digital input
• Check your DC system voltage range matches the CM-IWN.5 variant you've ordered — there are several flavours

The ABB documentation is decent but the application notes for IT systems are where the gold actually lives. Worth downloading before you start wiring.

Really useful thread. One thing worth flagging that I haven't seen mentioned yet — the CM-IWN.5 operates on a 0–10 kΩ measurement range, so you need to think carefully about your system capacitance to earth, particularly if you're running long DC cable runs or have significant PV array capacitance. In larger arrays this can cause nuisance tripping or skewed readings.

Also worth noting the relay needs a separate auxiliary supply (24–240V AC/DC) — don't assume you can pull that directly from your inverter output without proper isolation upstream. On my own Victron-based setup I feed the auxiliary from a dedicated small transformer precisely to avoid that headache.

The response time adjustment (t-on delay) is your friend here — dial it in carefully before going live rather than chasing phantom faults afterwards.

Good point from @Ozzy on the measurement range. Worth adding — on an IT system like most off-grid Victron setups, the CM-IWN.5 monitors both L1 and L2 to earth simultaneously, which is the whole point. Single fault won't trip your system, but you know it's there.

One thing I haven't seen mentioned: the response time delay is adjustable (0.1–10s), which matters if you've got switching transients causing nuisance trips. Had that issue with my Multiplus cycling loads in my garden office — wound the delay up slightly and it settled right down.

Also worth checking your local regs — in the UK, BS 7671 (18th Edition) has specific requirements around insulation monitoring on IT systems, so make sure your installation documentation reflects that. Some DNOs are funny about it on grid-tie hybrid setups.