The grid was over-voltage and the proof was already in his inverter

Kerry got in touch because his solar inverter kept shutting itself down — several times a day, most days. He was already fairly sure the cause was high mains voltage. What worried him was the next bit: how do you actually prove that to a network operator, and get them to do something about it? As it turned out, the proof had been sitting inside his own inverter the whole time.

The problem — a system that kept dropping out

Kerry's FoxESS hybrid kept throwing the same fault and disconnecting from the grid. Fox ESS's manuals call it 10min Volt Fault (grid voltage out of range over the last 10 minutes); Fox Cloud's alarm log showed it as Code 28, 10 Minute Average Grid Voltage Fault. To him it looked like the system kept breaking. In fact it was protecting itself. He suspected the grid voltage was simply too high — something that can happen as more solar goes onto a network, and especially for homes near the end of a long supply cable — but he had no voltage logger, no obvious way to evidence it, and a very reasonable fear that a phone call saying "I think my voltage is high" would go precisely nowhere.

Why “the grid is too high” is hard to prove on your own

A network operator is not going to re-tap a transformer or reinforce a street on a hunch. They want evidence: the voltage, measured, over time, breaching the limit. Most homeowners don't own a power-quality logger — and by the time anyone comes out to check, the voltage may happen to look fine. That catch-22 is exactly why people give up.

But there is a piece most people miss. A modern grid-connected inverter measures the grid voltage at its own terminals, and Fox ESS Cloud keeps historic records and alerts that can be viewed. The evidence isn't something you need to go and capture: it may already be recorded by the very device that keeps tripping.

What the inverter’s own data showed

We exported the inverter's grid-voltage history and its fault log from the FoxESS cloud and analysed them. Across a 19-day window the supply voltage:

• peaked at 255.9 V — nearly 3 V over the 253 V statutory limit;
• exceeded 253 V on 17 of the 19 days;
• sat at or above 250 V for roughly half the time — chronically pressed against the ceiling.

And here is the part that settles the argument. A large share of the breaches happened overnight — between roughly 2 am and 5 am — when Kerry's solar was generating nothing at all. That single fact dismantles the usual deflection that "it's your own solar pushing the voltage up." At three in the morning there is no solar export, so a reading above 253 V can only be the network's own voltage. During the day, his solar then stacks on top of an already-high base and pushes it further over the limit.

Eighty-nine shutdowns in a fortnight

The inverter's fault log recorded 89 over-voltage disconnections in 15 days — about six a day, and as many as fourteen on the worst day.

Every one of those is the inverter's grid protection at work: it measured the grid voltage out of range and disconnected. The inverter wasn't faulty — it was the canary, faithfully logging a supply that was out of tolerance.

What the supply is legally allowed to be

The voltage a domestic supply must be kept within is set by the Electricity Safety, Quality and Continuity Regulations 2002. A 230 V supply is permitted to vary by +10% / −6% — a band of 216.2 V to 253 V — and the network operator carries a legal duty to keep it there. A supply repeatedly reaching 255.9 V is outside that band, full stop. Once a customer reports it, the DNO must respond within five working days, or offer to investigate within seven, under the Guaranteed Standards of Performance — with a penalty payment due to the customer if it does not.

Turning the data into something the DNO can’t wave away

We pulled all of it together into a formal report on letterhead: the voltage logs, the fault log, a day-by-day breakdown, the relevant regulations, and a plain explanation of why the inverter is behaving correctly while the supply is not. Crucially, every figure is sourced from the inverter's own records — not opinion. That is the difference between "I think my voltage is high" and a documented breach the network operator has a statutory duty to act on.

The point, for Kerry, is that he didn't need to buy equipment or win a technical argument. The evidence already existed. It simply needed extracting, analysing against the right limit, and presenting in a form the DNO cannot reasonably dismiss.

If your inverter keeps tripping on over-voltage

You're probably not imagining it. A grid-tied inverter that repeatedly drops out on a grid voltage fault has measured a voltage outside its limits. A high or weak supply can cause this, though the inverter's own export and the cabling at your property can add to what it reads, so the voltage should be confirmed at your incoming supply by an electrician. Your monitoring may already hold records that help. We can look through them, compare them with the statutory limit, and build the evidence pack for the network operator.

See our Fox ESS grid fault guide for what each alarm means and what you can check, and the Fox ESS fault code index for other alarms.

Field note by Ron Vitova, Solar Tech Support. Customer name changed and identifying details withheld; all figures taken from the system's own monitoring records.