Energy

Dropping to Backup Is Now a Grid Code Breach

EirGrid measured 900 MW of demand vanishing during voltage dips. Its fix makes riding through mandatory, and the relief pins your load to 2025.

EnergyBy Servola Tech Desk2026-07-274 min read

AI-assisted, edited by humans. | Editor-in-Chief: Leon Soliman | Editorial standards

Dropping to Backup Is Now a Grid Code Breach

Key takeaways

EirGrid and SONI set an upper limit of 900 MW on how much demand can drop off during a transmission fault before system stability is at risk, and the fix makes riding through the dip mandatory, so the reflex of switching to backup power becomes the non-compliant act.
Three obligations apply to existing and future transmission-connected demand facilities: staying connected inside a defined voltage-against-time profile, tolerating rate-of-change-of-frequency events up to plus or minus 1 Hz per second measured over a rolling 500 milliseconds, and restoring at least 90 percent of demand within 500 milliseconds of voltage returning to 90 percent of nominal.
The group derogation for data centres buys 24 months and costs a Demand Utilisation Threshold, a fixed monthly limit in MVA on non-compliant demand calculated from 2025 consumption. The relief is priced in growth.
The CRU consultation closes on 29 July 2026 and the modification takes effect five weeks after approval. A site with no derogation on that date is non-compliant with the Grid Code and exposed to demand control.

What EirGrid measured when the voltage dipped

A transient fault on a transmission system is an ordinary event. A lightning strike, a tree, a piece of switchgear operating as designed. The voltage sags for a fraction of a second and recovers. Across Ireland, when that happens, data centres do exactly what they were built to do. They detect the dip, reduce their draw from the grid, switch to their own backup supply, and stay there for an extended period rather than returning the moment the voltage does.

EirGrid and SONI added up that behaviour and found a number. Beyond roughly 900 MW of simultaneous demand loss, system stability is at risk. Every individual site is behaving prudently. The sum of the prudent behaviour is the hazard, and it lands in the same instant, because every facility is responding to the same dip with the same automatic logic.

That reversal sits at the centre of Grid Code Modification MPID 345, now before the Commission for Regulation of Utilities. The protective action that keeps one facility online is the action that threatens the system every facility depends on. Ireland is writing the correction into its Grid Code, and the consultation closes this week.

Three obligations, all measured in milliseconds

Fault ride through is the headline requirement. A transmission-connected demand facility must remain connected and operate stably through a voltage disturbance, provided the deviation stays inside a defined voltage-against-time profile at the connection point. Inside that envelope, disconnecting stops being an engineering choice and becomes a failure to meet the code.

The second obligation covers frequency. Facilities must remain connected during rate-of-change-of-frequency events up to plus or minus 1 Hz per second, measured over a rolling 500 milliseconds. The third governs the return. Once the fault clears, a facility must restore at least 90 percent of its demand within 500 milliseconds of voltage recovering to 90 percent of nominal, with ramp rates agreed with the transmission system operator.

Read the third one twice, because it is the one that surprises people. Surviving the dip is not enough. The load has to come back fast and predictably, which makes the recovery behaviour of your cooling plant and your uninterruptible supply a regulated characteristic rather than an internal design detail. All three obligations reach existing facilities, not only new connections.

The derogation is a growth cap with a friendly name

EirGrid accepts that existing sites cannot comply on day one, so the framework offers three routes. A standard process, in which a facility submits a compliance plan for assessment by the transmission system operator. A group process for data centres, which requires plans within three months of the effective date and grants derogations lasting 24 months. And a group process for non-data-centre facilities, which allows 12 months to produce plans.

The group route is the one most operators will take, and it carries a condition worth reading closely. A data centre granted a derogation becomes subject to a Demand Utilisation Threshold, a fixed monthly demand limit in MVA covering its non-compliant demand, calculated from 2025 consumption using SCADA data, meter readings, or the higher of the two.

That is the sting. The relief is real, and it is denominated in last year's load. For 24 months the non-compliant portion of your demand is pinned to what the site was drawing in 2025, at precisely the point in the cycle when AI workloads are pushing rack density and site draw upward. The derogation does not buy time to grow. It buys time to retrofit, on a fixed ration.

What has to happen before Wednesday

The CRU consultation closes on 29 July 2026 and a decision is expected shortly after. The modification takes effect five weeks after approval, which makes the compliance clock short and knowable. A facility without a derogation on the effective date is non-compliant with the Grid Code and carries a raised risk of demand control, which means curtailment instructed by EirGrid rather than chosen by the operator.

Three actions are worth taking this week even if you never file a response. Establish whether the site is transmission-connected, because that is the trigger and the grid connection agreement answers it as a matter of fact. Pull 2025 consumption from SCADA and from meter records and compare them, since the threshold is set on the higher of the two and the gap between those sources is frequently material. And ask your electrical engineer what the protection settings actually do at 80 percent voltage, because most operators believe the answer is ride-through and the relay believes otherwise.

For operators outside Ireland, the reason to read this is not Irish law. It is that the underlying problem is not Irish. Any grid carrying a dense cluster of large loads with fast automatic transfer schemes has the same exposure, and Ireland is simply the market where the concentration made it arrive first.

Frequently asked questions

Does this apply to a data centre that is not transmission-connected?

The three obligations in MPID 345 are directed at transmission-connected demand facilities, so the first question for any site is which network it connects to, and the grid connection agreement answers it. Distribution-connected sites sit outside this particular modification, though the stability problem it addresses does not stop at the voltage boundary and connection policy in Ireland has been tightening across both.

What does the Demand Utilisation Threshold actually limit?

It sets a fixed monthly demand limit in MVA applying to the non-compliant portion of a derogated facility's demand, calculated from 2025 consumption using SCADA data, meter readings or the higher of the two. In practice it holds non-compliant load at the prior year's level for the 24 months the group derogation runs, so any growth has to come from capacity that already meets the new obligations.

Why is switching to backup power a problem if the site stays running?

Because the site staying running is not the concern. When many large facilities disconnect within the same instant in response to the same voltage dip, the transmission system loses a large block of demand at once. EirGrid and SONI put the upper limit at 900 MW before stability is at risk, which is why remaining connected inside a defined voltage-against-time profile is now an obligation rather than a preference.

Resilience engineering has always been written from the perspective of a single site. Detect trouble, disconnect, run on your own power, ride it out. At a few hundred megawatts of clustered load that logic stops being local, and Ireland has now measured the point where it turns and set it at 900 MW. Find out what your protection relays do during a voltage dip before a regulator tells you, because right now that answer is a design assumption rather than a tested fact.

EirGridSONICRUgrid codedata centresfault ride throughIrelandgrid stability

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