Arc flash studies for solar, wind and battery storage sites

An arc flash is what happens when electricity jumps through the air between two conductors instead of flowing through the cable it was meant to. It is an explosion of heat and light that lasts a fraction of a second and can burn a person standing two metres away. An arc flash study works out, for every switchboard, panel and inverter on a site, how much energy that explosion would release, so the people who open those doors know what to wear and how far to stand back.

This article is for the people responsible for solar farms, wind farms, battery storage sites and the industrial sites next to them: operations managers, asset owners, health and safety leads, and the contractors who maintain the equipment. It explains what a study produces, when the rules in the UK and the US expect one, why renewable energy sites are a special case, and why operators with more than one site usually have the work done by a specialist.

What an arc flash study produces

An incident energy figure for every point where someone might work live. Incident energy is the heat that would reach a person at working distance if a fault arced at that point. It is measured in calories per square centimetre, and it depends on the fault current available, how long the protection takes to clear it, and the geometry of the enclosure.

An arc flash boundary. The distance from the equipment at which the energy falls to a level that would cause no more than a curable burn. Inside that boundary, people need protection.

A protective clothing category. The arc-rated clothing and face protection needed to work inside the boundary, matched to the incident energy.

Labels. Each item of equipment gets a label stating the incident energy or the clothing category, the boundary and the nominal voltage, so the information is at the door, not in a filing cabinet.

Recommendations. A study almost always finds places where a change to protection settings, a faster relay or a different operating procedure brings the energy down. That part of the report is where the money is saved, because lowering incident energy is cheaper than dressing people for it.

Why solar, wind and battery sites are a special case

A study written for a factory switchroom does not transfer to a renewables site, for three reasons.

Inverters limit fault current. A transformer feeding a fault delivers many times its rated current. An inverter delivers only a little more than its rating, because its electronics will not allow more. That can make the AC side of a solar or battery site look benign, but a lower fault current also means the protection takes longer to trip, and a longer arc can release more energy than a bigger, faster one. The study has to model the inverters as they actually behave.

Batteries deliver fault current with nothing upstream to stop them. A battery string is a DC source that will feed a fault until something between it and the fault opens. The energy available at a battery rack, a DC combiner or a DC disconnect has to be calculated on the DC side, with different methods from the AC side.

Solar arrays now run at 1,500 V DC. Higher DC voltages sustain an arc more readily and over longer gaps. The DC combiners, inverter DC inputs and disconnects on a modern array all need their own assessment.

A study that models the site as a conventional AC network, with the inverters and batteries treated as ordinary sources, will get the wrong numbers at exactly the places where people spend the most time working.

When the rules expect one

In the UK, the Electricity at Work Regulations 1989 require every electrical system to be constructed and maintained so as to prevent danger so far as is reasonably practicable, and they define injury to include arcing and explosion. The Management of Health and Safety at Work Regulations 1999 require a suitable and sufficient risk assessment for anyone the work could put at risk. Together those two duties mean that if people work on or near equipment that could arc, the employer has to know how severe the arc would be, and the only way to know is to calculate it. The Health and Safety Executive's guidance on safe working practices, HSG85, covers the arc hazard directly. The study is not named in law; the duty to assess the risk is.

In the US, the workplace electrical safety standard NFPA 70E requires an incident energy analysis, requires it to be reviewed for accuracy at intervals of no more than five years, and requires it to be updated whenever the electrical system changes in a way that could affect the results. The calculation method it references is IEEE 1584-2018. Labels showing the incident energy or clothing category, the arc flash boundary and the nominal voltage have to be fixed to the equipment. Federal safety inspectors enforce the underlying duty.

In both countries, insurers, lenders and tender questionnaires increasingly ask whether a current study exists. An operator that cannot produce one is carrying a liability it has not measured.

Why operators with many sites outsource the study

The request that prompted this article came from an operator with sites across the country that could not do the studies in-house. That is the normal position, and the reasons are practical rather than technical.

The study needs current information that is scattered. Up-to-date single line diagrams, the protection relay settings as they are actually set, the fault level letter from the network operator for each connection, and equipment data for every inverter and battery. Gathering that for one site is a job; gathering it consistently for twenty is a project.

It needs modelling software and someone who uses it every week. The calculations are not done by hand. The software licences are expensive and the skill decays if it is used once a year.

It needs the same method applied everywhere. An operator with a mixed fleet wants one consistent set of labels and procedures across all its sites, not twenty interpretations by twenty contractors.

It needs to be repeated. Every protection change, every inverter replacement, every added battery container shifts the numbers. A five-year cycle with updates in between is easier to run through one provider who holds the models.

We offer arc flash studies through our UK consultancy team, who already run the power system studies that a study of this kind draws on: fault level, protection coordination and the compliance work described in our guide to what changes when a solar farm changes hands. A study that shares its model with the protection study is faster, cheaper and less likely to disagree with itself.

What to have ready before you ask for a quote

1. Single line diagrams for each site, as built, including any changes since commissioning.
2. Protection settings for every relay and breaker in the study scope, as set, not as designed.
3. The network operator's fault level data for the connection point, usually in the connection offer or available on request.
4. Equipment data for inverters, batteries, transformers and switchgear: ratings, fault contribution where the manufacturer states it, enclosure types.
5. A list of where people work live, so the study concentrates on the points that matter.

If some of this does not exist, say so. Reconstructing drawings and settings is part of the job, and knowing that up front changes the quote more than the size of the site does.

How to decide whether you need one now

1. Do people open live equipment on your site? If maintenance, inspection or fault-finding happens with the power on, the duty to assess the arc hazard already applies.
2. When was the last study, and what has changed since? Any new inverter, battery, transformer or protection setting can move the numbers. No study, or one older than five years, means the labels on the doors are guesses.
3. Could you produce the study if an insurer or a tender asked tomorrow? If the answer is no, that is the deadline.

---

Do you have sites without a current arc flash study? Renewable Energy & Drives runs arc flash, fault level and protection studies for solar, wind, battery and industrial sites, using one model per site so the results agree with each other. Start a consultation, email sales@renewable-d.com, or call +44 116 279 6131 (UK) or +1 414 562 6559 (US).