An arc flash study collects data on your electrical equipment, builds a computer model of the system, calculates fault currents and protective device clearing times, and from those works out the incident energy and arc flash boundary at each piece of equipment. The results go onto equipment labels and into a report that your electrical safety program relies on. Most of the effort, and most of the value, is in getting the field data right.
Why a study, not a table
CSA Z462 allows two broad ways to choose arc flash PPE: an incident energy analysis, which is what a study produces, or the PPE category tables in the standard. The tables only apply within stated limits of fault current and clearing time, and checking that equipment falls within those limits needs much of the same information a study does. For a building with more than a few panels, a study usually gives better, more specific answers. It also finds problems along the way, such as underrated equipment or breakers that coordinate poorly. For what the results look like on equipment, see reading an arc flash label.
The steps
1. Define the scope and gather records
The study starts by agreeing which equipment is included, usually everything from the service down to panels and motor control centres where energized work could happen. Existing single-line diagrams, past studies, panel schedules and as-built drawings save time, even if they are out of date. The utility is asked for the available fault current and protective device information at the point of supply.
2. Collect field data
Someone has to open and look at the equipment. For each piece of gear in scope, the data collector records:
- Manufacturer, model and ratings of switchboards, panels and transformers
- Every protective device: type, frame and trip rating, trip unit and the actual settings dialled in
- Fuse class and rating
- Cable size, material, number of conductors per phase and approximate length
- Large motors, generators, UPS units and other sources
- Equipment names, and the physical arrangement such as enclosure size and conductor gap, which affect the calculation
Collecting data inside energized equipment is itself hazardous. It must be done by qualified workers following the site's procedures and arc flash precautions. Some data can only be collected safely during a planned shutdown, so this step needs coordination with building operations.
3. Build the model and calculate fault current
The engineer enters the data into specialized software to build a model of the system. The software calculates the available fault current at each bus. See available fault current explained.
4. Check coordination and clearing times
The arc lasts until a protective device upstream opens. Clearing time, read from the device's time-current curve at the arcing current, is one of the biggest factors in incident energy. This is why the study usually includes a review of selective coordination using the same model.
5. Calculate incident energy and boundaries
Using a recognized method, commonly IEEE 1584, the software calculates the incident energy at the working distance and the arc flash boundary at each piece of equipment. It usually checks more than one operating scenario, such as normal utility supply, generator supply and reduced fault current from the utility, and reports the worst case.
6. Look at mitigation
Where incident energy is high, the engineer looks at options. Common ones include adjusting breaker settings, adding a maintenance switch that temporarily speeds up tripping while people work, using current-limiting fuses, or remote racking and operation so workers stand further away. Each option is tested in the model before it is recommended.
7. Labels and report
Labels are produced for each piece of equipment, with names matching the drawings. The report documents the data, assumptions, scenarios, results, recommended settings and any equipment found to be underrated.
What you need ready before the study
- Existing single-line diagrams, as-built drawings and any earlier studies
- Utility account and service address, so the utility contribution can be requested
- Access to all electrical rooms, keys and escorts
- A contact in operations who can plan any shutdowns needed for data collection
- Information on recent changes, such as new equipment or a service upgrade
- The site's electrical safety program, so the study's outputs fit how it chooses PPE
What the report should contain
- Scope, method, the standard and calculation method used, and the date
- The utility data used and its source
- An updated single-line diagram matching the labels
- Results tables for each bus and each scenario studied
- Recommended protective device settings, and a record of which were applied
- Equipment found to be underrated for the available fault current
- Mitigation recommendations with the expected effect on incident energy
- All assumptions, especially where data could not be collected
A typical case
A mid-sized manufacturing plant commissions its first arc flash study. The existing single-line diagram is twenty years old and missing several panels added since. Data collection takes longer than planned because two motor control centres can only be opened during the plant's annual shutdown. When the model is run, most panels show moderate incident energy, but the main switchboard and one large motor control centre show high values. The cause is a main breaker with no instantaneous trip and long delay settings. Testing options in the model shows that a maintenance switch on the main breaker, used only while people work downstream, brings the values at the motor control centre down substantially. The plant installs labels with the normal values, adds the maintenance switch procedure to its electrical safety program, and schedules a review for when it adds a new production line.
Common mistakes
- Assuming default breaker settings. The settings actually dialled in are often different, and they drive the results.
- Using an old utility value. Utility systems change, and an outdated value can understate or overstate the hazard.
- Leaving out the generator scenario. On generator supply, fault current is lower and breakers may trip much more slowly, which can raise incident energy.
- Putting labels up and forgetting them. The study must be kept current as the system changes.
A study does not make equipment safe. It describes the hazard so that work can be planned. The safest approach remains to de-energize equipment and establish an electrically safe work condition before work begins.
How ehZee can help
Our arc flash studies and labelling service covers scoping, data collection, modelling, labels and the report, and we walk you through the results and the mitigation options. We can also update an existing study when your system changes.
Common questions
How often does a study need to be updated?
Whenever the system changes in a way that could affect the results, and at the periodic review interval in CSA Z462. Check the current edition and your provincial requirements for the interval.
Will the study require a shutdown?
Not always. Much of the data can be collected from outside equipment or from records. Some equipment, though, can only be opened safely when de-energized, so a short planned outage is common for part of the work.
Can we use the study for anything else?
Yes. The same model supports short-circuit and coordination reviews, future service upgrades and planning for new equipment, which is a good reason to keep the model files.