Dealing with arc blast pressure

Dealing with arc blast pressure through calculations and preventive measures

Work on electrical installations is often accompanied by a risk of arc flash phenomena. For example, when opening a live cabinet to perform work on it, or when standing in the vicinity of electrical components with open busbars or conductors, or when performing work on a previously de-energized cabinet on which some parts have remained energized by accident. These events are rare, but the consequences can be disastrous. Besides blinding light and temperatures in excess of 19,000 degrees Celsius, an arc flash blast (caused by an arc flash) will generate a pressure wave with a deafening bang.  

In an enclosed space, a blast from an arc flash event can take a few milliseconds, or maybe half a second, and during that time, the pressure builds. The cut arc time depends on how fast the protection device (fuse or circuit breaker) reacts according to its settings or current-time characteristic curve. An arc blast can raise the pressure in a space to several times the standard atmospheric pressure. That’s not only extremely dangerous for anyone standing nearby, but it can also partly or completely demolish buildings. Anything within the blast radius can be destroyed in seconds. Windows may be broken, wood may be splintered, and brickwork and metal can even break. Apart from this pressure build, we also need to calculate the blast pressure someone would receive at a certain distance of the arc event.


Contributing to safety

Often, buildings haven’t been purpose-built for installations. Calculating the behavior of arc blasts before they take place contributes to safety, by providing insight into the risks and how these can be minimized. We have published extensively on risks related to arc flashes and PPE. However, it’s also vital to understand the buildup of pressure to ensure structural damage to buildings can be limited. This is done by taking adequate steps to adapt or reinforce structural elements or provide a path for pressure to escape with minimal damage.

The volume of a building is critical to arc blast pressure calculations. Depending on the volume available, the pressure wave can harm nearby employees (pushing them away) or lead to disintegration of physical structures such as motor panels or, in extreme cases, building rooms. Normally the design stage of an electrical installation should consider the arc blast scenario and define a proper vent / escape release. However, people aren’t always aware of how dangerous and destructive an arc flash event can be, or aware of the fact that savings made during construction might add significantly to arc blast risk. 

Peace of mind

Panels should be able to handle a pressure blast by design. If they are open, the pressure is released everywhere so the building or room if effect becomes a new ‘compartment’. It may be worth performing arc blast calculations on cabinet compartments into offer customers peace of mind regarding blast pressure.

For a ‘regular’ arc flash measurement you need the details of the cabinet and installation in question. Based on the resulting energy value, you can define PPE. For an arc blast calculation, you also need building data, such as measurements, layout, and volume. The results show customers what energy levels they can expect and the pressure that will grow inside a building, including forces at different distances and locations. Optimizing the moment at which the arc is cut out reduces overall pressure levels. The pressure to which a person in the vicinity might be subjected is affected by the initial blast and the common working distance from a machine.  

Despite the increase in awareness on incident energy in the event of an arc flash, far less research has been conducted into arc blast pressure. Pressure waves expand over the three dimensions from the source. The quantification of arc blast pressure is defined as the combination of the initial arc blast expected in the instant the arc occurs as well as the pressure rise or buildup over time until an arc flash is completely extinguished. Arc flash studies are based on established methodologies, but these aren’t as well-established and widely approved for arc blast calculations. HyTEPS has developed an approach based on scientific research and papers and an understanding of their limitations, discrepancies, and assumptions.

Practical measurements

Below are two recent examples of projects in which HyTEPS performed measurements and Arc Blast Pressure buildup calculations, based on customer documentation and our own measurements. Analysis is followed by customer recommendations to mitigate arc flash outcome.

Case 1: electrical transformer room

A new building was scheduled to be connected to the public grid with a strength at Point of Connection (POC) of 8.77 kA. The voltage is stepped down to 400 V via a 630 kVA transformer with a short-circuit Uk value of 6 %. A potential arc would occur at either Medium Voltage or Low Voltage side of an electrical transformer located inside a room of a large building. The client wanted to know whether the walls as specified by the builder and installed circuit breaker would in fact withstand a variety of blast scenarios.

As the arc blast pressure must be calculated at the transformer, it is important to determine the dimensions of the transformer room and the gap or distance between the bolts/electrodes where an arc can occur (for both Medium and Low Voltage).

HyTEPS gave the following recommendations and considerations

Investigate whether the obtained pressure values are safe and take further action if needed.
Study whether a pressure release valve to the exterior is or could be implemented.

Case 2: 5 MW vat inside a building

A vat located close to a staircase with medium voltage supply (open electrical cabinet with electrodes located on top). HyTEPS provided result calculations in case of an arc on the kettle electrodes: incident energy, initial arc blast pressure, arc blast pressure rise, temperature and heat transfer. High levels of energy were expected but the volume of the building has a positive impact on the calculation – the pressure cannot build up as much as it would inside a small cabinet compartment. The temperature hotspot is at the vat electrodes and, the further the object / person from the arc flash location, the lower the heat transfer of heat received.

It’s important to realise that even the sturdiest building isn’t indestructible – measurements we made at one client’s facilities showed that an atmospheric pressure increase on just 20 % could literally destroy the walls.

HyTEPS gave the following recommendations and considerations

Lower relay short-circuit time setting. The faster a circuit breaker trips in case of an arc flash event, the better (without increasing the risk of the breaker malfunctioning). There are no other cabinets downstream of the vat so operation would not be affected due to selectivity.
Study the possibility of installing a wall that could handle energy, pressure, and protect employees walking on the stairs nearby, ensuring they would not be subjected to a high initial arc blast.
These first two recommendations need to be performed in combination – it would not be advisable to install a wall without a vent or pressure release system connected to the outside and vice-versa.
Install a pressure and toxic vapors release ensuring there is a release in case of an arc.
Recommend the employees to wear earplugs near the staircase as an arc flash bang could result into temporary or even permanent deafness.

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