Working on your electrical installations inevitably involves the risk of arcing. This danger occurs, for example, when opening a live cabinet, or when employees are in the vicinity of open bus bars or conductors. Working on a de-energised cabinet whose components are inadvertently still live also poses a critical risk. Although arcs are relatively rare, the consequences can be disastrous for your staff and infrastructure.
In a confined space, an arc blast builds up in a fraction of a second; this process usually takes a few milliseconds to half a second. During this extremely short time, the pressure continues to build up continuously. The exact duration of this is highly dependent on the current-time characteristic and the reaction speed of your protection devices, such as fuses or circuit breakers. Within this time period, the pressure in the room can rise to a multiple of the standard atmospheric pressure.
This pressure build-up is particularly dangerous for anyone in the vicinity, and can even cause buildings to be completely or partially destroyed. Everything within a certain radius can be destroyed in seconds. Windows break, wood splinters and even heavy masonry and metal can break. Practical measurements show that an atmospheric pressure increase of just 20% can be enough to destroy walls. For safety reasons, it is therefore necessary to calculate how this pressure wave builds up and what pressure a person will experience at a specific distance.
Many buildings were not originally designed specifically for the heavy electrical installations they currently house. Moreover, people are not always aware how destructive an arc is, and do not realise that excessive frugality during construction can significantly increase the risk of damage. While the use of Personal Protective Equipment (PPE) is essential to mitigate immediate thermal hazards, it is equally important to understand the build-up of pressure to reduce structural damage.
Predicting the behaviour of these arc blasts through accurate calculations contributes directly to safety, as you gain precise insight into the risks and learn how to minimise them. The volume of your building or space is very important here. Depending on the available volume, an arc blast can push employees away, or even lead to the complete disintegration of physical structures such as engine panels and entire rooms. Panels should ideally be designed to withstand an arc blast. When cabinets are in an open arrangement, pressure is released on all sides, effectively causing the surrounding building or space to act as a new compartment.
By taking arc blast scenarios into account as early as the design phase of an installation, you can take appropriate measures. For example, structural elements can be modified or reinforced, and you can define vent routes through which pressure can escape with minimal damage. It is definitely worth performing these calculations on cabinet compartments, as this provides peace of mind regarding arc blast impacts.
While the focus in industry has traditionally been on the thermal energy of an arc, relatively little research has been done on the associated arc blasts. An arc blast expands from its source in three dimensions. Quantification of this pressure is defined as a combination of the expected initial pressure at the time of arc creation, and the further pressure rise until the arc is completely extinguished.
Although methodologies for arc blast studies are less established and widely accepted than those for thermal arc calculations, approaches based on scientific studies and publications are available. These approaches require a keen understanding of the associated assumptions, limitations and discrepancies.
A 'normal' arc blast measurement (aimed at determining the right PPE based on energy values) mainly requires details of the cabinet and installation. However, an arc blast calculation requires more: you also need detailed building data, including layout, dimensions and volume. The results of these calculations show exactly what energy levels can be expected and what pressures will occur in a building, including the forces occurring at different locations and distances.
To translate theory into your practice, we explain two cases where arc blast pressure rise has been accurately calculated based on measurements and documentation. These analyses result in targeted recommendations to mitigate the consequences of an incident. Here, it is crucial to mention that optimising the timing of arc blast shutdown directly reduces the overall pressure level. A person's exposure is further affected by the initial blast and the usual working distance to the plant.
In this situation, a new building was connected to the public grid with a short-circuit current of 8.77 kA at the connection point. Via a 630 kVA transformer with a short-circuit Uk value of 6%, the voltage was reduced to 400 V. The challenge was that an arc could potentially occur on the medium-voltage or low-voltage side of the transformer, which was located in a room within a large building. The client wanted to make sure that the walls specified by the builder and the circuit breaker installed would actually withstand various arc scenarios. To calculate the arc pressure at the transformer, it was important to determine the dimensions of the room and the distance between electrodes (for both medium- and low-voltage).
Recommendations from HyTEPS:
This case involved a 5 MW boiler located next to a staircase and supplied with medium voltage via an open electrical box with electrodes on top. HyTEPS calculations of incident energy, initial pressure, pressure rise, temperature and heat transfer showed that high energy levels were expected. However, the large volume of the building had a positive influence: the pressure could not rise as high as it would inside a small cabinet compartment. The hotspot of temperature was at the electrodes, with heat transfer decreasing the further the object or person was from the arc location. Here, it is crucial to realise that even an atmospheric pressure increase of only 20% could literally destroy walls.
Recommendations from HyTEPS:
In practice, calculating and mitigating arc blasts involves a fundamental increase in the safety and structural integrity of your facility. The insights gained from this provide you with the tools to make targeted investments in the right protection devices and venting systems. At the same time, you ensure the protection of your staff and prevent catastrophic physical damage or lengthy production stoppages after an incident. You thus ensure a safe installation. After all, no one should have to worry about the reliability and safety of electricity.
Want to dive deeper into the subject matter?
Want to know exactly how to understand and control the destructive forces of an arc blast in your own installation? Then follow our Power Topic: Arc & Arc Blast: From Theory to the Shop Floor. In it, our engineers share practical analysis and real-life examples from heavy industry.
The safety of your staff and the continuity of your processes do not tolerate assumptions. Because the volume of the room and the specific current-time characteristics determine the impact, every electrical installation reacts differently to an arc.
Here's a clear take on the situation surrounding your electrical safety: by understanding the risks in advance, you can prevent unforeseen structural damage. Let us explore how we can calculate and minimise the physical risks of an arc blast for your specific installation.
