Laajasalo reference case

An outage in electricity distribution, Laajasalo 23 august 2022.

In August of 2022 Helen Electricity Network had a significant power outage in the area covered by their Laajasalo substation, in the Finnish capital of Helsinki. The outage affected nearly 12,900 customers and lasted on average 88 minutes.

Arcteq’s AQ 100 series is a dedicated arc flash protection relays system, designed for the sole purpose of protecting against arc faults. The AQ 100 series is designed and tested according to the latest protection relay standards; this makes it suitable for installations in any environment, from utilities and power plants to heavy industry applications (e.g., offshore, marine, mining) as well as commercial and institutional electrical systems. Its modular design makes the AQ 100 series an excellent candidate for both new and retrofit installations.

For several years now Arcteq has supplied Helen Electricity Network Ltd with AQ 100 series arc flash protection products, used in many of their substations.

Event description, Helen Electricity Network Ltd

In August of 2022, Helen Electricity Network had a significant power outage in the area covered by their Laajasalo substation, in the Finnish capital of Helsinki. The outage affected nearly 12,900 customers and lasted on average 88 minutes. The outage lasted for two and a half hours at the longest, while some customers had their power restored within 25 minutes. However, SAIDI (the average total duration of outages per customer) was over two minutes, which is clearly higher than the combined index result of all other outages from 2022 up to this point.

Why did the power outage last as long as it did and why was it so extensive in a redundant distribution network? The Laajasalo substation was designed to have two separate networks, with Main Transformer 1 feeding one of the busbars and Main Transformer 2 feeding the other. This particular outage was a result of a previously unfamiliar event: a short circuit occurred in both of the two separate MV busbars in the substation at the same time. The outage began within a Duplex-type MV switchgear, when an earth fault occurred in the contacts of an absent withdrawable circuit breaker located within one of the substation’s incoming feeders. Within one second, the fault became a three-phase short circuit fault, and the resulting arc flash fault then used the open circuit breaker space to spread over to the energized tie cable end of the other busbar, causing a short circuit in there as well. Thus, two galvanically isolated distribution networks both had a short circuit fault at the same time.

The arc flash protection supplied by Arcteq Relays Ltd quickly and correctly triggered the main circuit breakers of both feeders, rendering the substation dead. Other protective devices and automation also functioned adequately. The total operating time for the protection —including the operating times of the circuit breakers themselves— was 60–70 milliseconds. The fault current was approximately 14 kA in total, with about 7 kA from each transformer.

The substation control quickly indicated a faulty switchgear, but the event caused personnel much confusion when trying to understand how both busbars could have a fault at the same time. It also took a while to locate the second fault. This meant that getting the substation back to operational had to be done very carefully, eventually done by only using a section of one busbar in the process. Electricity was restored to customers in the most part via the distribution network with the help of transformer substation automation.

The medium-voltage switchgear in which the original earth fault occurred was approximately 30 years old, and had functioned reliably without similar faults. It is possible that the tension spring of the contact’s L1 terminal had broken or loosened, thus first causing an arc fault to earth which then spread to an arc flash short circuit. While this particular switchgear had been inspected a couple of years before this fault event, in the future these contacts will be examined even more carefully during maintenance to prevent this from happening again.

According to the information received from the switchgear manufacturer, there have been no similar fault events anywhere else.

Mika Loukkalahti
Leading Expert
Helen Electricity Network Ltd


Mikko Manninen, Arcteq’s product manager for arc flash protection, commented on the Laajasalo substation double fault event: “Reports of arc flash events come very often and from a variety of applications. I’m glad to hear that our products have made the day safer.

Manninen is grateful that the Lead Expert from Helen, Mika Loukkalahti, provided a very detailed description of the event, as it allows everyone to learn from the event and share this information to others, which might help increase safety even further.

The AQ 100 series has been designed to be as fast and comprehensive an arc flash protection as possible. This is why it is able to function quickly and correctly even in difficult situations such as the one in Laajasalo. Within a short amount of time, I have seen many reports of similar incidents all over the world, situations where arc flash protection worked as intended and helped save human lives and prevent damage to equipment,” concludes Manninen.

Helen Electricity Network Ltd focuses on electricity network operations as defined by Finnish law. It offers its customers electricity distribution and transmission services in nearly all of the capital region of Helsinki. In terms of customer numbers, Helen is Finland’s third-largest company focused on electricity distribution, and it provides outstanding security of supply for its nearly 420,00 customers. Helen Electricity Network is part of the Helen Group, which is owned by the City of Helsinki.

AQ 100 arc flash protection system