Today, dependence on functioning digital infrastructure is one of the most significant economic and risk factors – and the requirements for data centre availability classes are correspondingly high. Companies, public authorities, medical facilities, critical infrastructure operators – they all face the same pressure: any unplanned failure of digital systems can bring operational processes to a standstill or even endanger human lives.
The figures speak for themselves. According to studies, just one hour of downtime costs 98% of companies more than £100,000. In large corporations, the damage caused by downtime quickly runs into millions – per hour. The average damage worldwide is around £9,000 per minute. In highly regulated or systemically important sectors, this figure can be significantly higher.
The increasing complexity of digital processes, the trend towards real-time processing and competition for immediate customer access are further exacerbating this dependency. Uptime is therefore becoming a basic business requirement – not only in Frankfurt, but also in Singapore, Dallas and São Paulo.
This makes the question of data centre availability a strategic one. Anyone planning or operating infrastructure today must be able to demonstrate how fail-safe and redundant the system is. So-called "data centre availability" is becoming a key performance indicator – comparable to a credit rating in finance or a CO₂ footprint in ESG assessment.
The global market for data centres is expected to grow to over one trillion US dollars by 2034. In this environment, availability classes help to strategically align investments: they structure requirements, make operational risks measurable and create a common language between IT, architecture, operations and management.
But what exactly is behind availability classes AC1 to AC4 according to EN 50600 and Tier I to Tier IV according to the Uptime Institute? Which specific redundancy models – N, N+1, 2N or 2N+1 – are behind them? And why have these systems become international benchmarks for reliability and digital resilience?
Standardised availability is not a technical detail, but a planning tool. Availability classes are standard-based categories that can be used to classify the operational readiness of data centres – regardless of specific technologies or providers. They do not indicate how a data centre is specifically structured, but rather the operational reliability and fault tolerance that its design should achieve.
The basis is the European standard EN 50600 (also: DIN EN 50600) or the internationally recognised Tier model of the Uptime Institute. Both models follow a four-level system – VK1 to VK4 (or AC 1–4) and Tier I to IV – and define target values for redundancy, maintainability and fault tolerance.
The real value of availability classes lies in their standardisation. While terms such as "highly available" or "redundant" were used inconsistently in the past, EN 50600 and the Tier models now provide an objective framework. This creates transparency:
A key advantage: each level – whether Tier or VK – represents a clearly defined level of availability that can also be translated into downtime per year. The models therefore enable internationally consistent evaluation, regardless of whether a data centre is located in Europe, Asia or North America.
Redundant components alone do not guarantee high data centre availability. Only when the entire infrastructure is consistently designed for reliability – including power supply, cooling, network connection and security concept – can a resilient level be achieved.
Availability classes help to structure precisely this complexity. They replace technical gut feeling with a comprehensible target architecture. EN 50600 forms the European framework and is also the basis for the international standard ISO/IEC 22237.
Both systems pursue a similar goal, but differ in their assessment approach and scope:
EN 50600 (VK1–VK4):
– Focus on design architecture
– Assessment via planning, documentation and infrastructure concept
– Holistic approach: also includes energy efficiency, security zones, operational management
– Considered the European reference and basis for ISO/IEC 22237
Tier system (Tier I–IV):
– Focus on specifically verifiable fault tolerance
– Measurable uptime values for each level: e.g. Tier III with 99.982% (1.6 hours downtime/year), Tier IV with 99.995% (26 minutes)
– Over 3,500 certificates issued worldwide
– De facto standard for international operators – especially in North America and Asia
EN 50600 dominates in Europe, partly because it can be certified by independent bodies such as TÜV. Internationally, the Uptime Institute is the market leader for Tier certificates. Many operators now pursue a dual certification strategy: VK certification for regional regulations, Tier certification for international customer requirements.
The two leading systems for classifying data centres – EN 50600 and the Uptime Institute's Tier model – differ in their methodological approach but lead to comparable levels of reliability in terms of content. One system is design-based, the other is key figure-driven. Both enable a structured classification of availability and form the basis for strategic infrastructure decisions.
The following tables show a direct comparison of how Tier levels and availability classes (AC) differ in terms of availability, redundancy model and typical areas of application.
| Tier level | Availability (year) | Downtime (year) | Redundancy model | Typical application |
|
Tier I |
99,671 % |
Approx. 28.8 hours |
Single supply, no redundancy |
Small businesses, development, test environments |
|
Tier II |
99,741 % |
Approx. 22 hours |
Partial redundancy (N+1), one supply path |
SMEs, internal services, non-critical systems |
|
Tier III |
99,982 % |
Approx. 1.6 hours |
N+1, multiple paths, maintainable during operation |
Enterprise IT, SaaS, e-commerce, healthcare |
|
Tier IV |
99,995 % |
Approx. 26 minutes |
2N or 2(N+1), fully fault-tolerant |
Banks, stock exchanges, critical national infrastructure |
| AC class | Availability (year) | Downtime (year) | Redundancy model | Typical application |
|
AC 1 |
99,0 % |
Up to 88 hours |
No redundancy, single supply |
Small businesses, test environments, laboratories |
|
AC 2 |
99,9% |
Up to 9 hours |
Partial redundancy (N+1), one supply path |
SMEs, internal IT, medium availability requirements |
|
AC 3 |
99,99% |
up to 53 minutes |
Full redundancy, multiple paths, maintainable |
Enterprise IT, cloud, critical business processes |
|
AC 4 |
99,999 % |
Up to 6 minutes |
2N or 2(N+1), fault-tolerant |
Financial sector, healthcare, KRITIS |
Note: While the Tier model defines specific downtimes, EN 50600 is based purely on the design architecture: The availability values are therefore for guidance only and may vary depending on implementation.
Not every industry needs VK4 or Tier IV, but every industry needs clarity. Availability classes in data centres are a control tool for investments, risk assessments and service level agreements (SLAs). They help to align business models with infrastructure requirements. After all, the risks associated with a system failure vary dramatically. One hour of downtime can cost tens of thousands of dollars in e-commerce or lead to life-threatening situations in healthcare. The choice of availability class is therefore highly differentiated.
Energy suppliers, waterworks, telecommunications providers: they operate facilities that are essential to the community. The requirements for data centre availability are correspondingly high. Systems must not only be redundant, but also fault-tolerant. A failure of such systems affects not only a single business, but entire regions, with consequences for security, supply, and public order.
In clinics, laboratories and emergency services, data centres secure medical technology systems, imaging procedures, patient records and surgical planning.
Automated production lines, supply chain control and machine communication rely on real-time data.
Government agencies and municipal services process highly sensitive citizen data, operate e-files, procurement platforms and internal systems.
Online platforms operate around the clock. Every second counts and every downtime is measurable.
Banks, insurance companies, stock exchanges and payment service providers are legally obliged to minimise IT failures.
Not every company needs to achieve VK4 or Tier IV. The decisive factor is the type of damage that could occur in the event of a failure: economic, legal or reputational. Choosing the availability class in the data centre helps to make these risks tangible and to negotiate appropriate SLAs with service providers.
Tip for decision-makers:
Don't automatically choose the highest class, choose the right one. Oversizing ( ) is expensive. Undersizing is risky.
Availability is not achieved through individual measures, but through the interaction of all infrastructure elements – from power supply and cooling to operational organisation. Anyone who wants to achieve Tier III, Tier IV or VK3, VK4 must plan reliability systematically and holistically. The focus is always on the question: How is redundancy achieved and how fault-tolerant is the overall system?
The decisive variable is the level of redundancy. Four models have become established internationally:
These concepts define the technical basis for the respective Tier or VK level:
High availability is not possible without a redundant power supply. In VK3/Tier III systems, power circuits are duplicated – with at least N+1 protection. Tier IV/VK4 requires completely separate power lines and UPS chains according to the 2N principle. Important: Redundancy only works if it is completely decoupled in terms of control technology. A supposedly dual supply without separation in the automation chain does not provide resilience.
IT systems are sensitive to temperature fluctuations, which is why cooling is one of the most critical infrastructure elements. High-availability data centres rely on:
In VK4/Tier IV environments, the following applies: The failure of a chiller or cooling unit must not lead to a rise in temperature. Here too, complete fault tolerance must be guaranteed – both technically and operationally.
Modern data centres protect their function with multi-layered security concepts – analogous to the "onion skin principle":
The higher the selected tier or VK level, the more detailed the security and emergency concepts must be documented – including tests, training and documentation.
Redundancy alone is not enough. The components used must themselves be fail-safe. An example: piping systems in cooling circuits. Faults here are rare, but have serious consequences: leaks lead directly to total failure. Component selection and material quality have a direct impact on the risk of failure, maintenance requirements and operational stability.
High-quality pipe systems made of polypropylene (PP) such as aquatherm blue offer decisive advantages over metal pipes:
They are established worldwide – in Tier III / IV and VK3 / 4 certified facilities as well as in scalable cloud data centres. aquatherm blue was developed specifically for closed cooling and heating systems, is designed for high operational reliability and meets the requirements for long-lasting, reliable infrastructure in data centres. The homogeneous connection technology (fusion welding) ensures permanently tight connections – without seals, adhesives or detachable connections that could represent potential weak points. All in all, the more critical the data centre, the more the quality of the infrastructure determines long-term success.
Availability classes are a strategic basis for decision-making. They help to systematically evaluate IT infrastructure, secure planning and align operational concepts in a resilient manner.
There is no one-size-fits-all solution; a tailor-made architecture is crucial here. Not every data centre has to achieve VK4 or Tier IV. The class selection should be based on the business model:
The availability class in the data centre determines all subsequent decisions: from the floor plan and the choice of energy supply to the design of the cooling technology.
Don't just plan a data centre. Plan availability.
aquatherm supports operators, planners and general contractors in the implementation of high-availability cooling infrastructures: with durable piping systems, consulting in early project phases and components that are designed to last for decades. Our products are used worldwide in data centres that have to operate around the clock.
Whether N+1, 2N or 2N+1: our systems meet the requirements of modern redundancy concepts and help to reliably achieve availability targets in accordance with EN 50600 and the Tier model: economical, resilient and future-proof.
We provide the technical foundation for global IT infrastructures in 24/7 operation.
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