Tier 4 data center: definition, gap with Tier 3 and Uptime certification
A tier 4 data center keeps its data halls powered and cooled despite the failure of a critical piece of equipment. In the Uptime Institute's Tier classification, this level remains the most demanding. Thésée Datacenter is one of the few Tier IV certified operators in France. Our Aubergenville campus (TDC01) is certified Tier IV Constructed Facility, alongside our other certifications. Source: Uptime Institute registry, consulted in September 2026.
Why does a Tier 4 datacenter protect your applications from an outage?
The Uptime Institute calls Tier IV "Fault Tolerant". The site detects a fault on its critical systems, then isolates it. It sustains the IT load without interrupting the power or the cooling of critical spaces. This requirement applies to planned maintenance as well as to unexpected incidents.
To achieve this, a tier 4 data center uses several independent systems that are physically separated. These systems also provide redundancy of components and of distribution paths. If an incident affects one of them, the other keeps the service running.
The classification assesses the topology of the site, not the quality of its operations. TDC01's certification letter states this unambiguously. A mistaken emergency shutdown or a malicious act can still affect the critical load. Procedures and team training therefore remain central.
From Tier 1 to Tier 4: what level of availability should you expect?
These percentages come from historical indicators that still appear frequently in tenders. The Uptime Institute's current Tier Standard no longer uses them. Instead, it defines topology requirements, set out in the Tier classification system. The service contract specifies a hosting provider's availability commitment.
Tier 3 or Tier 4: what does a failure change for your applications?
A Tier 3 datacenter allows maintenance without downtime. When a piece of equipment is taken offline for planned work, another path takes over the service. Its design does not, however, cover every sudden failure. Take the failure of an electrical switchboard: in Tier 3, it can reach the data hall. In Tier 4, a second independent path feeds the same equipment, with no human intervention.
- Unplanned failures: Tier 4 absorbs them by design, whereas Tier 3 mainly targets planned maintenance.
- Compartmentalisation: Tier 4 physically separates its paths, so that a local incident stays confined to a single one.
- Continuous cooling: Tier 4 keeps the data halls cool during electrical switchovers.
- Use cases: Tier 3 suits many enterprise applications; Tier 4 secures services that cannot tolerate even short outages.
How do you recognise a genuine Tier certification?
The Uptime Institute remains the only body that awards Tier certifications. Every certified site appears in its public registry of certifications in France. Three stages exist, each with a distinct scope.
- Design Documents: the institute validates the site's designs, without inspecting the completed building.
- Constructed Facility: its engineers verify on site, through live demonstrations, that the installation reaches the required level.
- Operational Sustainability: the institute reviews how the site is operated over time.
"Designed to Tier IV standards", "rated Tier IV" or "Tier III+" designate no certification at all. A site's level follows its weakest link. In this framework, a "+" therefore carries no value.
TDC01: which Tier IV certifications can you verify?
Our Aubergenville campus (TDC01) holds two Uptime Institute letters, each with its own scope.
- Constructed Facility, 24 January 2023: the institute certifies data hall S|1 of TDC01's first phase. Its engineers verified the constructed site.
- Design Documents, 12 February 2025: the institute validates the designs of the first two phases, for an IT load of 3,005 kW.
The institute inspected the site in June-July, then in November 2021. It went on to run remote demonstrations on 18 January 2023. The Constructed Facility certification remains valid as long as the installation stays unchanged. The Design Documents letter supersedes the 2023 one and remains valid until 12 February 2027.
On site, TDC01 uses a 2N electrical supply, that is two complete and independent chains. Two separate routes bring power to the building. Its Continuous Power generators run without any time limit. Its redundant Continuous Cooling keeps the halls cool during power outages. No single point of failure remains, and the site responds to failures automatically.
Both Uptime Institute letters are freely available to download. See the 2023 Constructed Facility certification and the 2025 Design Documents certification. Each letter states its date, its scope and its limits.
Where can you find a Tier IV certified datacenter in France?
As of 13 September 2026, the Uptime Institute registry lists seven French sites certified Tier IV Constructed Facility. All of them also hold the Design Documents certification.
Five other French sites hold the Design Documents certification only. Their designs have been validated, but the constructed building has not yet been verified. Thésée Datacenter's TDC01 campus is one of these seven Constructed Facility sites. The registry changes over time: check it on the date of your project.
Do your applications need a tier 4 data center?
The right level depends on what an outage costs your business. Three questions will guide the choice between Tier 3 and Tier 4.
- How much does one hour of downtime represent, in revenue, in penalties and in reputation?
- Can you tolerate an outage during maintenance, or must the service run without interruption?
- Do your regulatory or contractual constraints call for demonstrable service continuity?
Tier IV remains compatible with energy efficiency. TDC01 combines indirect free cooling and adiabatic evaporation. It targets a design PUE of 1.2. PUE (Power Usage Effectiveness) compares the total energy used by the site with the energy consumed by IT equipment. Our corporate social responsibility (CSR) policy sets out this approach. For a disaster recovery plan, you can also spread your workloads across our two datacenters in the Île-de-France region.
Are you comparing Tier 3 and Tier 4 for critical hosting? Our teams will build a tailor-made colocation offer with you, matched to your level of criticality. Contact us to discuss your project.
See also: HDS datacenter · Sovereign datacenter · Secure datacenter: ISO 27001, ISO 50001, PCI DSS
Frequently asked questions about Tier IV certification
Find the answers to the most common questions about the Tier classification, the availability you can expect and how to verify a certification.
Does Tier 5 exist?
No, the Uptime Institute classification stops at Tier IV. "Tier 5" is a commercial brand used by the US operator Switch, and has nothing to do with the Tier certifications.
What is the difference between tier 3 and tier 4?
A Tier 3 datacenter stays in service during planned work. A Tier 4 adds tolerance to sudden failures. There, a failure of a piece of equipment or of a distribution path interrupts neither the power nor the cooling of the data halls.
What availability does a tier 4 data center deliver?
The historical figure is 99.995%, or around 26 minutes of downtime per year. The current Tier Standard defines no percentage. The hosting provider's service contract sets out the availability commitment.
Do you need two electricity suppliers to be Tier IV certified?
No. The Uptime Institute treats the site's own generators as the reference energy source. What matters is having independent power paths and generators able to run for an unlimited duration.
How do you check that a datacenter is Tier IV certified?
Consult the Uptime Institute's public registry. Then ask for the certification letter. It states the type of certification, its date and its exact scope: building, phase or data hall.
Does a Tier IV datacenter consume more energy?
Not necessarily. Redundancy adds equipment, but consumption depends mainly on cooling. TDC01 combines indirect free cooling and adiabatic evaporation for a design PUE of 1.2.