Rack

What is a rack?

A rack is a standardised metal frame or cabinet in which equipment such as servers, switches, firewalls, power supplies and industrial control components is mounted one above the other on fixed dimensions, so that cabling, cooling, power and access can be managed centrally. The best-known form is the 19-inch rack, whose height is expressed in rack units (U) of 44.45 mm. Within IT OT Convergence the rack is where IT and OT literally meet: in server rooms, control rooms, MCC rooms and at edge sites, SCADA servers, historians and industrial switches sit side by side in the same cabinet. In automation the word also has a second meaning: the chassis or backplane of a modular PLC.

📏 What standard dimensions apply to a 19-inch rack?

The 19-inch rack was standardised by AT&T around 1922 for telephone exchanges and later laid down in the American standard EIA-310 (latest edition CEA-310-E from 2005). Internationally and in Europe the IEC 60297 series applies, the successor to the German DIN 41494. Thanks to these standards, a switch from one manufacturer fits in another manufacturer’s cabinet.

Equipment is fixed with cage nuts in square holes, with tapped holes (often 10-32 UNF or M6) or with slide rails for heavy servers. An 800 mm wide cabinet leaves room for vertical cable managers next to the 19-inch rails, which is particularly useful in network and patch cabinets. As a rule of thumb, switches and firewalls are usually 1U, rack servers 1U or 2U and storage systems 2U to 4U tall.

Besides 19 inch there are other formats. The ETSI rack from the telecom world (ETS 300 119, now EN 300 119) uses a mounting width of 535 mm and a 25 mm vertical pitch. The Open Rack specification of the Open Compute Project uses an internal width of about 21 inches and its own height unit, the OpenU of 48 mm. If you combine such equipment with standard IT hardware, you have to take this into account in the design.

⚙️ What types of racks are there?

An open frame gives the best airflow and access but offers no physical security. In a production environment the distinction with a control cabinet matters. A data centre rack usually has a low degree of protection (IP20 is common), whereas a cabinet on the shop floor often has to reach IP54 or higher under the IP Code. For switchgear and controlgear assemblies built into a cabinet, IEC 61439 also applies.

🏭 What does ‘rack’ mean in industrial automation?

In industrial automation, racks are found in three kinds of locations:

• Server room or OT data centre - SCADA servers, historians, domain controllers and virtualisation hosts at levels 2 and 3 of the Purdue Model.
• Control room or master station - operator stations, network equipment and UPS systems close to the operators.
• Field and production hall - cabinets with PLCs, DCS controllers, industrial switches and O, often in or next to an MCC room.

In the PLC world, ‘rack’ is also the name of the mounting frame with backplane into which modules slide. Rockwell supplies the ControlLogix 1756 chassis in versions with 4, 7, 10, 13 and 17 slots, and Siemens also refers to racks for the S7-300 and S7-400. A “rack fault” in PLC diagnostics therefore concerns the module chassis, not the 19-inch cabinet. In conversations between IT and OT it pays to make these two meanings explicit.

🌡️ How do you manage cooling and airflow in a rack?

Almost all IT equipment draws cool air in at the front and blows warm air out at the back (front-to-back). Racks are therefore placed in rows with alternating cold and hot aisles, optionally fully enclosed with aisle containment.

• Inlet temperature - the ASHRAE TC 9.9 thermal guidelines recommend 18 to 27 °C at the front of the equipment. Class A1 tolerates 15 to 32 °C as its allowable range.
• Blanking panels - covering empty U positions stops warm air from recirculating to the cold side.
• Cable management - cable bundles behind the servers must not block the exhaust.
• Non-standard equipment - some industrial switches and network devices cool side-to-side or back-to-front; plan their position or use air ducting.

An industrial cabinet in a hot production hall often has its own cooling unit or air-to-water heat exchanger. Temperature and door sensors are linked to DCIM or the BMS, so that monitoring raises an alarm in real time on overheating before equipment fails.

⚡ How is the power supply of a rack built up?

Power reaches the equipment through rack PDUs (Power Distribution Units). Servers and switches usually use IEC 60320 connectors: C13/C14 for up to 10 A and C19/C20 for up to 16 A.

For high availability a rack gets two independent feeds (A and B), each through its own PDU and preferably its own UPS. Equipment with dual power supplies (dual PSU) connects to both. Crucially, each feed must be able to carry the full load on its own: if you load A and B to 70% each, losing one feed still takes everything down. Intelligent PDUs measure current per outlet and can be read via SNMP or Modbus TCP. Do not forget earthing and equipotential bonding of the rack frame: this is required both for electrical safety under NEN 1010 and for EMC.

🔐 How do you secure racks physically and digitally?

A rack is a physical object, but anyone who can reach the rack can also reach the network ports, console ports and USB connections of critical systems. Physical access bypasses many digital controls, so cybersecurity and physical security start at the same cabinet door. A Zero Trust architecture likewise assumes that a physical network port earns no trust on its own.

IEC 62443 works with logical zones and conduits; physical security falls under the asset owner’s security programme (IEC 62443-2-1). In ISO 27001:2022, physical security sits in Annex A, section 7. For organisations under NIS2 and the Dutch Cyberbeveiligingswet, which entered into force on 15 August 2026, protecting the physical environment is explicitly part of the duty of care. An up-to-date asset inventory per rack, with position (U) and port usage, is a practical foundation for this.

🧭 How do you set up an OT rack step by step?

1. Inventory the contents - which equipment, how many U, what weight (mind the floor loading) and what power. Reserve roughly 20 to 30% space for growth, so that scalability does not immediately require a new cabinet.
2. Choose the cabinet type - a data centre rack in a conditioned room, or a closed industrial cabinet with the right IP rating on the shop floor.
3. Plan the layout - heavy equipment and UPS batteries at the bottom, patch panels and switches at the top, separate cable routes for power, network, control and safety signals, using patch panels, cable trays and hook-and-loop straps rather than cable ties.
4. Design power and redundancy - A/B feed, adequate PDU capacity, a UPS with sufficient autonomy (N+1 where needed) and dual uplinks or a ring topology for the network connections.
5. Arrange cooling and monitoring - blanking panels, temperature sensors and integration with DCIM via SNMP, MQTT or OPC UA.
6. Secure and document - locks, access registration, labelled cables and a rack elevation drawing in the documentation, as part of business continuity and disaster recovery.

❓ Frequently asked questions

How many millimetres is 1U in a rack?

One rack unit (1U) is 1.75 inches, or 44.45 mm. In practice a 1U front panel is about 43.7 mm tall, leaving some clearance between devices. A 2U server therefore takes up 88.9 mm of rack height.

Why is it called a 19-inch rack?

A 19-inch rack is named after the width of the front panel including the mounting ears: 19 inches, or 482.6 mm. The clear space between the posts is about 450 mm. The cabinet itself is usually 600 or 800 mm wide.

What is the difference between a rack and a control cabinet?

A rack is primarily a 19-inch mounting frame for IT and network equipment, usually in a conditioned room. A control cabinet is an enclosed housing with DIN rails and a mounting plate for PLCs, relays and protective devices, designed for conditions on the shop floor. OT environments often combine both, for example a 19-inch frame inside an industrial enclosure.

What is a PLC rack?

A PLC rack is the chassis with backplane that holds the modules of a modular PLC, such as the power supply, CPU, communication and I/O cards. Rockwell’s ControlLogix 1756 chassis, for example, comes in versions from 4 to 17 slots. A PLC rack has nothing to do with the 19-inch dimensions of IT racks.

How warm can a server rack get?

For IT equipment in a rack, ASHRAE TC 9.9 recommends an inlet temperature of 18 to 27 °C at the front. The allowable range for class A1 is 15 to 32 °C, but operating outside the recommended range for long periods increases the risk of failures. Always measure at the equipment inlet, not in the hot aisle.

Does a rack have to be locked under NIS2?

NIS2 and the Dutch Cyberbeveiligingswet do not prescribe a specific lock, but they do require appropriate measures to protect network and information systems and their physical environment. A lockable rack with access registration in a secured room is a common way to meet this. Without physical security, network controls such as NAC are much easier to bypass.

📌 In summary

A rack is the standardised 19-inch frame where IT and OT equipment comes together, and therefore the place where the availability, cooling, power and physical security of critical systems are determined. With the right dimensions (1U = 44.45 mm), A/B power feeds, aisle separation, monitoring via DCIM and locked, documented cabinets, you lay a reliable physical foundation under SCADA, PLCs and the OT network.