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Overspeed Protection CTo

Overspeed Protection

The Voith CTo overspeed protection system consists of an electronic signal pro­cessing unit that detects overspeed and an electrohydraulic assembly that actuates a hydraulic positioning cylinder directly. This positioning cylinder opens or closes a trip valve that releases or blocks the mass flow needed to drive machinery.

The CTo is a compact assembly and is intended for direct mounting to the actuator of the trip valve. This keeps signal paths short, making the unit immune to inter­ference.

The CTo features a simple design. This ensures reliable operation. The Mean Time Between Failures (MTBF) for the Voith CTo is over 200 years.


Voith Digital Solutions GmbH
Actuators and Governors
Voithstr. 1
74564 Crailsheim, Germany

Tel. +49 7951 32-470
Fax +49 7951 32-605

Technical data

Flow rate (A to T) 330 l/min [87gpm] at  Δp = 4 bar [58 psi]
Input pressure Up to 25 bar [362 psi]
MTBF (Mean Time Between Failures) 200 years (calculated in accordance to MIL‑HDBK‑217 and Siemens SN 29500, actual MTBF evaluations from installed products show even higher values)
Supply voltage 24 VDC
Current consumption 0.5 A (2.5 A for t < 1 s)
Fail-safe operation Internal return spring
Trigger frequency 4,000 to 10,000 Hz, adjustable
Ambient temperature -20 to +80°C [-4 to 176°F] (standard version, not rated for use in potentially explosive areas)
Explosion proof rating (optional)
  • ATEX (II 2G IIC T4, Ta = -30...+60°C)
  • IECEx (Ex db llC T4, Ta = -30...+60°C)
  • others on request
Protection class IP 65
Safety integrity level SIL 2

Advantages and benefits

  • The Mean Time Between Failures (MTBF) is over 200 years. Your equipment operates very reliably. Productivity increases and the costs associated with lost production remain low.
  • The Voith CTo is not sensitive to contaminated pilot oil. Turbine lubricating oil is suitable as the operating fluid. One oil supply system for both lubricating and pilot oil suffices. Maintenance is simple, which helps you lower operating costs.
  • The CTo features a simple design. Overspeed detection and tripping are de­cen­tralized. This is a cost-effective solution – for both new equipment and when retrofit to existing equipment.
  • A partial stroke test ("Trip-Sim") permits testing of the trip valve during turbine operation. This eliminates the need for additional hardware and keeps the system uncomplicated.
  • A separate speed feedback signal in the form of a 4 – 20 mA signal is available. This makes installation of an additional speed measuring device unnecessary.

Typical fields of use

Actuation of trip valves at

  • Steam turbines
  • Gas turbines
  • Compressors

Technical information


A Load connection (output)
T Tank connection
P Oil supply connection (input)
1 Control magnet housing
2 Plunger
3 Electrical connection
4 Hydraulic section
5 Control piston



The CTo consists of a 3/2 way hydraulic unit, a current-controlled electromagnet and the signal processing and control electronics.

If there are no trip criteria, the control magnet generates the force FMag. This magnetic force presses the control piston of the hydraulic unit against a return spring. In this position, the hydraulic unit connects the pressure supply to the actuator of the trip valve, causing it to open.

If there is a trip criterion (e.g. overspeed), the control magnet exciter current is interrupted immediately. The spring force FF exerted by the return spring shifts the control piston back to its original position, draining the actuator of the trip valve and causing it to close.