Knife-disconnect Terminals in Process Technology For a Good Connection between the Field Signal and Control Room

Author / Editor: Birger Diekmann / Dr. Jörg Kempf

Process engineering systems feature many sensors and actuators that allow ongoing processes to be measured and controlled directly from the control room. During commissioning, maintenance, or system errors, the signals of these field devices must be quickly, easily, and locally accessible or entirely isolated from the control loop. New compact-design knife-disconnect terminals allow wiring in the tightest of spaces.

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Disconnectable modular terminal blocks simplify the measuring of signals in the field.
Disconnectable modular terminal blocks simplify the measuring of signals in the field.
(Picture: Phoenix Contact)

Although many different alternatives based on bus technologies are available today, many users still count on terminal blocks to transmit the signals for their engineering processes from the field and route them to the so-called marshaling level. This is because terminal blocks still have clear advantages: they are robust and easy to use and offer outstanding clarity of arrangement and reliability.


The use of special terminal blocks allows signal circuits to be conveniently opened by means of disconnect elements arranged in the center. There are test sockets on both sides of the disconnect points. All types of measuring devices can be continuously integrated into the signal circuit without an error occurring at the control level. In this way, conventional troubleshooting signals can be read or, using a chart recorder, documented for long periods. If necessary, the devices can be permanently physically isolated from the control circuit in the field by opening the disconnect knife. A lock point at the stop position reliably prevents any accidental closing of the disconnect element and the contact point.

Knife-disconnect terminals also make good interfaces with Hart-enabled sensor technology. Through use of a portable Hart communicator, additional diagnostic data can be read out or field devices can be re-parameterized — even if access to Hart data is not possible directly from the control systems.

High Signal Density in the Tightest Spaces

In order to connect the extremely high number of signals from flow meters, control valves, and temperature and level sensors, a corresponding number of disconnectable terminal blocks is required. This is why large plants often have entire rows of control cabinets full of knife-disconnect terminals. Shortage of space is a problem not only during expansion of existing plants or in offshore applications, it is also becoming more important during planning of new plants because of the need to cut costs.

One way of saving space and reducing the number of control cabinets and marshalling distributors is to use double-level disconnect terminals, in which two independently disconnectable circuits are situated one above the other. They allow twice as many signals to be measured along the same length of mounting rail as single-level solutions. The new two-level terminal blocks from Phoenix Contact also support clear, color-coded pairing of the disconnect elements and ensure that the user always opens the correct control loop. This color-coded pairing is achieved with push-in connection technology with green and orange latches. For clamping blocks with traditional screw connection technology, the service port is labeled accordingly.