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What Is a Screw Terminal?

By Geisha A. Legazpi
Updated May 17, 2024
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A screw terminal is a device that can connect two or more conductors using multiple-twist fasteners. The conductors to be connected by the screw terminal are usually mounted with terminal crimp-type lugs that secure the bare conductor ends. The screw terminal can be found where there is electrical service and inside household, commercial, and industrial electrical equipment.

Electrical connectors include various types of plugs and sockets that allow anyone to connect or disconnect appliances. The screw terminal is a connector that may be mounted right on equipment chassis, brackets, or printed circuit boards, and is used for special purposes such as internal wiring and maintenance procedures. Screw terminals require that the bare wire be twisted, tinned, or installed with terminal crimp lugs or solder lugs, and as a best practice the copper surface of the bare wire should be coated with a layer of tin-lead alloy for protection from oxidation. The crimp-type terminal lug terminates the wire with a rigid metal eyelet that may be mounted on a terminal block. Without the terminal lug, the mechanical strength and connect/disconnect durability of the bare wire are not enough.

The barrier strip and terminal strip are each a set of screw terminals that come in pairs. The terminal strip, which may be either solder type or screw type, does not provide a barrier between terminals. The barrier strip usually has insulator barriers between terminals to prevent accidental shorting.

In modular wiring design, the screw terminal provides the flexibility and accessibility needed to avoid having to disconnect and reconnect parts of the circuit. For instance, there are several test points needed when testing a motor controlled by a relay that is controlled by a start-stop switch. The test points are readily available for voltage testing by a digital voltmeter, and the faulty one may be isolated easily.

The screw terminal may also be used in mobile transport. This application requires locking washers that ensure the screw does not loosen due to vibration. The luster terminal is also well suited for both fixed and mobile installations due to compactness and covered conducting portions. Setscrews are used in the luster terminal where the screw presses on bare or tinned wire.

A substitute for the screw terminal is the wire nut, which is a piece of insulator with a conductive gripper on the inside. It is used as a cap for wires that have been twisted together side-by-side. The exposed conductor is inserted inside the wire nut, which is twisted clockwise to secure the wire until there is no exposed conductor. Temporarily removing the wire nut allows for maintenance measurements and troubleshooting.

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