UL and CSA recognized or listed terminal blocks have rated torques. Traditionally UL expresses torque in pound-force-inch (lbfin) and CSA expresses torque in Newton-meter (Nm). For example 1 Nm = 8.85 lbfin and 1 lbfin = 0.113 Nm.
These torques are chosen from UL and CSA tables or by the manufacturer. All cases involve the screws being tightened to the rated torque and the product being subjected to mechanical and electrical tests. This rated torque must be taken as a rating. It must not be exceeded because this could break or deform the connector. It must not be taken as a minimum because this could crush some small soft wires and provoke circuit malfunctions.
Choosing the correct tightening torque for factory wiring or for field wiring involves the knowledge and application of the ratings, torque measuring instruments, torque limited screw drivers, correct screw driver bits, correctly sized screw drivers, and the human factors involved. Mechanically assisted screw drivers (electrical, pneumatic, hand held or fixed) are especially sensitive to misuse. Torque measuring instruments and torque limited screw drivers must be used properly. Improper use can give false readings and incorrect torques. They all have finite accuracies, precisions and resolutions. The human factors involve the installer’s strength, ability and choice of tool. In some jurisdictions electrical technicians must have the experience, education and tools required. Over tightening or under tightening can provoke failures that are not immediately obvious and cause overheating, fire or circuit failure. Screw driver bits must be correctly sized for the screw heads. Screw driver bit types such as Pozidrive, Phillips, Robertson and flat blade all perform differently. Screw heads such as Pozidrive, Phillips, Robertson and slot have differences and similarities of performance depending of what screw bit is use
