I have always used common screw wire nut connectors СИЗ. What is the difference between СИЗ and new version СИЗ-К?

Sometimes installation of screw wire nut connectors demands a lot of effort. The bigger the cross-section of the wire is, the more force you need to apply. And if during one day you need to do more than 10 installations, this work can seem to be a nightmare. СИЗ-K have special wings for an easier screw. The square profile of the inside spring and stronger torque effect make the connection reliable. Sometimes you can not make the screw till the end because the connector slips in your fingers, even if you have enough power to continue the screwing. The wings solved this problem. СИЗ-К have a wider range of sizes in comparison with common СИЗ connectors. If you want to compare yourself, take yellow and red connectors in СИЗ and СИЗ-K series; try to connect 3 sold wires with cross-section 2.5 mm². After such an experiment, most of the users do not mind the slight difference in price and choose СИЗ-К for their work.