This is a scratch, usually leads during installation to the formation of cracks, and then to a broken wire in the terminal. If you cleans insulation with a knife, make it so that the knife touched veins at an angle (as in ochinke pencil). But it's best to use a special tool for cleaning. There are many designs of such machines. The most common are arranged like the mites, but have a semi-circular cutting edges of the sample under different diameters of the veins. The most complex are arranged so that compression of handles first special sponges occupy space on the wire stripper, and then cutting edges with a groove cut through the isolation of the desired size, without touching the wires, and Finally, these cutting edges move away from the sponge, removing slotted piece of insulation. After stripping all lived them attached to the terminals. Modern cable manufacturers used to living color isolation.
For example, a three-core wire insulation can be blue, black and yellow-green color. Accepted for phase wire to use the blue vein to zero – a black and grounding – a yellow-green. It is important that all house from the switch board to the final electrical marking was the same. Attaching the ends of the cable cores to the terminals of socket (if the outlet without earth – phase and zero if the grounding – phase zero and the ground), it gently inserted into the mounting box, previously having put inside the loop wires and fixed.