Electricity flows from the supply to the load through cables. Cables are an integral part of all circuits. Every cable has to be connected to some part of the circuit. The connection of a cable to any part of the circuit is called a termination. There are many different types of terminations for different conditions and equipment.
The aim here is not to develop the complete technology of terminations inside cubicles, panels and junction boxes, but only to show the accessories such as lugs, terminals and tools to use, etc. If a termination is not made correctly it can cause a lot of problems. A bad termination may overheat and start a fire. A connection may have a very high resistance which can cause problems with the power supply to the equipment.
For the “other accessories”, i.e. the cable glands, the cable trays, the wall crossings, and the different cable laying devices, see next chapter. This present paragraph covers the most common types of cable terminations.
Cable terminations are an important part of the electrician’s job. Therefore they must be made correctly using the right tools and equipment.
Cable Terminations
The connection of a cable inside a device or piece of equipment is called a termination. All electrical terminations must be both electrically and mechanically secure.
The termination must be good enough to carry the load current of the circuit. This means that the connections must have a low resistance and the cable must be tightly secured. There should be no mechanical strain on the conductor connections.
The cable should be held firmly in the termination enclosure by a cable grip. Any mechanical strain should be on the cable grip, not on the conductors.
Different types of cable grips guarantee the level of mechanical strain exerted on the termination. Here are some special cable glands, collars and tension reducers adapted to different types of cables and termination enclosures.
If the conductors in the termination are under mechanical strain then they may become loose. A loose connection could overheat and cause a fire or it could disconnect and break the circuit.
It is important that all terminations meet the following conditions:
- A termination should be electrically and mechanically secure.
- The cable sheath should be intact and undamaged right up to the termination enclosure.
There should be little or no mechanical strain on the termination conductor connections.
The insulation should be intact and undamaged right up to the terminals. All the conductor’s strands must be intact and securely held in the termination. No loose wires.
REMOVING THE INSULATION
Before a conductor can be terminated, the cable insulation must be removed. Removing the sheath and insulation from a cable is called stripping the cable. Cable stripping can be done using side cutting pliers or a stripping knife.
Removing the sheath using side cutting pliers
- Split the sheath along the length of the cable. Be careful not to damage the insulation of the wires.
- Peel back the sheath and cut away the unwanted portion. (See figure 14-4).
- Check the conductor insulation for damage.
- Where there are two or three layers of protection, they must be removed separately.
Stripping insulated wires with a stripping knife
The insulation around a wire (conductor) can be stripped with a stripping knife. A stripping knife should have a short, wide blade with a flat end.
It is important to hold the knife at an acute angle when cutting the insulation.
Make two or three cuts from different sides of the wire. Then pull off the unwanted insulation with a pair of pliers.
Do not cut into the conductor.
A conductor with a nick (small cut) in it is doubly dangerous.
- It will break after it has been bent a few times.
- The cross-sectional area will be smaller so the resistance in that part of the conductor will be higher. This can cause overheating.
Use wire strippers to strip wire where possible. They do a cleaner, better job.
Wire stripping tools
You can use a special kind of pliers to remove the insulation from a wire. The jaws have Vshaped notches. When the jaws are closed the notches form a hole. You can adjust the jaws so that they only cut the insulation. When correctly adjusted they do not cut the wire.
How to use wire strippers?
- Turn the adjusting screw so that the grip in the jaws corresponds to the right wire diameter.
- Tighten the lock nut.
- Place the wire in the V of the bottom jaw and close the pliers on the wire (Part (A) in the figure).
- Turn the pliers and pull the wire out of the jaws to remove the insulation (Part (B) in the figure).
There are other kinds of wire stripping tools which work on the same principle. When the jaws close they form a hole so that they only cut the insulation. They leave the wire intact. See the “Automatic stripping pliers” pictures. Some tools even combine the stripping and crimping functions, but only for small wire cross-sections.
After the insulation has been removed, check that the conductor has not been damaged.
A conductor which has been damaged will break easily or it will increase the resistance in the wire. So you must be careful not to damage the conductor when removing the insulation.
If the cable has a stranded conductor then the strands should be twisted together tightly before making the termination. Use pliers (flat nose pliers preferably) to twist the strands in the direction of the existing twist (lay) of the cable.
You must make sure that all the strands are fitted into the termination. If not, the current carrying capacity of the cable will be reduced. Also, loose strands in the termination can cause short circuits.