|Handling and Connection of Cables and Lines for Fixed Laying - Course: Basic Skills and Knowledge of Electrical Engineering. Methodical Guide for Instructors|
|3. Recommendation for the Implementation of the Practical Vocational Training in the Working Techniques of Handling and Connection of Cables and Lines for Fixed Laying|
The introductory instructions for each working technique can be given in a class-room. During the instructions the trainees should be advised to note down necessary additions or answers into the Trainees Handbook of Lessons.
The arrangement of the contents of the Trainees Handbook of Lessons is adapted to the introductory instructions and the main points therein contained should be taught. The knowledge of the working techniques of making permanent joints and fastening joints should be repeated and referred to since a good command of those working techniques is a precondition for acquiring the working techniques of handling and connection of cables and lines for fixed laying.
The subject of Connection of Cables and Lines for Fixed Laying to Electrical Equipment, in particular the criteria for clamped joints in electrical installations, should be taught intensively using all teaching aids available.
Fundamentals for handling and connection of cables and lines for fixed laying
At the beginning the trainees should be informed on the aim to be achieved.
This can be done by showing the trainees finished products and pointing to the many applications of lines for fixed laying in industry, agriculture, public life and, last not least, in households.
Before the demonstrations and exercises the trainees must be made familiar with installation engineering terms not yet known. For this purpose the tables, lists etc. contained in the Trainees Handbook of Lessons may also be written on the blackboard or be used as transparency.
When explaining the new terms, some typical representatives of lines to be handled or connected should be shown and, if possible, also locations where they are installed.
The criteria for the selection and use of lines for fixed laying should also be dealt with in detail.
Usual working techniques to be applied prior to the connection of lines are to be repeated.
Handling of lines
It is to be made sure that all trainees can watch the demonstrations.
After the demonstrations the trainees should start with the exercises at their workplace.
Attention is to be paid to the fact that all trainees have to keep their workplace always in order and that
- tools are to be deposited in cleaned condition,
- tools are to be deposited so that they cannot be damaged,
- tools and materials are to be supported so that they do not protrude and cannot fall down,
- electrical tools are to be switched off after use with the mains plug to be disconnected.
Before the exercises the necessary material is to be handed out to the trainees.
The demonstration parts should be available to the trainees for comparison.
Any mistakes made in the exercises should be corrected immediately and be evaluated together with the trainees. The instructor should point out again and again that wrong operation of tools may cause waste material or re-working.
Stripping of the insulation
First the instructor should explain how to proceed with stripping of the insulation and what tools are to be used for stripping. The stripping length of the insulation sheath and conductor insulation is to be specified.
- Stripping of the insulation sheath by means of the cable stripper (knife) and by means of the sheath stripper
Here it is important that the cable stripper is applied at the correct angle and moved away from the body. (To avoid damage to the conductor insulation and to prevent accidents!)
- Removal of the filler
- Removal of the conductor insulation
When stripping the conductor insulation, special attention is to be paid to guiding the knife correctly since otherwise faults may be produced which cannot be repaired, such as:
- notching of the conductor
- cutting of the conductor sheathing (conductor insulation)
- injury of the trainee (stabs or cuts).
Vertical guidance of the knife on the conductor is a typical example of wrong guidance!
The use of insulation stripping pliers should also be dealt with since they remove the conductor insulation more quickly and more cleanly.
Insulation stripping pliers normally have several ground-in cutting edges, arranged side by side, and are suitable for cross sections of conductors from 0.75 to 6 mm2.
Only use the cutting edge intended and marked for the relevant cross section of the conductor to ensure clean removal of the conductor insulation and to avoid damage to the conductor! To avoid damage to the conductor, the pliers should cut into the conductor insulation no more than 2/3 up to 3/4! The rest is to be teared off.
Conductors with cross sections exceeding 6 mm2 are to be stripped by means of the cable stripper!
During the exercises the instructor should repeat again and again
- that the insulation layers are to be separated and removed cleanly and completely from the conductor beneath,
- that the conductor must never be cut.
The trainees must be strongly warned of the risk connected with a cut conductor.
- Reduction of the cross-sectional area at the joint may result in
· lower capacity,
· excessive heating,
· possible destruction of the connection and of the connected equipment.
- Risk of breakage of the stripped conductor (particularly of Al conductors)
This may have the following effects:
- Breakdown of the connected equipment.
- Loss of the protective function of the faulty conductor (short circuit, electrical accident by existing shock-hazard voltage).
- Bending of wire lugs
- Pressing-on of cable eyelets
Since these working techniques are already known to the trainees, it is recommended to repeat the knowledge already acquired based on the Trainees Handbook of Lessons for the lessons of Making Fastening Joints and Making Permanent Joints.
Connection of cables and lines for fixed laying to electrical equipment
At the beginning it should be made clear to the trainees that the rules elaborated hereinafter apply to all electrical appliances, devices, machines and other equipment and must be strictly observed. But is should also be pointed out that special installations, such as emergency lighting and explosion-proof installations are subject to special rules which are additionally to be observed. The relevant technological regulations are to be made known to the trainees. The demonstrations and subsequent exercises should start with simple electrical connections, such as switches, branch boxes etc.
It is not before the trainees have a good command of these working techniques that more intricate clamped joints can be dealt with, such as terminal boxes, distribution installations, machine and contactor connections.
Colour codes of cores are to be specifically discussed.
Making screw joints
The making of screw joints should be demonstrated to the trainees as vividly as possible.
This can be done by showing practical examples, such as screw joints in sockets, on individual terminals or other electrical equipment.
But it is also possible to prepare and show suitable examples on the blackboard or on transparencies.
When explaining the connection of conductors, the following points should be emphasized:
- The conductor insulation must end 1 mm before the point of clamping.
- One conductor only is to be connected to terminals of installation switches, sockets and lamps.
- The conductor must be put under the clamping screw in the direction of rotation only.
- The conductor must be supported by the whole contact area of the point of connection or joint.
- Cu and Al conductors are to be clamped so that they are not in direct contact.
- Al conductors are to be greased with anti-corrosive grease.
- If several conductors are connected to one connection bolt, it is to be made sure that a washer is inserted between the conductors and a lock washer after every third conductor.
- For connections of protective conductors or middle conductors one conductor per point of connection is admissible only.
(Series connection of the protective conductor is admissible only as uncut conductor loop within one unit!)
- When a cable eye and a lug are jointly connected to a connection bolt, they are to be separated.
- When connecting conductors to cap screws without clamping saddle or distributor plate, no more than three conductors, which may differ by up to two cross-sectional steps, must be connected with washers to be inserted between the conductors.
- When connecting conductors to cap screws with clamping saddle, one conductor only must be connected per saddle side and the conductors may differ by one cross-sectional step.
- When connecting conductors to cap screws with distributor plate, one conductor only must be connected per distributor plate and the conductors must have the same cross section.
- Washers used for connection to screws or bolts must cover at least two thirds of the conductor.
Criteria for clamped joints in electrical installations
Clamped joints in electrical installations are subject to special rules which must be quoted and explained to the trainees.
This refers in particular to the following rules:
- The switch-lamp-wire is to be connected to the floor contact and the neutral conductor is to be connected to the ring contact.
- Disconnection through switches is allowed for phase conductors only.
- Connection marks on switches are to be observed.
- For connection of sockets the phase conductor is to be clamped at the left-hand side and the neutral conductor with protective function (PEN) or the neutral conductor and protective conductor at the right-hand side.
- The phase conductor sequence is to be observed when connecting power sockets.
The points of clamping are to be firmly tightened!
Points of clamping of Al conductors, in particular, should be re-tightened after a few days since they might get loose because of flowing of the material!
The trainees are also to be made familiar with the TRR rule (TOP, REAR, RIGHT-HAND).
The necessity of having a consistent system of clamped connections is also to be underlined.
During the exercises the trainees should be constantly supervised.
Defective or poor-quality connections should be immediately reworked or repaired.
If certain faults are repeatedly found, the trainees should be called together at one working place and be advised on the causes and effects.
The aim should be to make clear to the trainees the proper relationship between expenditure of work and technology on the one hand and esthetics and effects of the faults on the other hand.
Here it is essential to point out that pressed joints are permanent joints.
Moreover it is necessary to discuss the criteria for connection by means of press-sleeves.
Press-sleeves must be fixed only by means of the pressing pliers intended for this purpose! Perfect pressing is to be ensured!
To avoid short circuit or body contact, perfect fit and vertical position of the insulation sleeves are to be ensured!
During the exercises the trainees should also be controlled and corrected, if necessary.
- Making wire-wrap joints
- Making soldered joints
These two working techniques should be dealt with in the form of a repetition since the trainees should already be familiar with these working techniques.
Here, too, Trainees Handbook of Lessons - Making Permanent Joints should be used.