|Electrical Installation - Basic Vocational Knowledge|
|3. Laying of underground cables|
Conductor connections on cable terminations are quite frequently made via cable terminals.
In case of the traditional methods of connection like soldering, welding or clamping of cable terminals to conductors no absolute safety for an unobjectionable connection will be given.
Stamped connections have replaced these traditional types of connection. Stamping is distinguished by reliability of the procedure, durability of the connection also under unfavourable conditions as well as by uncomplicated handling of the stamping tools. For the stamping of cable terminals mechanical hand stampers, or in case of bigger cross sections, hydraulic stamping devices are being employed.
1 stamping head, 2 safety valve, 3 pressure hose, 4 pressure reducing valve, 5 pump lever, 6 pressure tank, 7 pump handle, 8 bearing grip
1 Pressure tank of the stamping head, 2 space for stamping inserts, 3 hose connection, 4 screw cap, 5 piston
Stamping inserts being matched to the type of cable and cross section of the conductor will be fitted into the stamping tools.
There are two different types of stamping inserts:
- stamping inserts for V-grooving
- stamping inserts for hexagonal stamping.
V-grooving is on principle applied to copper conductors only!
Cable terminals are marked with information about the size of the diameter of the connecting pin and the conductors diameter.
1 V-grooving, 2 hexagonal stamping
The stamping inserts for the cable terminal are in any case to match the cable terminal.
The designations of suitable stamping inserts are to be taken from the tables in the valid regulations.
When stamping on cable terminals the following fitting process is to be adhered to:
1. Cutting conductor lengthwise
The conductor is to be matched to the length of the cable terminal up to the connecting pin. It is to fill out the cable terminal. The elongation of the cable terminals connection during hexagonal stamping has also to be considered. It has to be subtracted from the length of the conductor. Depending on the cross section in question this change of length may be between 3 and 5 mm.
2. Removal of the conductors insulation and cleaning of the contact surfaces.
The outside of the conductor and the inner contact surfaces of the terminals barrel must be metallic-bright. Contaminated conductors and barrels of terminals will have to be cleaned from adhering dirt and oxides by wire brush.
Every conductor has to be cleaned only just prior to the stamping process, max. 10 minutes beforehand!
3. Entering the conductor into the barrel of the terminal
Difficulties which may arise in that process are best overcome by chamfering the conductor laterally or by fixing a taut wire binding adjacent to the cut surface which will have to be removed after having entered the conductor into the cable terminals barrel. Segmental conductors are to stamped circularly by the corresponding stamping inserts prior to putting on the cable terminal.
4. Stamping on the cable terminal
V-grooving is to be performed so as to have the local impression situated at the upper or lower side of the terminals barrel aproximately in the centre. In particular, maintenance of the required stamping depth has to be observed. This will be achieved when the stamping jaws of the stamping insert will have closed or corresponding markings have been reached.
In case of hexagonal stamping of aluminium conductors the stamping insert has to be applied in between the stamping marks on the cable terminal. If two stampings are to be made, stamping will be started at the point nearest to the terminal lug. The burrs resulting from hexagonal stamping at the cable terminal will have to be removed.
If after closing of the stamp jaws further pressure is exerted on the stamping tool this will unconditionally lead to heavy damage at the stamp inserts or the stamping device itself (risk of accident!)!
Stamp jaws which are not completely closed do not warrant a durable electrical connection (restamping to be done!)!
Lapping the conductors, which has been dealt with in section 3.4. on processing of paper-insulated lead-sheathed cables, takes place after fitting the cable terminals.
For soldering of cable terminals there are some restrictions. Soldering of cable terminals should be turned to only if stamped terminals cannot be applied.
How to proceed when soldering cable terminal is to be taken from fig. 3.25. and 3.26.
a) prepared conductor, b) conductor with cable terminal slid on, c) soldering on the cable terminal: 1 flame-proof batting, 2 solder, 3 heating nozzle, 4 cable terminal
When soldering on cable terminals the connecting area around the lug of the cable terminal is continued to be heated until the soldering tin laid on starts to flow. So much solder is put on to the soldering surface that all parts of the conductor in the total area are covered by tin which must show a mirror reflection.
In no case so much solder is to be brought on the soldering surface resp. heating is continued so long that the solder flows through the cable terminal into the conductor!
Soldering cable terminals requires extensive special skills and is therefore to be carried out by especially qualified experts only.
1 plastic tube or tape