The present invention refers to a contrivance at mains cables for shielding the cables from radiation of electric alternating fields.

Almost all mains cables in application today are unscreened.

Every unscreened mains cable is radiating an electric alternating field. This field influences other electric equipment in its surrounding. For example, we get hum in our hifi equipments if we do not screen the low signal interconnect cables. We get disturbances in data transference and process controllers if we do not screen the transmission cables and sensitive parts of the equipment. Disturbances may as well emanate from transients on the mains net work, caused by other mains equipment, for example thyristor controlled lamps and machines.

There is also a discussion going about the electric field and its influence on human beings. The influence is a physical fact, but there are different opinions about whether it is injurious or not. Thus, it has not been proved to be quite harmless.

One way to avoid these problems is to screen the mains cables. There are, in fact, screened cables on the market. However, these are troublesome to install since they, besides the ordinary earth conductor, are provided with an extra screen connection conductor. This in order to enable the earth conductor to be insulated separately and marked with green/yellow colour in compliance with the safety regulations. These are based on the IEC-statutes .

These exisiting screened mains cables are constructed as an ordinary 3-conductor mains cable, but provided with an exterior screen of aluminum foil and inside

this aluminum screen there is a naked wire in contact with the screen, along the entire cable. This naked wire serves as a screen connection conductor.

The main purpose with the present invention is to achieve a contrivance regarding mains cables that enables a screening of the cable without the additional screen connection conductor, and that the cable can be handled as any ordinary mains cable.

The mentioned purpose is achieved by means of a contrivance in accordance with the present invention which is mainly characterized in that between fields of insulation arranged to encircle an earth conductor which is received on the inside of an along the entire cable running screen, an electric conductor is received, which is arranged to connect the earth conductor with the mentioned cable screen.

A variant of the invention is characterized in that a jacket preferably made of a multi-coloured, for example green/yellow, electrically semiconductive or conductive mixture of polymers, which for example is formed by additive polymer materials, is extending around an earth conductor, which is received on the inside of an along the cable extending cable screen, which earth conductor jacket is arranged so as to connect the earth conductor with said surrounding cable screen.

The invention is described below as two pre¬ ferred embodiments with reference to the attached drawings, in which

Fig. 1 shows a cross sectional view of a screened mains installation cable in accordance with the present invention and which comprises three conductors,

Fig. 2 shows a perspective view of an earth conductor in accordance with the invention, and

Fig. 3 shows a variant of a screened mains installation cable in cross section.

With the new cable the extra screen connection is eliminated and the cable can be handled as any ordinary 3-conductor cable.

The idea is to insert a black marking line between the green and the yellow field on the insulation of the earth conductor. This marking line consists of semi- conductive plastic. Thus, it constitutes a connection between the earth conductor and the surface of the insula¬ tion along the line, which in its turn is in contact with the screen. In this way we have got an automatic screen connection.

Then it remains to make it electric shock safe, for instance in case one is holding the earth conductor with one hand and a phase connected part with the other hand. Furthermore, a dangerous current must not be allowed to flow if the earth conductor is touching against any other current carrying part. Measurements of electric alternating fields and screen effects show that the connection between the screen and the earth can be rather high resistive without the screen effect being taken away. Furthermore, the screen connection surface of the semiconductive plastic is consid- erably larger than the surface that might touch another current carrying component or touch a part of the human body, and thus enabling the semiconductive plastic to keep a high value of Ohm x meter.

Hence, the purpose is to adapt the conduct ^¬ ivity of the semiconductive plastic down to a level of harm- lessness which can be approved by SEMKO after exemption from the authorities of electric safety.

The cable is also to be secured against interior electric surge breakdown. PVC insulation can

exhibit pores that go all the way through and such pores might cause electric surge breakdown. Therefore, an insula¬ tion similar to the insulation of the two other conductors is required also on the earth conductor. Thus, there is always a double insulation between any two conductors and it is extremely unlikely to get two through-going pores connected to each other. However, the periphery of the earth conductor of our cable is not entirely PVC insulated owing to the semiconductive line. This is solved by putting a double insulation on each of the two current carrying conductors instead. Then the probability of two through- going pores in contact with each other remains equally small.

In accordance with the invention a contriv- ance 1 at a mains cable 2, which is meant to make a screen¬ ing of the cable from electric alternating field, what is called the E-field, is designed in such a way that between fields 3, 4 of electric insulation, which is arranged to encircle an earth conductor 5, for example a common concen- trie multistrand copper conductor with strand diameter and oh ic resistance in compliance with the regulations, and which is interiorly received in a tube shaped cavity 6 in an along the cable 2 extending connecting screen 7, a conductor 8, 9 is received, which is arranged to connect the earth conductor 5 with the surface 10A of its insulation 10, for conductive connection with said connecting screen 7 of the mains cable 2.

More exactly said conductor 8, 9 preferably consists of semiconductive plastic which can be designed as a sector shaped line extending to the connecting surface between the earth conductor 5 and the insulation 10 along¬ side the line 8, 9.

Preferably the connection conductors 8, 9, which preferably are two conductors located opposite each

other on the earth conductor 5, run helically around the earth conductor 5.

Said insulation 3, 4 is made of plastic material, preferably PVC and more exactly comprising two differently coloured fields 3, 4 of insulation, green/yellow which lenghtwise are limited by the connection conductors 8, 9 along lateral edges 8A, 8B, 9A, 9B, respectively, extend¬ ing along the earth conductor 5 for surface connection with the lateral edges 3A, 3B, 4A, 4B of the insulation fields 3, 4. The intermediate connnection conductors 8, 9 are black coloured.

The earth conductor 5 as well as additional conductors 11, 12 can be surrounded by individual layers 13, 14 of insulation, preferably with double layers 13, 13A; 14, 14A of PVC.

A semiconductive nylon screen 7 preferably forms a cable screen exteriorly around all the conductors 5, 11, 12, and the conductors 5, 11, 12, which are evenly distributed inside the semiconductive nylon screen 7, are also surrounded by an around said nylon screen 7 extending, exterior insulating jacket 15 of electrically insulating material, preferably of PVC.

Measurements carried out have shown that the insulation resistance, i.e. the contact resistance, between the screen 7 and the earth conductor 5, can be rather high resistant without the screen effect being lost. Measurements have been carried out on 6 m cables, whereby resistors of up to 1 Mohm have been connected between earth conductor 5 and screen 7 and have still given acceptable results. (Cf. the IEC requirement for insulation resistance for H05W-F which is 0.011 Mohm x km) . Therefore, the conductive plastic may have a rather high Mohm x km value.

Thus, a connection stretch of some cm would make some hundreds of Mohm of resistance to the earth con-

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ductor through the semiconductive plastic at an unwanted connection between un unjacketed earth conductor and other electric parts, which would be considered safe for human beings. The purpose with this mains cable design was initially to address the recording studio market as well as the radio/TV stations for clearing the studios of inter¬ ference noise, but an even larger market is the clearance from electric alternating fields in surroundings where people suffering from electric allergy stay. Also, for new installations there should be a considerable market share.

The most important difference between the variant of the invention shown in Fig. 3 and the previously described embodiment of electrically screened mains cables is the lack of lengthwise black coloured connnection con¬ ductors limited by the cable. Thus, the contrivance 101 at a mains cable 102, which is intended for screening of the cable from electric alternating field, comprises a jacket 150 extending around an earth conductor 105, which is received interiorly in a cable screen 107 extending along the cable 102, which jacket 150 is preferably made of multi¬ coloured, for example green/yellow, electrically semicon¬ ductive or conductive polymers, i.e. plastic. Said plastic is formed by polymer materials, preferably PVC containing for example a number of organic additives in order to obtain the desired properties of electric conductivity as well as the wanted colour of the product. Said jacket 150 of the earth conductor is likewise arranged so as to via connecting surface 151 between screen 107 and electrically conductive or semiconductive jacket 150 electrically connect the earth conductor 105 with said cable screen 107.

Also this cable 102 may exhibit an electri¬ cally semiconductive nylon screen 107 forming a cable screen

exteriorly around the earth conductor 105 and two conductors 111, 112.

Further, said conductors 105, 111, 112, which are evenly distributed inside the semiconductive nylon screen 107, may also be surrounded by an around said nylon screen 107 extending exterior insulating jacket 115, prefer¬ ably made of PVC.

Cables exhibiting the desired properties, i.e. concerning electric conductivity and colour, can be produced for example by adding suitable additives to the electrically conductive or semiconductive polymer material in question which is possible to colour. In order to obtain mixtures of polymers which are coloured green and yellow, respectively, one example is to add compounds with the designation CEE 0841 and CEE 0851, respectively, from the company Borealis Compounds AB.

The design is that a semiconductive nylon screen surrounds a mains cable with three conductors, where the earth conductor is not insulated, but provided with a jacket of yellow/green semiconductive plastic. The yellow/ green plastic bears against the screen, whereby the screen is automatically connected.

The bending fatigue limit of the nylon screen is far higher than that of a common copper twist. Also the environmental immunity is higher as far as the capacitive influence of humidity is concerned, and also the interference immunity is higher.

The idea of the invention is to colour it yellow/green and in this way automatize the screen connec¬ tion and still comply with the requirements.

A screened cable, where it is not necessary to connect the screen by means of a separate secondary conductor, but which is automatically screened by means of the yellow/green earth conductor. The very idea of the

invention is t _h e use of yellow/green coloured semicon ^d uctive plastic.

Regarding the safety as far as interior electric surge breakdown in the cable is concerned, this can _b e solve _{d b} y means of a double insulation of the two current carrying conductors.

To make use of conductive plastic as fill ⁱ ng material inside the cable would also increase the contact with the screen even if one should increase the resistance in the yellow/green semiconductive earth conductor insula- tion.

The invention is, however, not limited to the embodiments described above and shown in the drawings, but can be varied within the scope of the patent claims without departing from the idea of the invention.

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