GB2096411A

GB2096411A - An improved mineral insulated electric cable

Info

Publication number: GB2096411A
Application number: GB8209662A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1981-04-02
Filing date: 1982-04-01
Publication date: 1982-10-13
Also published as: GB2096411B

Abstract

To protect a mineral insulated wiring cable 6 and other components of an inducted circuit against voltage surges, the cable has secured to at least one of its ends a sealed termination 1 housing a varistor 9 which is electrically connected between the rectilinear conductor 5 of the cable and the cable sheath. The varistor 9, which is of resistance material that has a non-linear current voltage characteristic and that is capable of protecting circuits with operating voltages from ten to thousands of volts may be a preformed body having at least one throughbore in which the rectilinear conductor of the cable is a sufficiently tight fit to effect good electrical contact, or it may be a quantity of resistance material which is compacted into the termination at the time the cable is terminated. <IMAGE>

Description

SPECIFICATION An improved mineral insulated electric cable This invention relates to mineral insulated electric cables comprising at least one rectilinear conductor extending lengthwise within and insulated from a surrounding metal sheath, and where there is more than one rectilinear conductor from the other rectilinear conductor or conductors, by compacted mineral powder, usually but not necessarily magnesium oxide. The invention is particularly concerned with mineral insulated wiring cables in which the or each rectilinear conductor is made of copper or copper alloy, aluminium or aluminium alloy or other metal or metal alloy of high electrical conductivity.

Inductive circuits can produce surge voltages greatly exceeding the line voltage when the current flowing in them is suddenly interrupted and, under adverse conditions, these surge voltages can be suf ficient to cause a breakdown in a circuit component.

The factors influencing the generation of surges of dangerous magnitude are many and complex; they include the point in the current cycle at which switching occurs, the exact moment of seperation of the contact points and the presence of stray damping and other losses in the inductive circuit. Installations most likely to give trouble are those which are most frequently switched and these include airconditioning installations, refrigeration systems and other installations where equipment is cycled every few minutes, installations incorporating three-phase motors and/or contactors and fluorescent lighting installations.

When a mineral insulated wiring cable is used in an inductive circuit and the circuit is subjected to a voltage surge, in some circumstances the cable may be the circuit component with the lowest surge vol tagewithstand and, in such a case, a cable failure may result.

To protect a mineral insulated wiring cable and other components of an inductive circuit against voltage surges, it is the general practice to interconnect in the inductive circuit, usually at the equipment likely to generate a voltage surge, a separately formed suppressor device made of a resistance material having a non-linear current-voltage characteristic. Although the provision of a separately formed suppressor device is relatively inexpensive, it has the disadvantage that it is dependent on the contractor or user of the equipment likely to generate a voltage surge recognising the need for such a device.

According to the present invention, we provide an inproved mineral insulated wiring cable having secured to at least one of its ends a sealed termination housing a varistor, as hereinafter defined, which is electrically connected between the or each rectilinear conductor of the cable and cable sheath.

By the term "varistor" is meant a body resistance material which has a non-linear current-voltage characteristic and which is capable of protecting circuit with operating voltages from ten to thousand of volts.

The varistor housed in the termination may be a preformed body having at least one throughbore in which the rectilinear conductor of the cable is a sufficiently tight fit to effect good electrical contact, or it may be a quantity of resistance material which is compacted into the termination at the time the cable is terminated.

When the pot or other container ofthe sealed termination is of metal or metal alloy and is mechanically secured to the cable sheath, the varistor may be in direct physical contact with the pot or other container so that it is effectively electrically connected between the cable conductor and the metal sheath of the cable. Where the pot or other container of the sealed termination is of an electrically insulating material, the varistor may be in direct physical contact with an earth tail which is electrically connected to the cable sheath and which extends through and out of the sealed termination.In each of these cases, where the varistor housed in the termination is a preformed body, at least two spaced surfaces of the body that are in physical contact with the pot or other container or an earth tail end with at least one rectilinear conductor of the cable each has a coating or other covering layer of electrically conductive metal or metal alloy.

The invention also includes a kit of parts for effecting a sealed termination to an end of a mineral insulated wiring cable, which kit of parts includes a sealing pot or other container and, for housing in the pot or other container, a varistor, as hereinbefore defined, comprising a preformed body which has at least one throughbore and is made of resistance material having a non-linear current-voltage characteristic, or consisting of a quantity of such resistance material in a readily compactible state.

The invention is further illustrated by a description, by way of example, of two preferred sealed terminations of mineral insulated wiring cables with reference to the accompanying drawings, in which: Figure lisa sectional side view of the first sealed termination, and Figure 2 is a sectional side view of the second sealed termination.

In the sealed termination shown in Figure 1, the sealing pot 1 is made of metal and is of generally cup-like form, having a thick base 2 and a relatively thin peripheral wall 3 of cylindrical form. At the centre of the base is an internally screw threaded aperture 4 perpendicular to the outer surface of the base. The end of a mineral insulated cable 6 is stripped back and the apertured base of the pot 1 is screwed onto the cut back end 7 of the cable sheath so that it projects a short distance into the interior of the pot.Sealing of the cable end is completed by firstly partially filling the pot 1 with a waterproof non-drying insulating compound 8, inserting a preformed apertured varistor disc 9 over the conductor 5, the disc being in physical contact with the peripheral wall 3, and securing a closure disc 10 against a shoulder provided nearthe open end of the pot, the cable conductor, enclosed in an insulating sleeve 11, passing through an aperture in the closure disc.

In the sealed termination shown in Figure 2, a tubular body 21 is made of heat shrinkable polymeric material and is closed at one end by a wall 22 having two openings each bounded by an outwardly extending sleeve integral with the wall. One of the openings and its associated sleeve 23 is for accommodating the tail of a cable conductor 31 when the sealing device is applied to an end of a mineral insulated cable 30. The other opening and associated sleeve 24 has extending therethrough a single earth wire 26 which at one end is soldered to a copper collar 25 housed in, and extending around a major part of the periphery of, the bore of the body 21. A part of the internal surface of the tubular body 21 and of the internal surface of each of the sleeves 23 and 24 is lined with a coating 27 of heat sensitive adhesive.The mineral insulated cable 30 is cut back to expose the tail of the cable conductor 31 and a preformed aperture varistor disc 29 is applied over the tail of the cable conductor 31 so that it abuts the end face of the cut back cable. The body 21 and each of the sleeces 23, 24 have internal diameters such that when the sealing device is applied to the cut back sheath 32 of the mineral insulated cable 30 with the tail of the cable conductor 31 projecting through the sleeve 23 and heat is applied to the device, the body 21 shrinks onto the cable sheath 32 and varistor disc 29 to urge the collar 25 into electrical contact with the sheath, to urge the single earth wire 26 into electrical contact with the varistor disc 29 and to make a substantially fluid-tight seal with the sheath, and the sleeves 23 and 24, respectively, shrink onto and make substantially fluid-tight seals with the conductor tail 31 and earth wire 26. The heat to which the sealing device is subjected is also suffi cient to cause the coating 27 of heat sensitive adhe sive to melt and, on setting, to bond the body 21 and sleeves 23, 24, respectively, to the cable sheath 32 and conductors 31 and 26.

Claims

1. A mineral insulated wiring cable having sec ured to at least one of its a sealed termination housing a varistor, as hereinbefore defined, which is electrically connected between the or each rectilinear conductor of the cable and the cable sheath.

2. A mineral insulated wiring cable having secured to at least one of its ends a sealed termination housing a varistor, as hereinbefore defined, in the form of a preformed body which has at least one throughbore in which the rectilinear conductor of the cable is a sufficiently tight fit to effect good electrical contact and which is electrically connected to the cable sheath.

3. A mineral insulated wiring cable having secured to at least one of its ends a sealed termination housing a varistor, as hereinbefore defined, in the form of a quantity of compacted resistance material, which compacted material is electrically connected between the or each rectilinear conductor of the cable and the cable sheath.

4. A mineral insulated wiring cable as claimed in any one of the preceding Claims in which the pot or other container of the sealed termination is of metal or metal alloy and is mechanically secured to the cable sheath, wherein the varistor is in direct physical contact with the pot or other container.

5. A mineral insulated wiring cable as claimed in any one of Claims 1 to 3 in which the pot or other container of the sealed termination is of an electrically insulating material, wherein the varistor is in direct physical contact with an earth tail which is electrically connected to the cable sheath and which extends through and out of the sealed termination.

6. A mineral insulated wiring cable as claimed in Claim 1 or 2 in which the varistor is a preformed body and in which the pot or other container of the seal termination is of metal or metal alloy and the preformed body is in direct physical contact with the pot or other container or in which the pot or other container is of an electrically insulating material and the preformed body is in direct physical contact with an earth tail electrically connected to the cable sheath, wherein at least two spaced surfaces of the preformed body that are in physical contact with the pot or other container or an earth tail and with at least one rectilinear conductor of the cable each has a coating or other covering layer of electrically conductive metal or metal alloy.

7. A kit of parts for effecting a sealed termination to an end of a mineral insulating wiring cable, which kit of parts includes a sealing pot or other container and, for housing in the pot or other container, a varistor, as hereinbefore defined, comprising a preformed body which has at least one throughbore and is made of resistance material having a nonlinear current-voltage characteristic, or consisting of quantity of such resistance material in a readily compactible state.

8. A mineral insulated wiring cable having secured to at least one of its ends a sealed termination substantially as herein before described with reference to and as shown in Figure 1 of the accompanying drawings.

9. A mineral insulated wiring cable having secured to at least one of its ends a sealed termination substantially as herein before described with reference to and as shown in Figure 2 of the accompanying drawings.