Electrical devices with water-blocking insulation

Permeation of water between inner and outer insulating jackets of an elongate electrical heater or other device along a metallic braid between the jackets is limited by means of a water-blocking section. The water-blocking section is generally placed near the connected end of the device to prevent water from reaching the connection. In the water-blocking section, the outer jacket is removed and an insulating sleeve, preferably a heat-shrunk polymeric sleeve, is placed over the exposed length of the inner jacket and the ends of the outer jacket. A water-proof sealing material is placed between the sleeve and part only of the exposed inner jacket. In this way, water which permeates between the jackets to the water-blocking section is prevented from passing through the section but is permitted to escape from the device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to elongate electrical devices comprising two 
spaced-apart insulating jackets and means for limiting the damage which 
can result from water permeating along the device between the jackets, 
e.g., along a metallic braid between the jackets. 
2. Introduction to the Invention 
Elongate electrical devices of many kinds, in particular heaters and 
sensors, have been described in prior publications and in co-pending, 
commonly assigned, patent applications. Particularly useful are 
self-regulating heaters which comprise two or more elongate electrodes 
having a plurality of heating elements connected in parallel with each 
other between the electrodes; for example, heaters comprising two or more 
electrodes embedded in a PTC conductive polymer element are commercially 
available and give excellent results. Other heaters comprise a resistive 
heating element composed of a metal. Reference may be made for example to 
U.S. Pat. No. 4,242,573. 
Many elongate electrical devices comprise inner and outer insulating 
jackets which are spaced apart from each other, typically by a metallic 
braid which provides an electrical shield as well as added resistance to 
physical abuse. A disadvantage of this type of device is that if water 
enters the space between the jackets, e.g., as a result of damage to the 
outer jacket, the water can wick down the heater between the jackets. This 
can occur as a result of capillary action even when there is no 
hydrostatic pressure. If the water reaches either end of the device, there 
is a risk that the device will malfunction, e.g., by shorting the two (or 
more) electrodes of a parallel heater. So long as the water does not reach 
either end of the device, it does not usually have any adverse effect on 
the performance of the device because the inner jacket excludes the water 
from the electrically active part of the device. 
SUMMARY OF THE INVENTION 
I have now discovered that the risk of malfunction can be substantially 
reduced by providing the device with at least one water-blocking section 
through which water cannot wick and from which water can escape from the 
space between the insulating jackets to the exterior of the device. 
In one aspect, the invention provides an elongate electrical device which 
comprises 
(a) an electrical element; 
(b) an inner insulating jacket surrounding the electrical element; and 
(c) an outer insulating jacket surrounding but spaced apart from the inner 
jacket so that water entering the space between the jackets can wick along 
the device between the jackets, 
the device comprising, at at least one point along its length, a 
water-blocking section in which the outer jacket has been removed to 
provide an exposed length of the inner jacket, said water-blocking section 
comprising 
(d) an insulating sleeve which surrounds the exposed length of the inner 
jacket and the ends of the outer insulating jacket, and 
(e) a sealing material which provides a water-proof seal between the sleeve 
and part only of the exposed length of the inner jacket.

DETAILED DESCRIPTION OF THE INVENTION 
Since the purpose of the water-blocking section or sections is usually to 
prevent water from reaching one or both ends of the device, the 
water-blocking section is preferably located near one end of the device, 
with the sealing material providing a waterproof seal not only between the 
sleeve and part of the exposed length of inner jacket, but also between 
the sleeve and the end of the outer jacket which is nearer the adjacent 
end of the device. 
Suitable sealing materials are known in the art. Hot melt adhesives are 
preferred. The sealing material can be applied to the device (after 
removing a length of the outer jacket) before the sleeve is applied. 
Alternatively the sealing material and sleeve can be applied 
simultaneously by means of a sleeve having part of its inner surface 
covered with sealing material. It is important that the sealing material 
cover part only of the exposed inner jacket, so that water wicking along 
the device can escape from the water-blocking section. 
The sleeve is preferably one which is applied by shrinking a shrinkable, 
preferably heat-shrinkable, polymeric member around the water-blocking 
section. The sleeve can be a generally tubular member of closed cross 
section which is slipped over the end of the device or a wrap-around 
sleeve having the two edges secured to each other in any suitable way. 
Refer now to the drawing, FIGS. 1 and 2 show the water-blocking section of 
a self-regulating heater which comprises two electrodes 11 and 12 embedded 
in a PTC conductive polymer strip 13 which is surrounded in turn by an 
inner insulating jacket 14, a metallic braid 15 and an outer insulating 
jacket 16. In the water-blocking section a length of the outer jacket 16 
has been removed and part of the axial length of exposed metal braid has 
been impregnated around its entire cross-section with a hot melt adhesive 
17 which provides a waterproof seal between the inner jacket and an outer 
sleeve 18 which has been heat-shrunk around the heater. When water wicks 
up the metallic braid from the right hand end of the heater, it cannot 
pass through the water-blocking section to the left hand end of the 
heater, but can escape from the heater between the sleeve and the end 161 
of the outer jacket.