Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
1999-11-03
2001-09-11
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S553000
Reexamination Certificate
active
06288372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical devices. More particularly, the present invention relates to electric cables, heating cables, and the like, having a ground plane layer of conductive polymer and drain conductor for providing ground fault detection.
2. Introduction to the Invention
Heating cables are well known in the art. These electrical devices typically comprise an elongate resistance body of an organic polymer such as a polyethylene or polyvinylidene fluoride having a particulate conductive filler such as carbon black effectively dispersed therein. The body is typically melt-extruded over two or more suitably gauged stranded metal (e.g. nickel or tin-coated copper) wires to produce an inner heater having a generally rectangular, oval or dog-bone cross-section. Many of these types of known electrical devices include a metallic braid which is provided to act as an electrical ground path and also to provide some mechanical reinforcement of the cable device. In many instances the heating cable has a resistance element manifesting a positive temperature coefficient (PTC) which renders the heater self-regulating about a desired temperature generally irrespective of its particular length. Self-regulating heating cables are commonly used as heaters for bodies such as liquid-containing vessels, and structures or substrates such as pipes, within chemical processes or other systems requiring temperature maintenance. Since heating cables may be used in a wide variety of applications and configurations, it is highly desirable that the heating cables manifest a sufficient degree of mechanical flexibility in order to be wrapped around pipes to be heated as well as providing a sufficient degree of toughness, wear resistance, and longevity. Heating cables powered by single phase AC power may extend for up to 1200 feet in length or longer, for example. Three-phase strip heaters may extend much farther, up to 12,000 feet in length or longer, for example.
It is useful and important to monitor the condition of a heating cable that may have been improperly installed in the first instance, or may have sustained physical damage or degradation after installation, such as a cut, puncture, tear, break, abrasion or other failure mode of the outer insulation, or of a ground braid element of the heater, in response to external impact or other externally caused abuse or misuse. By monitoring the heating cable condition one can increase the safety and reduce the possibility that a damaged heating cable will be used or remain in service and protect against hazards to personnel and equipment posed by any continuing use of damaged heating cables such as, for example, an explosion or a fire, particularly within hazardous environments. In order to protect against continued use of damaged heating cables, ground-fault protection devices (“GFPDs”) may be employed. GFPDs generally function to sense a current imbalance, trip, and thereupon interrupt a source of electrical power to the strip heater as by opening a circuit breaker or a set of contacts at a power distribution circuit breaker panel. GFPDs may be included within breaker switches. Discrete GFPDs may alternatively be installed at branch circuit breaker panels. GFPD equivalent functions may also be included within temperature/operational control or monitoring apparatus to which a heating cable may be connected. GFPDs for protecting apparatus and equipment are designed to trip at a relatively low fault current detection level, such as 20 mA to 360 mA or higher, and most typically 30 mA. GFPDs typically include, but are not limited to, ground-fault circuit interrupt (GFCI) devices which provide ground fault protection for personnel against shock. GFCI devices are typically set to trip at a 5 mA current level.
One example of a method of monitoring a heating cable for faults is described in U.S. Pat. No. 4,698,583 to co-inventor Chester L. Sandberg, entitled “Method of Monitoring a Heater For Faults”, the disclosure thereof being incorporated herein by reference.
With reference to
FIGS. 1 and 2
a conventional self-regulating heating cable
10
is shown as including two stranded electrical conductor
12
and
14
. In this particular example, the conductor
12
is denominated the phase lead and conductor
14
is denominated the neutral (return) lead. The conductor wires
12
and
14
are effectively and intimately embedded within a heater body
16
most preferably comprising a matrix polymer and conductive particles effectively dispersed therein. The heater body
16
most preferably manifests a positive temperature coefficient (PTC), so that the heating cable
10
is self-regulating about a design temperature following application of operating power, such as about 120 volts (alternating current) for example.
An inner jacket
18
of nonconductive thermoplastic or elastomeric material, such as polyethylene or ethylene-propylene-diene monomer (EPDM), respectively, is extruded over the heater body
16
, preferably using a tube-down extrusion technique. The innerjacket
18
and body
16
are then exposed to an electron beam or other ionizing radiation source at a selected energy level and for a controlled time period as to promote polymer crosslinking.
A metal wire braid
20
is woven or otherwise placed over the inner jacket
18
. A standards-specified ground plane braid, such as wire braid
20
, has a woven strand mesh density such that a 1 mm diameter probe passing through an outer jacket
22
at any arbitrary location will necessarily come into electrical contact with one or more strands of the braid. The braid
20
forms a ground plane for the heating cable
10
.
Using a tube-down extrusion technique, an outer jacket
22
of nonconductive material, which may be of the same type as the inner jacket
18
, is extruded over the wire braid
20
. Accordingly as shown in
FIGS. 1 and 2
, the conventional self-regulating heater cable
10
includes (progressively from its periphery to its center) the outer insulative jacket
22
, the wire braid
20
, the inner insulative jacket
18
, and the conductive polymer matrix heater body
16
which envelopes and electrically connects to the phase and neutral conductor wires
12
and
14
.
An alternative conventional heating cable construction
25
is shown in the
FIG. 1A
view. In this example, the phase and neutral stranded copper bus wire electrodes
12
and
14
are spaced apart by a nonconductive polymeric spacer
15
. A plurality of self-regulating conductive polymeric-fiber heating elements
17
are wrapped around, and connected to, the phase and neutral electrodes
12
and
14
. The construction
25
includes a conventional tinned-copper wire braid jacket
20
, and a nonconductive outer jacket
22
of e.g. fluoropolymer. Heating cables in accordance with the
FIG. 1
cable construction
25
are described in greater detail in U.S. Pat. No. 4,459,473 to Kamath, entitled “Self-Regulating Heaters”, the disclosure of which is incorporated herein by reference.
As shown in
FIG. 3
, electrical power is supplied to the cable
10
from a breaker panel
24
including a circuit breaker
26
for selectively connecting the phase conductor
12
to a phase bus
28
. The neutral conductor
14
is typically returned to a neutral bus
30
at the breaker panel
24
. A GFPD
32
typically located at the breaker panel
24
is connected to the conductors
12
and
14
, and to the neutral bus
30
. Braid
20
is then connected to ground. Any imbalance in current between the phase conductor
12
and the neutral conductor
14
is detected by the GFPD
32
, and if the imbalance is above a predetermined trip threshold, such as 30 mA, the GFPD
32
trips breaker
26
which thereupon disconnects the phase conductor
12
from the phase bus
28
. One reason for a current imbalance is an unwanted ground fault between the wire braid
20
and the phase conductor
12
, such as a current-leakage path
34
at some location along the cable
10
. The current-leakage path
34
may be the resul
Aune Ted M.
Gamarra Jose
Highe Albert J.
Orecchia Frank
Sandberg Chester L.
Hoang Tu Ba
Tyco Electronics Corporation
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