Note: Descriptions are shown in the official language in which they were submitted.
HEAT SENSITIVE CIRCUIT INT~RRUPTER
The present inventlon relates to a heat
sensitive circult interrupter.
There are ~any applications in which a reliable
heat sensitive circuit interrupter can be used to
adYantage. For example an interrupter whlch operates
to ~nterrupt a c~rcult when exposed to a temperature
at or above a predetermined critical temperature
can be used to trigger an alarm or any other appro-
priate response. One possible use of an interrupter
is to monitor the temperature o~ an item of equ~p-
ment and to shut down that equipment when the
critical temperature is detected.
Thermostats of conYentional type can perform
the function of a clrcuit interrupter. Thermostats
do suffer however from the limitation that they can
sense the temperature in their immediate vicinlty
but cannot detect overheating outside that vlcinity.
Thus in many circumstances thermostats can only be
used if the expense of installing a large number o~
them can be Justified. For example, it ~s highly
desirable to be able to detect overheating o~ cables
whether these cables are themselves used for heating
purposes or are simply used to carry power or informa-
tion slgnals. Thermostats cannot be used to detect
localised overheating in cables at acceptable cost.
Heating cables which are used for example to
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protect process plant against frost are generally
referred to a~ heating tapes. Such tapes are wrapped
around pipewor~ and covered in insulatlon. It is
not possible to detect "hot spots" reliably ln the
tape by monltoring current supplied or the tape
resistance and thus the tapes and the systems in
which they are incorporated must be deslgned to be
"fail safe" if they are to be used in hazardous
areas~ A ~ail safe design is one in whlch any
predictable fault cannot result in overheating.
A ~t-.5~fe design is expensive because lt requires
a higher degree o~ complexlty and a higher nom1 n~l
capacity than would be the case if the design was
not required to accommodate a variety of possible
fault conditions.
It has been known for many years that the
heating of an electrical circuit above a critical
temperature can be dstected by lncorporating in
the circuit a circuit interrupter in the form o~
a wire which melts at the critical temperature.
For example British Patent No. 336 270 dated 1929
- proposes a heating element energised via a wire
which melts to break the supply circuit when the
heating element becomes overheated. Such arrange-
ments have not found acceptance however becauselt ls generally necessary to cover the wire ln
lnsulation and when the wire melts its insulation
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often retalns the molten metal in i~s initial
position at least for some time, maint~n~ng
electrical continuity.
This molten metal retention effect is
described in Brltish Patent Nol 1 164 238 which
proposes to overcome the problem by supporting
the meltable wire without ~nsulation inside a
stiff insulating tube defining sufficlent space
lnternally to allow the molten metal to flow
easily away from its init~al position. One way
of providing this space is to ~ill the interior
of th~ stiff tube with an insulating substance
that is non-flammable and disintegrate~ or melts
at a temperature lower than that at whlch the
meltable wire melts. Examples of such ~lllers
giYen are silicon grease or a paste flux.
British Patent No. 1 141 234 also refers to
molten metal retention, and sugge~ts overcoming
the problem by providing a body which is capable
of absorblng the molten metal.
~oth the above suggested solut~ons to the
problem of molten metal retention are undeslrable .
as they require non-standard extra feature~ which
cannot be included in cables at low cost.
More recently, proposals have been made
as described ln published PCT Applicat~on WO
83/01138 to provide a monitoring cable in which
a meltable conductor ls separated from another
conductor by a ~ermeable insulator. When over-
heating occurs molten conductor diffuses through
the insulatcr and the resultant drop in resistance
between the two conductors is detected by suitable
monltoring equlpment. Accordlngly this device
relies upon the fact that the molten portion of
the meltable conductor re~o ~ n-~ in electrical
contact with the unmelted portion o~ the meltable
conductor leading to the monitoring Pquipment.
Flux ~s used in conventional meltable alloys
such as solder to help the molten metal "wet" a
sur~ace to which lt ls to adhere. Accordingly it
could reasonably be assumed that introducing flux
lnto a sheathed meltable wlre would increase the
probability o~ any molten portion of the wire
maintalnlng electrical continu~ty. Surprisingly
it has been discovered howeve~r that this ls not
the case.
It ls an ob~ect of the present invention to
provlde an lmproved circuit interrupter.
According to the present invention, there
is provided a heat sensitive circuit interrupter compris-
sing an electrical conductor made from a material of
predetermined melting temperature and supported
by an electrically insulating member which ls able
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5.
to withstand temperatures higher than the said
melting temperature~ wherein the material of the
electrical conductor and the material of the pol~ion
of the insulating member with which it ls ~n contact
are such that when the electrical conductor is
melted the contact angle between the molten
conductor and ths insulating member is sufficiently
large ~or the molten conductor to flow into separated
drops and thereby break the electrical continuity
of the conductorO
Preferably, the conductor is made from a
solder which incorporates flux. Solders consisting
of 60% tin, 40% lead and lncorporating longitudinal
cores of flux have proved particularly successful,
such solders being used conventionally for r~kin~
electrical connections. The solder m~ be rolled to
form a flat strip.
The present invent~on is based on the known
theory of the behaviour of a liquid when placed
on a solid flat surface, which behaviour is
dependent upon the contact angle. The contact
angle is defined as the angle subtended by the
flat surface and a tangent to the liquld surface
drawn from the edge of the liquld in a plane
Z5 perpendicular to the flat surface and the edge of
the liquid. If this contact angle is small, the
liquid wlll "wet" the flat surface. If the contact
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angle i8 large~ the liquid will form drops or
bubbles.
The contact angle is the resultant of three
thermodynamic forces F1, F~ and F3 that act on
each interface in the llquid/solid/surrounding
vapour system. These forces are related as follows:
F1 ~ F2 ~ F3 cos C
where C ~ contact angle
~ 1 = surface free energy of the solid/~apour
interface
F2 S sur~ace free energy of the solid~llquid
interface
F3 ~ surface free energy of the liquid/vapour
interface (surface tension)
The presence of a flux ln a molten solder
modifies these thermodynamic forces and hence will
af~ect the contact angle. It has been discovered
that a melted solder without flux will ~e as a
l~quid film and not contract into separated drops,
whereas a melted solder with flux will contract
into bubbles.
In con~entlonal solder, the flux is used to
help the liquid to wet the surface. In contrast,
in the present inYention, flux is used to cause
the llquid solder to separate into separated drops
or bubhles. This contrast can be explained by
consldering the 3 possible ef~ects that the flux
has .
~I ~L9~9~?~
1~ It chemically remoYes ( or corrodes )
any oxides or imperfections in the flat
surface that tend to give the surface a
high surfac~ energy.
2) It chemically removes (or corrodes) any
oxldes of lead and tin that may have
formed on the solder surfaoe.
~) Excess flux provides a thln low energy
film upon which the liquid solder can
move more easily.
In convent1onal ~oldering the removal of
deposits on the solid surface is the ~o~ n~nt
effect and hence the solder will wet the surface,
whilst in the present invention ths removal of
oxide film in the solder ~s the do~ nAnt effect
and hence the surface tension of the liquid solder
wlll cause it to flow lnto low energy forms 9 that
is drops or bubbles.
An embodiment of the present invention will
now be described, by way of example, with re~erence
to the accompAnh1n~ drawing which ls an end view of
a heating tape incorporating a heat sensitive
clrcuit interrupter according to the inventlon.
The illustrated heating tape comprlses a
sheath 1 within which two copper foils 2, 3 are
encasedO A woven heating element 4 is positioned
beneath the foils but electrically ~nsulated from
8.
them by a web 5 of lnsulatlng materlal. A palr of
foils 6, 7 of solder are posltioned on a support
fllm 8 above the copper foils 2, ~ so as to be
separated ~rom the copper ~oils by a web 9 of
insulatlng material. The support film 8 may ~e
of glass fibre or th0 plastics marketed as 'tKapton"~
Connections are made between the copper foils 2,
3 and the heatlng element 4 by lnserting rivets
through the heating elements and the copper ~oils
at spaced locations along the length of the tape.
For example rivets could be placed at one metre
intervals along each foil, the r~vets on one
foil being staggered by 50 cm relat~ve to the rivets
on the other foil. A heating tape having a woven
heating element and foil conductor structure of
this type is descr~bed in British Patent No.
1 523 129.
The illustrated heating tape is made up by
forming a core comprising the foil conductors 2, 3
embedded in an insulating body including webs 5
and 9. The outline of the core is indicated by a
dashed line 10 in the drawing. The solder foils
6, 7 are ~hen adhered to the film 8. The heat~ng
element 4 ls pressed against one side of the core,
and secured by rivets to the folls 27 ~,and the
fllm 8 ls pressed against the other slde. The
resulting assembly ls then encased ln the sheath.
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1 by an extrusion proces~.
The tape may have any convenient dimensions,
e. g. 20 mm wide and 4 mm thick. The solder foils
6, 7 may be formed by rolling out con~entional ~ine
multi-core lead/tin solder wire as used for ~k~ n~
connection~ to electron~c components to form a strip
approxlmately 4 mm wide. It has been found that
using such a solder foil a break of some 10 mm
width occurs ln the foll as soon as it is heated
to lts melting point, the molten solder flowin~
away ~rom the break to thicken the ends of the foil
on either side of the break.
Solders can be easily prepared whlch melt
at well defined temperatures over a wide range o~
temperatures, e.g. 100C to 300C. Thus the
illustrated tape can be used for a wide variety
of purposes.
It is possible to dispense with the copper
foils 2, ~ and use the solder folls 6, 7 to supply
ener~y to the heating element. In some circumstances
this might not be so advantageous however as i~
power is supplied via the solder sparks might occur
when it melts and breaks. In contrast in the
illustrated arrangement a low voltage monitoring
circult could be connected between the foils 6, 7
at one end of the tape, the other enfls of the ~oils
6, 7 being connected together. With a low voltage
10.
monitorlng clrcuit there is no risk of sparklng.
It will be appreciated that one of the
solder foils 6 9 7 could be replaced by a non-
fusible conductor of for example copper~
It will be appreciated that the lnvention has
appllcations not related to heating tapes. For
example a monitoring tape could be produced having
only one or two solder conductors withln lt and
no heating element or separate supply conductors.
The monitoring tape could then be placed in areas
where it is desired to detect excessi~e temperatures~
e~g. in electrical cable conduit, or in the ae~ e
o~ a warehouse, and co~nected to a simple circuit
adapted to sound an alarm i~ the solder conductor
~5 breaks. The monitorin~ tape could also be incorpora~
ted ~n equipment, e~g. the w~ nding5 of electrlc
motors, to automatlcally shut the equipment down
in the-event of overheating~
The lllustrated embodiment shows the A~older
conductors in the form of thin foils. It will
however be appreciated that the solder may be ln
other forms to suit particular applications provid-
lng that once molten it ls capable of flowlng to
form a break.
Experlments have shown that both single and
multl-core fluxed solder work satls~actori~y
although multl-core solder ls particularly good
as it flows more freely to form separate balls of
molten metal. Simple unfluxed solder generally
does not work as it melts but does not flow easily
to form a break. Unfluxed solder lying in flux
powder will also not work effectively if ~he fl~x
powder is allowed to oxidise.
The described embodiment of the invention
utilizes a solder in which flux is pro~ided ln the
form of` cores. The solder could however be
externally coated wi*h flux.
The term "solder" is used herein to mean any
electrically conductive fusible material~ Generally
solder will be ln the form of a low melting po~nt
fusible alloy. The flux can be of any suitable type~
but care must be taken to ensure that the flux ls
stable at the normal temperatures to which it is
in use exposedO
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