A time-lag fuse constructed so that a fuse element is wound on a core member made of a ceramic material having high thermal conductivity, for example, a material comprising high weight per cent aluminum oxide (Al.sub.2 O.sub.3, 85 - 100 wt.%). The fuse has excellent time-lag characteristics imparted by the strong heat-absorbing action of said high weight per cent aluminum oxide.

BACKGROUND OF THE INVENTION 
This invention relates to a time-lag fuse in which a fuse element is wound 
on an elongated core member having high-thermal conductivity. 
Time-lag fuses used in the past have some defects such that they are 
inferior in temperature characteristics to widely vary in time-lag 
characteristics. As an improvement developed and eliminate such defects, 
there are time-lag fuses in which a fuse element is spirally wound on an 
elongated core member made of a sintered ceramic material. This invention 
is an improvement made further on such time-lag fuse as a fuse for motor 
protection. 
One object of this invention is to provide a time-lag fuse in which a fuse 
element is wound on an elongated core member having high thermal 
conductivity, said fuse protecting a motor without melting even when a 
large drive current generated at the initial stage of the current flow. 
Another object of this invention is to provide a time-lag fuse having 
excellent time-lag characteristics in which the elongated core member is 
made of a material comprising high weight percent aluminum oxide. 
SUMMARY OF THE INVENTION 
The time-lag fuse of this invention is constructed in such a manner that a 
fuse element is wound on an elongated core member having high thermal 
conductivity (more than about 14 kcal/m.hr..degree. C). In this invention, 
the core member used for winding the fuse element therearound and in close 
contact therewith should have a strong heat-absorbing action for improving 
the time-lag characteristics. That is, a material to be used as the core 
member is selected from among those which have high thermal conductivity; 
therefore, when a large current generated at the start time of a motor 
flows, the high temperature generated from the fuse element is absorbed by 
the strong endothermic action of the core member in contact with the fuse 
element, so that it is possible to realize excellent time-lag 
characteristics on the order of several times the rated current capacity, 
whereby it becomes possible for the fuse to protect the motor without 
melting. 
The relationship between the composition of aluminum oxide and the thermal 
conductivity thereof is shown in the following table. 
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Thermal 
Conductivity 
Water 
Composition at 100.degree. C, 
absorptivity 
Material wt. % kcal/m.hr..degree. C 
% 
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Fused alumina 
Al.sub.2 O.sub.3 100 
26.0 0 
" Al.sub.2 O.sub.3 99 
18.0 0 
Fused alumina 
Al.sub.2 O.sub.3 96 
18.0 0 
" Al.sub.2 O.sub.3 85 
14.4 0 
Pure alumina 
Al.sub.2 O.sub.3 99 
14.4 5 - 12 
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As shown in the table, a ceramic material made of high weight percent 
aluminum oxide is very superior in high thermal conductivity. Therefore, 
the object of this invention, that is, to improve the thermal 
conductivity, is achieved by using a ceramic material made of high (85 - 
100) weight percent aluminum oxide as the core member.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to drawing, there is shown the construction of a time-lag 
fuse according to this invention in which a fuse element 2 is wound on an 
elongated core member 1 made of a ceramic material comprising high weight 
percent aluminum oxide, for example, 96% fused alumina, said core member 
being disposed in an insulated tubular member 3 diagonally and in intimate 
contact with sealing means 4 at both ends. 
When a large current generated at the initial stage of motor driving flows 
to the fuse element, the core member 1 having high thermal conductivity 
absorbs the heat from the fuse member, so that it is possible, without the 
fuse melting, to drive the motor; the fuse is strikingly superior in 
time-lag characteristics. 
In one tests, the time-lag fuse of this invention having a rated current 
capacity of 4.3 amperes remained unchanged even when a current of 16 
amperes was applied for 3 seconds, and, also, did not melt even when a 
current of 22 amperes was applied for 1.5 seconds. 
Thus, the time-lag fuse of this invention has really excellent time-lag 
characteristics imparted by the strong heat-absorbing action due to high 
thermal conductivity of the ceramic material consisting of high weight 
percent alumina.