| 摘要 |
<p>With reference to Figure 3, an electrically-powered heating panel 1 of flexible construction, in the form of an under-carpet heater comprises, a dual wire heating cable 2, with inner 3 and outer 4 heating wires thereof connected in series together, with fuse means 5, the inner and outer heating wires 3, 4 being of substantially equal electrical resistance. The inner and outer heating wires 3, 4, which are of coiled form, are electrically insulated from each other by inner insulation 6 of thermoplastics material. (Polyethylene.) Under normal conditions, the current flow from line L to neutral N is by way of fuse 5, inner heating wire 3, and then outer heating wire 4. As the inner insulation sheath 6 is of polyethylene, this sheath will soften/melt and allow substantial electrical contact, (creating a short circuit), to occur between the inner and outer wires 3, 4 in the event of excessive overheating at any point along the length of the heating cable 2. If a short circuit takes place at point "A" current will flow from the line L, through fuse 5 and inner wire 3 and onto the neutral N. Thus only the inner heating wire 3 remains in use, the outer wire 4 being by-passed. This halves the nominal resistance of the heating cable 2, and doubles the current flow. The increase in current flow may not be sufficient to blow the fuse 5 at once. However, the increased current will cause the inner insulation sheath 6 to further overheat, (and soften/melt), so as to cause further short circuits between the heating wires 3 and 4, and increase current flow sufficiently to ensure that the fuse 5 does indeed blow. The current supply is therefore terminated. Wherever an initial short circuit occurs, the nominal resistance is halved. <IMAGE></p> |
