Electricity: electrical systems and devices – Safety and protection of systems and devices – Circuit interruption by thermal sensing
Patent
1992-12-24
1993-12-28
DeBoer, Todd E.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Circuit interruption by thermal sensing
337 87, 337364, 337371, H02H 504
Patent
active
052745252
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the thermal protection of an electrical line between a high temperature and a low temperature. The invention relates in particular to franking machines which involve electronic counting. At each franking operation, the franked value is stored in a memory backed-up by a battery having a long service life (for example, 10 years in the case of a lithium cell).
Retention of data can therefore be guaranteed if the batteries are replaced every five years on condition that the memories have not been subjected to any electronic or thermal shock.
Protection against electric shocks is achieved by placing within the franking machine at least two battery-backed memories which are continuously monitored for coherence of contents, in particular the identity of price totalizing counters. As soon as an abnormal condition appears, the machine goes into an error state, thus prohibiting its subsequent use but making it possible to determine the contents of at least one of the memories, considering that simultaneous destruction of both memories by electric shock is improbable.
This does not apply in the case of thermal protection of a machine in which all the elements of the machine are subjected simultaneously to the same shock.
Recording of data may be disturbed beyond a limiting temperature whilst the other functions of the machine are still valid. In order to prevent any fraud by heating of the machine, a thermal protector has therefore been placed in series on the franking control unit in order to prohibit any subsequent franking in the event of overstepping of a safety temperature for the recording of data such as 75.degree. C., for example.
Up to the present time, attention has been devoted only to heating which anyone can readily perform by means such as kitchen ovens.
There is employed for this purpose a manual-reset thermostat in which the heat-sensitive element is a dished bimetallic disk, the curvature of which is reversed at an operating temperature TN and which is restored to its initial curvature only by manual resetting or at a restoral temperature TR below 0.degree. C.
By way of example, TN is equal to 77.degree. C..+-.3.degree. C. whilst TR is lower than 0.degree. C.; the differential temperature or hysteresis equal to TN-TR is therefore higher than 77.degree. C.
Metal disks of this type are manufactured by the Comepa Company which adapts their temperatures by combination of the metals and of their curvature.
In regard to cooling, recording incidents occur only below -40.degree. C. and it was considered as difficult to maintain and employ a franking machine below -40.degree. C. This consideration has now been disproved since deep-freeze units have become widely available.
It therefore proves useful to protect data-recording machines also against low-temperature conditions.
The aim of the present invention is to permit the use of an electrical line, especially on a data-recording machine, only between two limiting temperatures, namely a high temperature and a low temperature.
In accordance with the invention, the device providing thermal protection of an electrical line between a high temperature TNC and a low temperature TRF is of the type comprising a two-state bithermosensitive element which controls a contact placed on the electrical line, the thermosensitive element being intended to change from a first state to the second state by heating to a first temperature TNC and to change back by cooling from the second state to the first state at a second temperature TRC lower than the first, the first and second temperatures being intended to form a hysteresis interval TRC, TNC.
In accordance with the invention, the protector comprises a second bithermosensitive element which is identical in design to the first, the hysteresis intervals (or differential temperatures) of the two elements being intended to include the high temperature TNC and the low temperature TRF, the first element being preconditioned so as to change state at the temperature TNC whilst the second is preconditi
REFERENCES:
patent: 3500277 (1970-03-01), Nardulli
patent: 3878499 (1975-04-01), Concin
patent: 4035756 (1977-07-01), Schmitt
Deboer Todd E.
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