The invention relates to a selective electronic tripping device associated to a circuit breaker and comprising:
means for measuring the current flowing in the circuit breaker,
processing means connected to the means for measuring the current and comprising means for determining a quantity representative of the peak value of the current, means for comparing said quantity with an instantaneous tripping threshold so as to provide an instantaneous tripping signal when said quantity exceeds said threshold.
Known electronic tripping devices comprise a processing unit, generally with a microprocessor, performing in particular long delay, short delay and instantaneous tripping functions. To achieve selectivity between two circuit breakers connected in series or cascading, it is known to provide a high rating for the up-line circuit breaker and a lower rating for the down-line circuit breaker. In this case the instantaneous tripping selectivity is of the current intensity type, the instantaneous tripping threshold of the down-line circuit breaker being lower than that of the up-line circuit breaker.
It is also known to use a time-based selectivity between two circuit breakers. In this case the tripping device of the up-line circuit breaker has a greater instantaneous tripping delay than the instantaneous tripping time of the tripping device of the down-line circuit breaker.
These two types of selectivity are not always efficient when short-circuit currents are very high, since high current thresholds are reached in both circuit breakers and a too long time delay of the associated tripping device would be liable to lead to premature wear of the contacts of the up-line circuit breaker.
To reduce these drawbacks a selective tripping device has been proposed, in particular in the document EP-A-128084, comprising a counter enabling the successive opening and closing cycles of the circuit breaker contacts in case of a short-circuit to be counted.
It has also been proposed, in particular in the document EP-A-872939, to disable an instantaneous tripping order if repulsions are detected for primary currents lower than a preset threshold.
Another type of selective instantaneous tripping, which will be described in greater detail with respect to FIGS. 3 and 4, uses one time delay when the current has exceeded a first, low, threshold, and a second, higher, threshold after this time delay.
In all known selective tripping devices, it is in practice necessary to wait for at least two short-circuit current peaks before making an instantaneous tripping decision. This leads to detrimental wear of the circuit breaker contacts.
The object of the invention is to provide a selective tripping device not presenting these shortcomings. The selective tripping device according to the invention has to be faster than known selective tripping devices, while at the same time achieving selectivity with a down-line circuit breaker so as to reduce wear of the contacts as far as possible in the event of a short-circuit.
This object is achieved by the fact that the processing means comprise means for determining a second quantity, representative of the time necessary for the current to reach the peak value, and means for determining the instantaneous tripping threshold according to a decreasing function of the second quantity.
According to a development of the invention, the instantaneous tripping threshold varies according to the second quantity according to a curve disposed between first and second curves representative of envelopes of the peak values of the current, the first curve being obtained when the associated circuit breaker is not connected in series with another circuit breaker connected down-line and the second curve being obtained when the associated circuit breaker is connected in series with another circuit breaker connected down-line. The first and second curves are preferably determined experimentally.
The tripping device according to the invention thus enables the associated circuit breaker not to be opened for no purpose when the latter is connected in series with a down-line circuit breaker. However, in the absence of a down-line circuit breaker, the absence of the latter is detected in less than one current half-period, which enables the destructive effect of repulsion of the contacts in case of a short-circuit to be minimized.