The invention relates to time-division exchanges in which the connection between the subscribers is established successively in time at the same frequency as a sampling of telephone signals. It consists in carrying out the spatial multiplex switching stage simultaneously on k words in series mode, each word using a separate spatial multiplex switch. A spatial multiplex switching stage with k independent elements is thus provided.This invention relates to time-division exchanges, in which the connection between the subscribers is established successively in time at the same rate as a sampling of telephone signals. The invention also relates to methods reshaping an exchange of this kind.These signals are generally sampled at a frequency of 8kc/s, after which the value of each sample is coded by a number of 8 bits. In most cases, this is followed by concentration which makes it possible, for example, to reduce the number of subscribers capable of being simultaneously connected from 256 to 32, the others awaiting a free connection. The degree of concentration is governed by the traffic expected on the subscriber lines which is known statistically. In general, the concentration circuit then delivers a frame of 32 channels multiplexed in time which is supported by a so-called multiplex junction circuit. It is also possible to carry out concentration with analogue samples, followed by encoding.The above mentioned numerical values are not critical, although they are generally used because of national and international standards.One significant problem is to guarantee adequate safety of operations in the event of failure of an element so that, instead of all the lines served by the automatic switching system having to be taken out of service, the blocking level is merely increased to a small extent.One solution to this problem is described in French Patent Application No. 75.05799. It comprises dividing shaping of an exchange of the type in question into n identical and independent sections connected at the level of the space-division switches by n busbars. Considering these spatial multiplex switches thus connected as a whole, it is apparent that they could be placed in the form of a matrix with n inputs and n outputs comprising n.sup.2 connection points.In the case of a high-capacity exchange this number of connection points would be too high. One known solution to this problem comprises using a multistage spatial multiplex switching network made up of a large number of matrices of small dimensions. Finally the number of connection points of these matrices as a whole is distinctly below n.sup.2, but it is no longer possible to return to the structure described above and the preceding protection system is no longer applicable.However, protection is essential, especially at the level of the spatial multiplex switch, because switching matrices are integrated in one and the same housing in which the failure of one element has consequential effects on all the others. Taking as an example a very simple integrated circuit comprising 4 multiplexers with 8 multiple inputs and used in the normal way in a connection network operating in parallel mode at a frequency of 2 MHz, this circuit will be used for more than 1000 calls. A failure of a circuit such as this is extremely serious.Various processes for replacing all or some of the safety elements of the switching network have already been proposed. These processes are attended by the disadvantage of requiring a large number of switches which, even if they are of the static type, may themselves be the origin of failures. In addition, the rescue switching procedure is complex and involves the loss of a large number of calls.In accordance with the present invention a time-division exchange is provided for switching binary words of k bits from a plurality of incoming parallel-mode PCM junctions to a plurality of output parallel-mode PCM junctions, said exchange comprising:first time-division switching means connected to said incoming PCM junctions for delivering said binary words in parallel mode;parallel-series conversion means for receiving said binary words from said first time-division switching means and delivering said binary words in series mode on k + n pluralities of input connections;matrix switching means connected to said k + n pluralities of input connections for delivering said binary words in series mode on k + n pluralities of output connections; said matrix switching means being divided into k + n independent distinct elements, each of said elements being connected respectively to one of said pluralities of input connections and to one of said pluralities of output connections and presenting to said binary words a plurality of paths between said input connections and said output connections;series-parallel conversion means connected to said k + n pluralities of output connections for delivering said binary words in parallel mode; andsecond time-division switching means connected to said series parallel conversion means for delivering said binary words to said plurality of output parallel-mode PCM junctions.