sheet 50 must be thin enough to be capable of transmitting heat while still being thick enough to ensure electrical insulation the cap 5 is fixed to the support 2 by three polyamide screws 35 39 and another screw not shown but screwed into tapped hole 31 the support 2 is fixed to a spacer 45 by three screws 41 to 43 the receptacle 3 and the receptacle 4 are fixed to the support 2 by pegs 20 to 23 they bear against the spacer 45 via bearing surfaces 54 and 58 the spacer 45 bears against the support 2 via a shoulder 33 fig5 is a fragmentary section through the support 2 when associated with receptacles 4 and cap 5 the section being as seen looking at line bb this figure shows that peg 23 is constituted by a set screw whose head is the same diameter as its threaded shank this head is received in a smooth hole formed in the support 2 the cap 5 is screwed against the support 2 on its side opposite from the receptacles 4 by means of a screw 39 having a countersunk head such that the head of the screw is received within the thickness of the cap 5 the cap 5 is installed after the screw 23 and corresponding screws constituting the pegs 20 21 and 22 have been put in place these screws do not project outside their holes in the support 2 and they are masked by the cap 5 once it has been put into place the general shape of the support 2 is a plane shape occupying a sector of a disk having an angle of 120.