near vicinity and upstream of the electrode wheels 10 12 for clarity the sensor 91 is not shown in fig1 a delay circuit 92 between the sensor 91 and the gating circuit 90 delays the blanking pulses in accordance with the time taken for a gap as sensed by the sensor to move from the sensor to the weld location after modification by the gating circuit 90 the pulse train from the high pass filter 88 is compared in a comparator 92 with a reference value entered on a line 93 it is also passed to a filter 94 which passes only the relatively long duration pulses of the pulse train that is to say those having a duration of more than 2 ms or 2 of the weld cycle time these latter pulses are compared in a further comparator 95 with a further reference value entered on line 96 it is to be noted that the reference value on line 96 is substantially smaller than that on line 93 on receipt of a pulse from the filter 94 having a magnitude greater than the reference value on line 96 the comparator 95 produces a signal on an output line 97 in response to which the blank is rejected by means not shown after leaving the electrode wheels in this way blanks which are marred by the particular faults 80 and 84 previously discussed in relation to fig4 b are identified and rejected likewise a signal in response to which the respective blank is rejected appears on an output line 98 whenever the comparator 92 receives a pulse having a magnitude greater.