The proving unit proves the satisfactory condition of the voltage indicator, or at least that the lamps have not failed. As Zoomup has said, any malfunction would be obvious in use. I prefer the Drummond voltage indicator, which has 50, 120, 230 and 400 V lamps. What you really want to know is whether no lamps really means < 50 V so I can see the sense in checking the calibration at that level. I am not sure that the other levels are critical. After all, some voltage indicators go in steps of 50, 100, 200, 400 V. If you got 2 lamps when you expect 3, or 3 when you expect 4, I don't think that it would matter much and you could always check the voltage with your MFT, or even a suitable multimeter.
The proving unit proves the satisfactory condition of the voltage indicator, or at least that the lamps have not failed. As Zoomup has said, any malfunction would be obvious in use. I prefer the Drummond voltage indicator, which has 50, 120, 230 and 400 V lamps. What you really want to know is whether no lamps really means < 50 V so I can see the sense in checking the calibration at that level. I am not sure that the other levels are critical. After all, some voltage indicators go in steps of 50, 100, 200, 400 V. If you got 2 lamps when you expect 3, or 3 when you expect 4, I don't think that it would matter much and you could always check the voltage with your MFT, or even a suitable multimeter.
