Hello.

I am wondering about a problem which I faced in two dc drives.
Yesterday, a fault occurred in two dc drives in a flexo machine that their LED type operation displayers not working. It was looked as their power was off. I checked and found fuses of control supply blown out. There were separated cabinets for each drive. I changed the fuses and investigated what happened. A technician was working with a contactor of a motor in the cabinet of drive #1, he accidentally touched the 110VAC control wire with 460VAC power line.
Actually, we have installed a step up transformer (TR1) 400V/460V for providing power supply to dc drives, motors and a step down transformer (TR2) 460V/110V which is used for control supply. The out put of TR1 (460V) touched with out put of TR2 (110V) which was isolated and caused the fuses to blown out.
This is the cause of my wondering that how can that possible? Can any body explain?
Thanks for any input.

If you dumped 460V down the 110 volt line, I am not surprised that you blew the control fuses, I am surprised that you did not do much further damage than that!!

The 110 Volt control circuit probably feeds a transformer and the 460 volts will have caused a excessive current to flow in the transformer primary and this would blow the fuse (and/or the transformer). Sounds like you were very lucky!!

Best regards,

Hello.

I think I couldn't explain my question well. I have tried to explain in a circuit diagram. Please find the attached file.
Thanks a lot.

Hello AB2005

Yes, I do understand, but if the transformer T2 is an isolating transformer, and the control input to the drives is also completely isolated internally, there may be a chance of survival, however if T2 is a standard step down transformer which is an auto transformer, and if there is any connection to the 110 internally other than to the primary of an internal isolation transformer, I would expect there to be a problem and you are lucky that there was not a lot of smoke to put back!!

In other words, I would expect there to be a problem unless the 110Volt circuit was completely isolated from all other circuits in all circuits connected to it.

Best regards,

Hello Marke

Thanks for your inputs.
I checked TR2 but there was no any information typed on it about its type, Step Down or Isolation. But according to your arguments, TR2 is an auto transformer because if it would a step down transformer, fuse blown fault not occur. Am I right?
Now I am going to replace this transformer with a Step Down transformer.