Quoting gardenyu : > > First time user, thank you for any suggestion: > > > We have a motor controller built here: there are two turn-on and > fwd/rvs press buttons on a case top, which connects to the PCB > inside via some signal cable. The input to the board is 110V AC, > since there is no earth ground presents, the high and low input > voltage could float to higher than 110. "Float" implies that there is only leakage current possible, but it sounds more like the circuit ground is electrically "hot" (connected to the mains voltage). > The problem is: if the buttons are somehow shorted to earth > ground at the case top (even if we have built almost perfect > insulation on the chassis), there will be about 40mA comes from the > switch button, goes through human body, which kills. Is there > anything we can do to this situation? > > I'm just a junior engineer, so it might seems a stupid question > to you guys, but thank you very much for any help. Two options: 1) Galvanically isolate the part of the circuit that goes to the buttons (eg. optoisolators, transformers, that sort of thing). 2) Make sure the buttons are insulated so that accidental contact with mains voltage is just about impossible.... even if the product is abused or mis-used. For example, perhaps you could use a cheap tact switch, but put a long plastic operator on it. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" s...@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist