mike wrote:I have a rectifier that has 47.7 volts dc output but no mV or mA reading across the shunt. What kind of problem should I be looking for.
Mike - I will assume that there is nothing wrong with your shunt, ie. it was working before. In that case, remember that the current you get is the result of the your voltage and resistance (ohms law tells you that). Since you have voltage (and a lot of it) its telling me that your resistance is too high. What coud cause this high resistance? A very common problem is for either the anode or cathode cables to be broken or disconnected from the circuit.
I would first check the posts where the anode and cathode cables are connected to on the rectifier. Make sure that the cables ares securely fastened to the posts (with the power off of course). If so, most times the cathode cable may have been cut. If you suspect this then try to see if you can run a new cathode cable to the rectifier from the pipe and try the rectifier again.
Things can get very complicated if the problem lies on the anode side of the circuit. If the anodes are very old it could be that they have all been consumed and represent a very high resistance. If you know that the anodes were in good condition (based on your recent records indicating a sizeable current output) then it could mean that the header cable was cut or broken. If it wasn't cut by mechanical means it could have had a small cut in the insulation when it was installed and the cable has since corroded. This happens sometimes because in an impressed current system you are driving the current and the anode wire acts as part of the anode. Hope this helps. -gil