Looking at my lighting earlier and see that my multimeter is measuring 230v from live to neutral at the ceiling light, but only70 ish volts from live to earth. Am I right in thinking the earth may be disconnected somewhere and the 70ish volts im reading is due to capacitance ? I cannot get my head round why its like this The live is definately at 230v so if the earth was disconnected I should not be getting any reading atall. Where is this 70v reading from ???? and between what ?? I need to check the continuity of the earth tomorrow but it cannot be continuous or i would be getting 230v, even a loose earth connection would still read full voltage would it not ??
This has become a classic 'gotcha' with digital meters. The capacitance of modern cables is perhaps 50-100pF between cores per metre. so the earth if it is not properly tied down then it floats in the middle of an L-N voltage divider of two capacitances . For few metre of cable, and a meter of a few million ohms input impedance you do not measure the half mains voltage but you do see a significant fraction of it. When you put the meter across the other part of the accidental divider you see a similar partial voltage. do not expect the two parts to add up to the total voltage, as you will always read lower on the side loaded by the meter.
This has become a classic 'gotcha' with digital meters. The capacitance of modern cables is perhaps 50-100pF between cores per metre. so the earth if it is not properly tied down then it floats in the middle of an L-N voltage divider of two capacitances . For few metre of cable, and a meter of a few million ohms input impedance you do not measure the half mains voltage but you do see a significant fraction of it. When you put the meter across the other part of the accidental divider you see a similar partial voltage. do not expect the two parts to add up to the total voltage, as you will always read lower on the side loaded by the meter.
