#### mica

##### New member

Martin Cornfoot, the author, explained how to make leakage current and resistance meanssurements the ship's system isolated.

Values of resistance so measured were listed as follows:

10,000 Ohms - High Resistance - no problem

5,000 Ohms - Slight leakage, but not serious

1,000 Ohms - Significant leak, should be identified and eliminated

500 Ohms or less - serious and potentially catastrophic leak

I carried out resistance measurements on my own boat with a high impedence digital multirange test meter and the results were as follows:

With main switches off and ancilliary connections, Adverc sensor lead and shorepower leads, disconnected, measured resistance is 17,000 Ohms, so no problem. With the main fuse (100 amp) removed the resistance is 970,000 Ohms - even better. The problem arises when I measure the leakage current under these circuit conditions. The equivalent figures are 1) 8.5 milliamps and 2) 3.5 milliamps.

Now, applying Ohms Law which as every schoolboy and most sailors will know, relates to resistance by the formula E (voltage) = I (current) x R (resistance), the figures do not stack up.

In case 1) resistance 17Kohms is equivalent to a current of 12 volts divided by 17,000 ohms = 0.7 milliamps. The actual measured current is however much higher than this. The same applies to the second measurement. 12 volts divided by 970Kohms = 0.01mA.

To work it the other way round : 12 volts divided by 8.5mA = 1400 ohms, a worrying lower resistance figure. Even with the main fuse removed, the resistance by Ohms Law (12 volts/3.5mA) is about 3400 Ohms, not 970Kohms.

Which set of results should I believe, and why is there this discrepancy between measured current and measured resistance?

Can anyone explain or tell me where I am going wrong.

MICA