AC Earth Bond Test (Rigel 277)
Test Voltage.....6V rms nominal (no load)
Test Current.....100mA, 1A, 10A, 25A (0 - 0.2Ohm range as per IEC60601)
Range....1A, 10A, 25A.....0.00. - 19.99Ohm.
100mA.....0.00. - 4.99Ohm
Resolution....0.01Ohm
Accuracy....1A, 10A, 25A …+/- 5% of reading, +/- 2 digits
0.10A……...…..+/- 5% of reading, +/- 4 digits
As far as my reckoning on the back of a piece of scrap A4 goes, at typical 2m IEC320 lead resistances of around 0.075Ohm and at 100mA test current, the worst-case uncertainty in the measurements in the Rigel 277 indication may be as much as +/-60%, i.e. the indicated earth bond reading could be up to 1.20Ohm (This assumes there is good contact resistance at connections that does not require high "wetting current" to clear oxidation, etc, and no thermal effects in the earth). At best uncertainties of up +/-15% may be evident for earth bonds of 2.0Ohm (BS EN 60601-1 limit) at 25A test current and they may potentially be up to +/-25% at 0.10Ohm.
Perhaps my arithmetic is a bit shaky but I think this demonstrates, that for a tester in widespread use, limited resolution of the tester and errors in the measurement may limit the accuracy and most likely the accuracy & repeatability of earth bond measurement at lower currents and for lower resistances compared to higher resistances measured at higher test currents - even for "state of the art" technology available currently. Look at the published specifications for the Rigel 277, for example, in the operators' manual.
After looking at the information provided by JB it seems to me that the problem of poor contact resistance exacerbates the problem of poor accuracy & repeatability when insufficient wetting current is available - thus spurious "fails" are more likely to be produced at low current. Even at high currents measurements may be more repeatable (due to the wetting current maintaining low contact resistance) but not particularly accurate, as outlined above.
I think RICK was correct about this wetting current business - although a well-trained and experienced colleague I've worked with for a few years has commented on it in the past. How's the training coming along Alan? Have you found the degree course you're looking for yet?
