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Apple iMac earth bond problem

PostPosted: Fri Feb 22, 2013 11:42 am
by Matt Macdermott
I have just tried to test an Apple iMac and cnnot get a earth bond reading low enough to pass; I tried all the bare metal I could find ie all the USB ports etc on the back and the case itself. I got readings from all of these but in the range 0.3 to0.42 ohms.Dunno what to do any ideas??

Re: Apple iMac earth bond problem

PostPosted: Fri Feb 22, 2013 1:30 pm
by Grizzly
What have you calculated as the resistance contributed by the supply cord?
When this is subtracted from your results above, is the remainder 0.1 ohm or less?

Re: Apple iMac earth bond problem

PostPosted: Fri Feb 22, 2013 4:18 pm
by Matt Macdermott
With csa of 0.75mm the supply cord resistance is 0.039 ohm (cord is 1.5m) so the readings are still too high for a pass (0.261 the lowest) so I'm still stuck as to what to do

Re: Apple iMac earth bond problem

PostPosted: Fri Feb 22, 2013 5:56 pm
by stu at ats
Sometimes a tarnished earth pin can give high earth bond readings.
A gentle polish with an emery board (nail file) can solve the problem.
Also mains lead do pull out from apples (trendy iec leads), a wiggle can help.

Re: Apple iMac earth bond problem

PostPosted: Mon Feb 25, 2013 11:49 am
by Patnik
Agree with stu that tarnished (oxide layer) earth is probably the culprit. I sometimes find the problem is where the IEC lead connects to the computer and plugging/unplugging a few times cleans the connector and gives a good reading.

Re: Apple iMac earth bond problem

PostPosted: Mon Feb 25, 2013 7:56 pm
by Alan Freeman
What is the earth bond resistance of the power cord on it's own?? Add 0.1 to that figure - are you in now??