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Thread: Test & Tag Machine Calibration
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19th May 2021, 07:13 PM #46Most Valued Member
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Without commenting on the rights or wrongs of T&T requirements, I threw out a heap of old Australian made laboratory power supplies today for a variety of reasons. Some failed the visual inspection miserably. Others failed the earth conductivity test. On many of them the aluminium front and rear panels had >20 ohms of resistance to earth (exposed metal has to be <1 ohm). It could have been more- the machine tops out at 20 ohms. I also failed a few IEC leads due to poor earth continuity. So there is some benefit to the T&T process.
Chris
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19th May 2021, 07:44 PM #47Diamond Member
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19th May 2021, 08:58 PM #48Most Valued Member
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Nope. Put an Out of Service tag on it closest the plug end of the cord and give it back to the owner is the safest option.
At the site I work on electricians get sacked if they cut cords off for failing appliance testing.
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19th May 2021, 08:58 PM #49Member: Blue and white apron brigade
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Reminds me off the time when I went round to my sisters place to find accountant BIL play electronics with his 4 sons at the dining table.
I'm not sure why. but they needed an earth so BIL had taken an earth pin out of a standard plying and had it naked poking out of a mains GPO with a wire attached running up to the table top.
Apparenty they had been doing this fopr a few months and could not understand why I was speechless.
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21st May 2021, 01:22 PM #50Most Valued Member
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21st May 2021, 03:56 PM #51Most Valued Member
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For Class 1 appliances the test checks the resistance between the earth pin on the plug and a metal part of the appliance. You plug the appliance into the machine and touch a probe onto the metal part.
Chris
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21st May 2021, 05:51 PM #52
Hi Guys,
I believe that they stuff a known current into the earth connection and measure the voltage across it, about 1 amp I think. The only problem with this is it doesn't actually tell you where the high resistance is ! In the days of hand wired plugs it very often was the earth wire that was loose in the plug.
A multimeter will give you an idea of the earth wire resistance, but it doesn't put enough current through the circuit to ensure that the reading is valid.Best Regards:
Baron J.
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30th May 2021, 04:43 PM #53Member
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1st Jun 2021, 11:29 PM #54Senior Member
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Possibly not..
Parallel series pair will average out the variation between them, but if they were all from the same batch, they are likely to be all very close anyway, even if on the edge of their tolerance.
If you could get 4 completely random components you could end up with something closer.
(unless manufacturing has changed a lot in the last 30 years
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