Wikipedia:Reference desk/Archives/Science/2018 March 6

= March 6 =

earthing
is there a way to test if something (a washing machine, say) is earthed properly (or at all)? Can I just measure the resistance between PE (earth clips, it's a Schuko type outlet and plug) and some exposed metal part with a DMM? I did mine and it was something like 700 Ohms - that's not ok, it should be very low, should it? 78.50.151.26 (talk) 17:04, 6 March 2018 (UTC)
 * Here is a basic outline on how to test if something is properly earthed (grounded in AmEng). -- Jayron 32 17:09, 6 March 2018 (UTC)
 * That's not the same thing. That article is about testing the fixed premises wiring, not the appliance. Andy Dingley (talk) 21:52, 6 March 2018 (UTC)
 * This is meant to be done with a dedicated earth continuity tester, that can inject 10s of Amps, and measure the voltage rise. They are commonly called Meggas, after the company that makes them. We don't seem to have an article on the device, or the company. LongHairedFop (talk) 21:49, 6 March 2018 (UTC)
 * "Meggers" aren't the earth impedance testers, but the high resistance insulation testers. We could certainly use an article on them, although we do have one on the company. Andy Dingley (talk) 21:51, 6 March 2018 (UTC)


 * I don't know German domestic regulations, but in the UK this would be part of portable appliance testing, usually known tautologically as 'PAT testing'. A quick guide to the requirements for it. A detailed guide to how to organise testing in a business, (requires sign-up). This is normally outsourced to the innumerable 'cottage industry' PAT testers: it costs a few hundred for a few days' course, a few hundred more for a test machine and then almost anyone can set themselves up as such a service. Cost of having a test done mostly depends on the number of items to test in a day: per-item costs are cheap when doing a whole office, but a call-out charge to test one item is a whole call-out charge.
 * Technically, the requirements are in here: Very simplified, the steps are:
 * Visual check for damage
 * Earth continuity test (low impedance measurement from the plug earth to the case)
 * Insulation resistance test (high impedance Megger test from Live & Neutral pins of the plug (connected together) to the Earth pin
 * Leakage current test (high impedance test from Live & Neutral to the case) - this may involve a high voltage 'flash' test.
 * Functional check
 * It's not necessary to use a specific PAT tester instrument for this, although it does need properly calibrated instruments, one of which is a Megger, a high-voltage resistance meter and many tests need to be carried out at current which aren't those that a normal ohmmeter works at. PAT testers are much quicker for bulk testing though. Competent PAT testing does need more than just "poking at it a bit with a Megger" though.
 * For your washing machine, the first thing is to eyeball check it for what's wrong - it should have an earthed cable with a clear earth connection to the case, just inside the cable entrance. If this is still connected, then I'd strongly expect that the cable or plug has a broken earth connection. Then do a continuity test  where you need a fractional ohm ohmmeter that can use a high testing current. This current can be 20-200mA for one testing method, or as much as 25A for an appliance like a washing machine  (this is why a PAT tester is often needed, as there aren't many other test instruments which can do this easily). The earth resistance must be no more than 0.1Ω (for the connection to the case), plus an allowance (usually even less than this 0.1Ω) for the cable resistance. [I'm not going to re-state the rest of PAT testing]
 * Your washing machine is a simple device, earthing wise. So I'd expect to be able to get the loop impedance right by no more than just looking at that one connection and probably swapping a damaged cable. Andy Dingley (talk) 21:50, 6 March 2018 (UTC)
 * Thank you everyone so much! I'll report back about the results 78.50.151.112 (talk) 14:51, 11 March 2018 (UTC)

Frozen-thawed eggs - yolks remained solid
I froze and later thawed some whole eggs and when I cracked them open, all three yolks were solid (they'd thawed over 12 hours at ~20 °C so they weren't still frozen). Was this caused by the freeze-thawing? — Preceding unsigned comment added by 185.222.219.151 (talk) 22:41, 6 March 2018 (UTC)
 * See also: —2606:A000:4C0C:E200:1D14:5B23:6FC6:8569 (talk) 00:22, 7 March 2018 (UTC)
 * See also: —2606:A000:4C0C:E200:1D14:5B23:6FC6:8569 (talk) 00:22, 7 March 2018 (UTC)
 * See also: —2606:A000:4C0C:E200:1D14:5B23:6FC6:8569 (talk) 00:22, 7 March 2018 (UTC)