Probes for working with mains - Page 1
Forum section: electronics / testgear

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 10:41:38 am
content: Hi guys,I am planning to work with circuits connected to mains directly (in fact, through an insulation transformer), no differential probes, though. The scope will be hmo1002.I plan to use 1:10 probes (not switchable), are there any concerns regarding this? Or is it suggested to use 1:100 probes rather?Please share your thoughts, I am pretty new in this field. Thanks

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2016, 11:37:04 am
content: Quote from: pxl on March 07, 2016, 10:41:38 amYes, there are concerns - as you will find if you do even a little research.Start with the references onhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: Fungus
author_rank: Super Contributor
timestamp: March 07, 2016, 12:12:29 pm
content: Quote from: pxl on March 07, 2016, 10:41:38 amNo problems with the 10x probes, as such. Plenty of problems in other areas. Isolation transformers don't magically make it safe to work with mains electricity. There's still plenty of voltage and power in your circuit, remember to keep one hand in your pocket whenever you go near it.PS: Remember toisolate your 'scopeas well or you may as well not bother with the transformer.

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 12:27:18 pm
content: Quote from: Fungus on March 07, 2016, 12:12:29 pmHmm, I planned to isolate only the circuits and using common ground for both channel (ie. using the ground wire of only one channel, the other ground wire will be disconnected.This way the floating circuit will be grounded on one point and all measurements will be relative to this point.

author: Fungus
author_rank: Super Contributor
timestamp: March 07, 2016, 01:22:55 pm
content: Quote from: pxl on March 07, 2016, 12:27:18 pmBoth channels can have common ground but your 'scope has to be powered from the isolation transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2016, 01:37:17 pm
content: Floating the scope is strongly discouraged nowadays. Doing so is preferable to re-introducing a ground if working live on an appliance containing a SMPSU, but the *ONLY* acceptable points to connect the scope ground clip to are chassis or the -ve side of the reservoir cap, and if you connect *ANYTHING* else, even an aerial lead, you must be careful not to re-introduce a ground. If you cant make the measurement you need with these limitations, you *MUST* get a differential probe.N.B. doing any 3 phase power or US 120V-0-120V phase to phase measurements with a floating scope is *UNSAFE*. Don't even *THINK* about doing that.

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 02:56:30 pm
content: Thanks, it would be just regular 230V/50Hz circuits (Europe), nothing fancy, like 3 phase motor drive circuits.I know that the proper solution would be using two, proper differential high voltage probes, but that would cost a fortune. I could buy cheaper, chinese probes, of course, but, well, I have no trust in those at all.

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2016, 03:00:51 pm
content: I wouldn't even consider it unless *ALL* probing is on the appliance side of its power switch and fuse. What do you actually need to measure?

author: nctnico
author_rank: Super Contributor
timestamp: March 07, 2016, 03:07:14 pm
content: Quote from: Fungus on March 07, 2016, 01:22:55 pmNOOO! Please don't even think about doing this! There are still way too many things to go wrong in this setup. Isolation transformers are a thing from the past for this kind of work.The best way is to have a bench with a GFI (ground fault interrupter) and a differential probe with the proper CAT ratings. Please leave all your test gear grounded.

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 03:11:02 pm
content: Quote from: Ian.M on March 07, 2016, 01:37:17 pmI am ok with these limitations. The question, however, remains, is it ok to use 1:10 probes, or I would need go for 1:100? Theoretically, 1:10 should be enough, the max voltage will be 33 volts, a way lower than the 200V of the scope, still I am not that confident.

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 03:14:53 pm
content: Quote from: nctnico on March 07, 2016, 03:07:14 pmThanks, I will investigate this question deeper, there are no hurries here, and all of your experiences are appreciated!

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2016, 03:37:37 pm
content: A RCD (GFCI) upstream of an isolating transformer doesn't provide *ANY* protection downstream of it. If you are working on the primary side of a SMPSU, fed by an isolating transformer, the choices are: use differential probes, float the scope independently or work live. If you do float the scope(against manufacturer's advice and good workshop practice), for %DEITY%'s sake don't use a cheater cord or other method of breaking the earth connection, that relies on the functional insulation of the scope's own PSU transformer as the isolation barrier. Use a proper isolating transfomer, and the same care and precautions should be taken as for working live, with the whole scope case treated as a live conductor. A simple grub-screw in a knob could kill you if your hand slips while you are holding the probe in one hand and adjusting the scope with the other. Don't even consider it in a commercial environment or OSHA, HSE or whatever national safety body has jurisdiction will have cause to close you down.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2016, 04:06:34 pm
content: Quote from: pxl on March 07, 2016, 03:14:53 pmYou would be a fool to completely trust any safety-related answers on a forum such as this. I've already shown you where proper, professional safety-related information can be found.You would be wise tounderstandas many professional safety-related references as possible. They will all provide slightly different information in different ways, and it is up to you to choose what is safe for you and the people around you.

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 04:44:37 pm
content: yes, of course, I keep in mind this

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2016, 06:06:36 pm
content: Quote from: pxl on March 07, 2016, 02:56:30 pmLess than your funeral.Research "probe derating with frequency" and get 100:1 probes at least.Differential probe are the preferred option and while you have "expected" test connection methods in mind ATM, differential probes will allow previously "no go" POI to be connected to.

author: macboy
author_rank: Super Contributor
timestamp: March 07, 2016, 06:09:52 pm
content: Quote from: Fungus on March 07, 2016, 01:22:55 pmThis is extremely dangerous advice. You should never defeat the ground on test equipment by powering it through an isolation transformer.The DUT can be powered through an isolation transformer if one understands the few benefits and many remaining risks of doing so.

author: mos6502
author_rank: Frequent Contributor
timestamp: March 07, 2016, 07:06:07 pm
content: Quote from: Fungus on March 07, 2016, 01:22:55 pmAll the channels on the scopearecommon ground. It's not like you have an option.

author: pxl
author_rank: Regular Contributor
timestamp: March 07, 2016, 07:38:02 pm
content: I dont know guys, but this looks everything, but safe:http://jeelabs.org/wp-content/uploads/wp-content/uploads/2011/11/DSC_2773.jpgand look at that sine wave :/http://jeelabs.org/wp-content/uploads/2011/11/17/ac-mains-measurements/index.html(ok, proper probes are much different, I know)

author: D3f1ant
author_rank: Frequent Contributor
timestamp: March 08, 2016, 06:38:57 am
content: I've had a DP25 for a while and never had a problem with it, probably still is the best bang for the buck on the market.Also, mine doesn't look like that on the inside. This is what current models look likehttps://www.eevblog.com/forum/testgear/teardown-pintek-dp-25-differentail-probe/msg169543/#msg169543