Oscilloscope grounding for power electronic (+ measuring DC rectifier voltage) - Page 1
Forum section: electronics / beginners

author: mike buba
author_rank: Contributor
timestamp: July 10, 2016, 06:33:49 pm
content: Again a question about oscilloscope grounding...So, there is Dave's video about oscilloscope grounding where he basically says Device Under Test should be isolated (i.e. floating).Than there is Tektronix document (www.tek.com/dl/51W_10640_1.pdf) where they say oscilloscope should not be connected through the isolating transformer.BUTthan this video uses floating osciloscope?? Othervise the fluse blows; around 2:25 min mark.Now I have couple of questions:There should be onlyONEgrounded point in the whole system. Sowhy did his fuse blow? If he connected AC input wires to L and N? Because oscilloscope is earth grounded this only means that his (earthing) system is TN-C or TN-C-S. Right?For power electronics applications (> 10 kW)mains isolating transformer is quiteexpensiveand not easily commercially available (probably have to be order directly from the factory!). I have been to several testing laboratories and have never seen people using isolating mains transformer, only small (250 W and 500 W) ones for oscilloscopes (even Tektronix), signal generators, etc. So why does Tektronix says no to floating oscilloscopes?

author: BobsURuncle
author_rank: Regular Contributor
timestamp: July 10, 2016, 07:04:13 pm
content: "So why does Tektronix says no to floating oscilloscopes?"On a scope there are exposed electrical surfaces, such as the probe BNC connection. If the scope is floating and you connect your probe ground clip to a hot spot in the device under test, then the exposed BNC jacks are now hot and present a serious shock hazard.He blew the fuse because he put the ground clip onto the hot 170Vp AC. He could have used two probes (without the ground clips) instead and used the math function to look at the difference voltage.

author: mike buba
author_rank: Contributor
timestamp: July 10, 2016, 07:15:55 pm
content: Yes, I'm aware of that.But if the scope is floating, then I have the freedom to define live and ground of measuring system, from a scope point of view, as I like it?Similar to using a voltmeter, the voltage will be +VD or -VD depending on how I turn the probes (red and black or black and red).Also, I do not mean to touch measuring equipment while there is voltage present in the system.

author: Kilrah
author_rank: Unknown
timestamp: July 10, 2016, 07:19:39 pm
content: It will work technically, but it's considered dangerous and thus not recommended. You do what you want, but people who are giving training advice will always want to stay on the safe side and say "don't do that, rather use proper equipment that allows you to do the job without compromising safety".Just don't touch that other probe ground clip you forgot to remove/isolate...

author: BobsURuncle
author_rank: Regular Contributor
timestamp: July 10, 2016, 07:21:46 pm
content: Quote from: mike buba on July 10, 2016, 07:15:55 pmThat's what everyone says before they get zapped or their negligence causes someone else, that wasn't expecting to touch 120V when they hooked up their probe, to get shocked! A big part of safety is developing good work habits so when you are distracted and make a mistake you don't pay too high a price. Everyone makes mistakes.

author: saturation
author_rank: Super Contributor
timestamp: July 10, 2016, 08:33:39 pm
content: Floating a mains powered DSO by defeating a ground plug has been done by many for decades but its unsafe practice for reasons the OP has already linked, the Tektronix document and Dave's video explains why itshould not be done.If the operator knows the whole path of the DUT and are correct, you get away with it. If the operator is wrong, consider test equipment are used to explore faults they know-not-where, then a shock risk exists that is wholly preventable. In addition, there is a risk of fire if a short circuit ignites the test leads or burns the DSO.In USA workplaces, defeating a ground plug on any equipment so equipped isillegalby Federal law. Naturally, you can do anything you want in a non-work setting and take the consequences.See 1910.334(a)(3)https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9911In non-workplaces, injury caused by unsafe practice can be held liable by either local laws or civil action, if for example, you injure anyone but yourself, or cause a fire or explosion and injure your neighbors property.The safest way to use a grounded DSO is with a separate differential probe, and there is the poor man's differential probe using channel A-B approach described in the Tek document. Keep in mind if you are probing mains using the A-B approach, you have limited CAT rated protection also limited by the probes response; typical DSO are CAT 1 rated, some are CAT II, whereas most mains power will be CAT III worse case and CAT II best case.

author: bitseeker
author_rank: Super Contributor
timestamp: July 10, 2016, 09:54:44 pm
content: In addition to the points already covered, if the scope is floated, someone might not realize or simply forget that this does not isolate the channels from one another. All of a standard scope's grounds are common and the ground clips still have to be attached wisely. Otherwise, dangerous short circuits are still possible.Of course, floating the DUT still has its dangers. So, either way, knowledge, understanding, and care are still necessary.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2016, 10:42:48 pm
content: Quote from: mike buba on July 10, 2016, 07:15:55 pmYou do but there are limits. The oscilloscope relies on galvanic isolation through its off-line power supply and this is limited. There is still considerable capacitive coupling between the AC line and the oscilloscope ground either through the oscilloscope or through an external isolation transformer.QuoteVoltmeters have some of the same problems to one extent or another. Excluding specific designs, the inputs have asymmetrical capacitive coupling either to the environment for a portable meter or to the AC line for a bench meter.QuoteNobody does but accidents happen.

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2016, 11:18:27 pm
content: Quote from: mike buba on July 10, 2016, 07:15:55 pmSnort. Clearly you are young and inexperienced.