Floating handheld (battery power) oscilloscope newb questions - Page 1
Forum section: electronics / beginners

author: ColdSteel
author_rank: Contributor
timestamp: September 23, 2012, 05:14:08 pm
content: Hello forum.I bought an Uni-T UTD1025CL portable/handheld oscilloscope and was wondering about using it safely.Now, I know some of the Fluke scopes have isolated and insulated connections that allow safe floating measurements (no ground ref). However, I am wondering if this is just a marketing gimmick from Fluke to boost their sales, since my understanding is that all portable handheld battery powered scopes are safe to float.Please help me understand more.Basically, I only bought a scope to be able to visually see sine waves output from a Honda generator (just to see how sinusoidal it is). Being paranoid about 240V, I made sure to work behind a HPM RCD and MCB GPO (it is actually one of them "industrial worksite approved 4 double pole units). I am in no way an electrician, but rather just starting out to learn more about electrical stuff.Now, my understanding is with how to use a scope is to attach the GROUND LEAD on the probe to the EARTH on the generator, and then attach the actual PROBE to the ACTIVE, and from here I can get a 50Hz wave reading. However, I am unsure if firstly this is safe to do and if it is also okay to do using a portable battery powered scope that is not grounded itself (I don't connect the scope to the actual household mains power). I don't know if that is the right jargon to express. Nonetheless, I have successfully got measurements and frequencies from both the Honda Generator comparing the results without changing any parameters with household 240VAC 50Hz mains power as control reference, but I am unsure if it is safe and figured I better ask you experts if it is safe et cetera. I don't want a cardiac arrest over ignorance. I was that paranoid using the scope during measurements I used a wooden broomstick with probe on end to keep my nimble body at bay, all whilst wearing thick gumboots and poor man leather gloves!------------------------------QUESTIONS:How do I use the scope properly for my intended purpose of finding out how many Hz (Hertz) and visually see the waveform is being generated from my Honda Generator, and what leads to connect to what. I want to do it safely for both myself and the scope (obviously don't want to blow a fuse unnecessary).------------------------------Thanks.p.s. The Uni-T UTD1025CL is a nice DSO/DMM. My only gripe is the delay it takes to switch DSO and DMM. However, I am new to scopes and I have only had the scope for one week approx.

author: ColdSteel
author_rank: Contributor
timestamp: September 23, 2012, 05:19:05 pm
content: I'm wondering if I need to bother hooking up the ground clip (scope) to anything? I have not tried this and neither would I know if there would be any results.I blame Dave with his "how to blow up a oscilloscope" youtube video for freaking me out about being electrocuted.To ground or not to ground, that is the question.

author: IanB
author_rank: Super Contributor
timestamp: September 23, 2012, 06:14:38 pm
content: Perhaps a few pointers to help you out.Firstly, while mains voltages are of course dangerous, it can be even more dangerous to be excessively paranoid. It is enough to know and understand the risks, and then be careful and methodical in your actions. After all, you plug stuff into 240 V sockets every day, and operate highly dangerous 240 V light switches with your bare fingers. The electricity doesn't leap out and attack youAbout generators: if your generator is standing alone and not connected to any mains circuits, then it is already isolated and floating. It will not be connected to earth unless you physically make an earth connection for it. So it already has a degree of safety and RCD breakers will not add any extra safety. (RCD breakers work by detecting any current that flows in a "third way" outside the circuit. If a generator is floating and isolated, no "third way" or earth return path exists.)About the scope: the main parameter you need to know about is the maximum safe input voltage on the probes. The operating manual and specifications will tell you this. You need it to be 600 V or higher to make sure the scope can withstand mains voltages.To measure the mains frequency on the scope the safest way to proceed is to attach the scope probes between L and N while the circuit is switched off (only with a portable battery powered scope, not a bench scope!). Put the scope in automatic mode, then close the mains switch while not touching the scope. This should keep you safe. You can actually do the same with the real mains as long as the portable scope is sitting on a wooden bench or similar surface.A possible hazard might occur if the scope develops a fault and allows a large current to flow through the probes. In that case something might explode and produce flying fragments. So make sure to wear safety glasses and don't put your face inches away from anything energized.

author: ColdSteel
author_rank: Contributor
timestamp: September 23, 2012, 06:53:00 pm
content: Thanks IanBOh, I should of expressed; this is 240VAC Australian 50Hz.Does your rule still apply (re: Live and Neutral)? Not Earth and Live/Active?My scope:- 1 x Channel (one probe)- Scope CAT II 300V, Meter CAT III 600VI bought it off Carmates (ebay)http://www.ebay.com.au/itm/UNI-T-Handheld-OSCILLOSCOPE-25MHz-200Ms-s-TFT-LCD-UTD1025CL-Sydney-1-Year-WRT-/261077863093Did I buy the wrong scope? Reason I ask is cause it says it is only rated CATII 300V for the scope.I don't know if it is okay to use or not.

author: ColdSteel
author_rank: Contributor
timestamp: September 23, 2012, 07:08:45 pm
content: I forgot to add IanB,My scope also supports and has both the x1 and x10 attenuation (option in the user interface of scope and also there is a switch thing on the probe itself). In all tests I conducted I had always used the x10 attenuation as my basic understanding is that it reduces the voltage by 10.So, the scope only receives 24V from a 240V source, thereby coming under the CAT II 300V maximum safe voltage allowed via scope mode.Theoretically bumping the scope up to CAT II 600V maximum when using the 10:1 mode on both the probe and unit itself (if the following Agilent PDF article is anything to go by even though I actually have a Uni-T scope/product:http://cp.literature.agilent.com/litweb/pdf/U1602-90015.pdf).Please correct me if I am wrong. :/..

author: IanB
author_rank: Super Contributor
timestamp: September 23, 2012, 07:15:02 pm
content: OK, I was just going to mention about the x10 probe attenuation. Yes, always use the probes in x10 mode.My perspective on 240 V 50 Hz is from the UK, but I think the Australian system is essentially the same.When you have a battery operated portable scope, you can in principle probe between any two points in the circuit. Therefore you don't need to worry about earth connections and can just probe directly between live and neutral. As long as the scope meter is not connected to anything else at the time it will be fine as one AC voltage is just like another.A scope input of CAT II 300 V is not perfectly ideal for measuring mains voltages, but as long as you keep the probes in x10 mode you should be OK. That scope is intended mainly for automotive applications rather than mains applications.

author: ColdSteel
author_rank: Contributor
timestamp: September 23, 2012, 07:33:28 pm
content: IanB,You my friend are a gem. I take heed to your advice. Brilliant stuff.I shall use this scope till I save up for a portable Fluke or Agilent that is rated to minimum 600V CAT III (instead of this Uni-T Cat II). Just in case I accidentally forget or bump the attenuation. O_OPlease feel free to add more.Cause I stuff around with small engines so much (re: fiddling and tuning) I wanted a scope that was able to instantly tell me if I have high RPM or if I had overlooked Brush/AVR et cetera. Something to help diagnose and check sine/modified wave quick helps aswell.I never saw this coming, always thought I would just stick to mechanical things in life.

author: Unknown
author_rank: Unknown
timestamp: September 23, 2012, 09:11:23 pm
content: Quote from: ColdSteel on September 23, 2012, 07:08:45 pmThe probe in 10x mode acts just like a 10x voltage divider. This should reduce the voltage under test and any transients by 90%. The catch is that the probe has to be able to handle this. The probe + scope system can only be used for CAT II 600 V circuits if the probe itself is rated for CAT II 600 V. If a transient shorts the attenuator in the probe, then the scope input will still see the full input voltage.Attenuation setting in the scope does not matter for safety, it only multiplies the displayed voltages by 10.

author: ColdSteel
author_rank: Contributor
timestamp: September 24, 2012, 12:44:44 am
content: Quote from: alm on September 23, 2012, 09:11:23 pmDid you do typo here? Are you sure it is multiplied and not divided. If so; that would literally mean a 240VAC steps up to 2400VAC. Is this correct? Something is not right here.The probe that came with my scope has the following information written on the nib thingy that you hold in your hand since I am a layman that clearly is still learning all the jargon but don't let me stop you:- 600VDC- PEAK AC- CAT IIWhat you said only confused me more.If my scope and probes are dangerous to use then please say so. I made it clear what my only intention and use of this instrument is for, so all I need to know is the correct and safe practice/use for my only intended purpose.Unless IanB is wrong with his/her advice and safe use I shall stick with his/her advice.

author: IanB
author_rank: Super Contributor
timestamp: September 24, 2012, 12:56:45 am
content: Quote from: ColdSteel on September 24, 2012, 12:44:44 amWhen the probe switch is placed in the x10 setting it divides the measured voltage by 10. So 240 V at the probe tip becomes 24 V at the scope input terminal. Then in order for the scope to display the correct voltage, it has to multiply by 10 internally to show the correct voltage on the screen.The danger is that if any fault develops in the probe, then that 240 V at the probe tip could cause the probe to fail, and could cause the full 240 V to appear at the scope input terminal. That is why I advised you to wear safety glasses and keep your face away from the probes. With a low power circuit a failure might just cause a bit of smoke. But with a high power circuit like the mains a failure can cause a flash and a bang and flying debris. Best to be safe and not be too close if that ever happens...

author: IanB
author_rank: Super Contributor
timestamp: September 24, 2012, 01:00:33 am
content: Note: when considering maximum voltages, remember that 240 V AC will give a peak voltage of 340 V at the top of the cycle. This is why a 300 V max rating is not enough to handle a 240 V mains circuit.

author: ColdSteel
author_rank: Contributor
timestamp: September 24, 2012, 01:39:06 pm
content: Thank you IanB for clarifying.Never thought about dangers of flashes a debris. Gives me good excuse to fancy dress up.I am curious though, even if I had the right safe and recommended tools for the job, should insulated gloves and eye glasses always be worn when dealing with 240V+ ?I have seen many of these top tier DMM and DSO manufacturers/sellers with ads showing the person wearing PPE (Personal Protective Equipment).Cheers.

author: IanB
author_rank: Super Contributor
timestamp: September 24, 2012, 02:42:35 pm
content: Quote from: ColdSteel on September 24, 2012, 01:39:06 pmSafety glasses yes. Gloves are really not necessary for testing ordinary domestic mains circuits. You see them worn when industrial electricians are working with really high energy circuits to protect against the danger of arc flash. Without appropriate PPE you could get severe burns from the intensity of the arc during a short circuit.

author: ColdSteel
author_rank: Contributor
timestamp: September 25, 2012, 11:03:16 pm
content: Thanks mate.I think to be on the safe side I will always wear PPE (incl. gloves and broomstick).Topic closed.