Observing 3P Mains with an Oscilliscope? - Page 1
Forum section: electronics / general

author: SimpleOne
author_rank: Contributor
timestamp: November 06, 2020, 09:25:46 pm
content: Hi All,As part of a solar project I'm planning, I wanted to check the phase rotation on the lines coming into the property. I have a 4 Channel Rigol MSO5074. I also have one set of HV differential probes, and one single phase isolation transformer...none of which is immediately helpful in my mind to looking at 3 sets of mains line voltages safely and simultaneously (other than perhaps being able to float the scope's earth from the mains itself....)The major question is:What are the risks of spiking transient voltages, and is there anything I can connect inline (between mains and the oscope probes) to help filter/fuse transients before they hit the probes?Another option in my mind, is that one can probably do this fairly safely with three sets HV Differential probes, but that's a rather expensive undertaking. Another is probably a fully isolated oscope like a PC based one to a laptop on battery or a Cleverscope etc....Can anyone offer any other advice on how to go about it that might be somewhat more cost effective than those approaches?

author: ejeffrey
author_rank: Super Contributor
timestamp: November 06, 2020, 11:01:13 pm
content: Set the trigger input on the scope to "AC line" and then use the HV differential probe to look at each phase in turn.

author: SimpleOne
author_rank: Contributor
timestamp: November 06, 2020, 11:37:58 pm
content: Wouldn't that retrigger on each phase though, when you move the probe from line to line?i.e. I wouldn't be able to tell how each line was phase shifted relative to the other lines would I?I'm trying to determine the order of the phases on the supply lines, as that is not known. I was planning to use an arbitrary phase as the initial reference, and then compare the lead/lag of the other two lines to it. I assumed I would have to trigger on the first phase, align it's 0 crossing point to the LHS of the scope and then display the others simultaneously in order to determine their relative phasing. Can I somehow keep that same initial trigger reference, even as the probe is moved to the next two lines (thus avoiding needing to have 3 HV probes)?EDIT: Hang on, maybe I misunderstood, does the 'AC Line' trigger off the power supply line that scope itself is being powered from does it, rather than just an AC coupling on one of the probe channels (thus allowing the single HV's probes measurements to be relative to the supply of the scope)?

author: sandalcandal
author_rank: Unknown
timestamp: November 06, 2020, 11:46:17 pm
content: Anything you use to probe mains needs to be appropriately CAT rated (CAT IV in your case? CAT III if you're probing behind your distribution board) and thus should be appropriately designed to handle any transients. Best don't touch it with anything that isn't.If your differential probe is appropriately rated you'll be able to do as you require by following ejeffrey's suggestion. AC Line trigger will cause the scope to trigger off it's mains power input which should be one of your phases you are trying to measure.

author: SimpleOne
author_rank: Contributor
timestamp: November 06, 2020, 11:57:36 pm
content: Thanks fellas, that makes sense. I'd literally never looked at what that AC Line trigger did....obvious in hindsight!The probe is CAT III, so that's fine, I'll do it on the household side of the distribution board

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 07, 2020, 12:31:25 am
content: I don’t understand why direction matters but... please... keep the scope safe on the bench.Buy a phase rotation meter. Non-contact is the safest. There are scores of different models out there. There are very cheap contact models (<$20) but I wouldn’t trust my life to the cheesy probes and little plastic boxes they are packaged in. But, hey, that just me.https://www.amazon.com/DLG-PT01-Non-contact-Rotation-Tester/dp/B01NAH1M07/ref=pd_lpo_469_t_2/144-0316645-2756139?_encoding=UTF8

author: james_s
author_rank: Super Contributor
timestamp: November 07, 2020, 02:41:20 am
content: Couldn't you accomplish what you're trying to do by simply taking 3 stepdown transformers and connecting the scope to the low voltage secondaries using normal scope probes? The bandwidth will be seriously limited by iron transformers compared to differential probes but if you just want to see the phase relationships that shouldn't matter.

author: richard.cs
author_rank: Super Contributor
timestamp: November 07, 2020, 12:07:50 pm
content: If this is something you intend to do more than once build yourself a phase rotation tester, it's just a couple of cheap neon lamps and a RC phase shifter. Easy enough to package up safely into a plastic box with either a suitable 3-phase plug or some probes (as you probably have less than 3 hands you may want probes with croc clips, which also allows a much safer approach of connecting and disconnecting before power is applied). Or just buy one, they're very cheap.

author: nightfire
author_rank: Frequent Contributor
timestamp: November 07, 2020, 12:19:07 pm
content: Still better: Get yourself a voltage tester like the Duspol type (german brand/name) or Fluke T110 or similar. With that, you can probe voltages without the risk of having some multimeter accidently plugged in the amps section and you can usually put some load on the circuit by pressing some buttons to have a "Low-Z" measurement.Those testers have usually also some phase rotation indicator on them, you need to take several measurements as they only can compare two poles at each time.