Mains and the oscilloscope

[Ian.M]

OTOH if the ground wire is broken you probably wont be able to get enough current to trip the RCD. .You cant get a L-N imbalance if the current's got nowhere to go!

[David Hess]

OTOH if the ground wire is broken you probably wont be able to get enough current to trip the RCD. .You cant get a L-N imbalance if the current's got nowhere to go!

If that is the case, then you also cannot shock yourself except between the line and neutral which a RCD will not help with anyway.  Often the problem involves the sink or tub and for that, the RCD will work just fine.  At least here in the US, areas around water like the kitchen and bathroom are where RCDs are required.

[Neukyhm]

I have finally decided (until I buy better probes) to build a voltage divider with two big 10 Mohm resistors.

By the way, the oscilloscope I bought is the Siglent SDS1102CML+

[tggzzz]

I have finally decided (until I buy better probes) to build a voltage divider with two big 10 Mohm resistors.

So, before you do that you should consider the ways that could fail. And if you don't understand them all, you should reconsider.

Hint: it is unlikely that a beginner will be able to simply think, and reproduce all the hard-won experience gained over the decades. Hells teeth, I doubt I could think of all the ways.

[Neukyhm]

I have finally decided (until I buy better probes) to build a voltage divider with two big 10 Mohm resistors.

it is unlikely that a beginner will be able to simply think, and reproduce all the hard-won experience gained over the decades. Hells teeth, I doubt I could think of all the ways.

That's why I want to do things. As I said, I'm a physicist. In college, I learnt how transformers work but no idea about how to build one, so I had to learn that by myself. I have built some 100KV transformers, and I have blown up some capacitors in the way (it's fun) and I'm proud of it (the capacitors, I mean)

But with the oscilloscope is different, it's my first one. That and the high power of mains have led me to this forum so I can be extra sure that I don't break the oscilloscope or kill myself. Hey, I know what's 340V AC through my body for 10 seconds (it wasn't my fault, the inverter circuit of an automatic door was broken and the RCD too, it didn't jump).

I may have no experience with oscilloscopes but I'm into electric stuff.

PS: guys, you really have to test your RCD once a month at least xD

[Brumby]

Well, you have obviously dabbled a bit - and no doubt understand our concerns ... so I will finish by saying:  Stay safe.



But we are still a bit worried....

[tggzzz]

Hey, I know what's 340V AC through my body for 10 seconds (it wasn't my fault, the inverter circuit of an automatic door was broken and the RCD too, it didn't jump).

There are many types of "RCD" and different installations. However, in the UK I would expect an RCD to trip if you had >30mA through your body to ground. It wouldn't trip if the current was from live to neutral, nor if <30mA.

[Neukyhm]

Hey, I know what's 340V AC through my body for 10 seconds (it wasn't my fault, the inverter circuit of an automatic door was broken and the RCD too, it didn't jump).

There are many types of "RCD" and different installations. However, in the UK I would expect an RCD to trip if you had >30mA through your body to ground. It wouldn't trip if the current was from live to neutral, nor if <30mA.

But the thing is that my RCD was completely broken. I was 11 years old then (now 22) and I almost die. An electrician found the RCD to be dead, it didn't respond to the test button.

Well, you have obviously dabbled a bit - and no doubt understand our concerns ... so I will finish by saying:  Stay safe.

But we are still a bit worried....

I appreciate that, you guys are worrying about somebody who you don't even know.

[StillTrying]

By the way, the oscilloscope I bought is the Siglent SDS1102CML+

Why did you choose a CML+ ?
Did you find an online review anywhere ?

[Neukyhm]

By the way, the oscilloscope I bought is the Siglent SDS1102CML+

Why did you choose a CML+ ?
Did you find an online review anywhere ?

Is it a bad oscilloscope?

[StillTrying]

Is it a bad oscilloscope?

The jury (of about 1) is still out.  https://www.eevblog.com/forum/testgear/siglent-cml-firmware/

[tautech]

Is it a bad oscilloscope?

The jury's still out.  https://www.eevblog.com/forum/testgear/siglent-cml-firmware/

Yours is ~3rd that I know of that has had a problem, one of which I sold. 
The Plus version does seem to have some issues but I am very confident Siglent will fix them in short time. They must....there are many of this model in the marketplace.

SDS1102CML+ is my best seller.

[StillTrying]

Yours is ~3rd that I know of that has had a problem, one of which I sold. 

After I'd gone away, I was trying to remember which thread I'd just posted that in because I remembered that I'd forgotten to say that currently the CML+ jury consists of only about 1. 

The Plus version does seem to have some issues but I am very confident Siglent will fix them in short time. They must....there are many of this model in the marketplace.
SDS1102CML+ is my best seller.

In normal modes, being able to trigger on parts of a waveform 1mV or less high is 20 times better than my aging CRO, so I won't be throwing the CML+ away just yet!

[raspberrypi]

I think this is where I got the 50/50 idea from:

Never remove the Neutral (White wire) from the Neutral t
erminal Bar in the Service Entrance panel /
load Center / Distribution panel if the phase conductors are energized — it could be part of a Multi-
wire Branch Circuit, and removing it could damage the connected electrical loads / cause a fire.
removal of the Neutral wire will amount to the two 120 VAC loads being connected in series across
240 VAC. If the impedances of the two 120 VAC loads are not equal (have different Watt ratings), the
load with the higher impedance (higher watt rating) will see a dangerous over voltage condition >
120 VAC and may reach 240 VAC in case the other load is shorted (please see example in Fig 2 on
page 3).

[Neukyhm]

I have finally tested the oscilloscope, brand new and calibrated in factory last month, so I suppose it has the latest firmware. I have also measure mains with a 2x 10Mohm resistors and I'm still alive!

Just one thing, I have tested the "-" math function, it displays with a white cursor the result but nothing more, I mean, where is the voltage measured? I don't know how to make it display for example the Vrms of the subtract operation.

[tautech]

I have finally tested the oscilloscope, brand new and calibrated in factory last month, so I suppose it has the latest firmware.

In Utility/System info you can check the firmware version is the same as this one:
http://siglentamerica.com/gjjrj-xq.aspx?id=5074&tid=15

Just one thing, I have tested the "-" math function, it displays with a white cursor the result but nothing more, I mean, where is the voltage measured? I don't know how to make it display for example the Vrms of the subtract operation.

Spend some more time to become more familiar with the scope and time reading the manual on the CD.
IIRC you can turn on something in the Measurements menu to get it up on the OSD.
If I get a chance later I'll get one out and have a look.

[StillTrying]

Just one thing, I have tested the "-" math function, it displays with a white cursor the result but nothing more, I mean, where is the voltage measured? I don't know how to make it display for example the Vrms of the subtract operation.

Congratulations, it's your first bug find!
Ignore me, I've got all the math functions working somehow!

[Neukyhm]

Just one thing, I have tested the "-" math function, it displays with a white cursor the result but nothing more, I mean, where is the voltage measured? I don't know how to make it display for example the Vrms of the subtract operation.

Congratulations, it's your first bug find!  I'll add it to the other thread soon, unless you want to.

You can subtract two waveforms by using '+' and having one channel inverted, but these math functions are very rough.

With "white cursor" I ment that I can see the result waveform in white, but I can't or don't know how to display information about that waveform.

In Utility/System info you can check the firmware version is the same as this one:
http://siglentamerica.com/gjjrj-xq.aspx?id=5074&tid=15

I confirm it's the latest firmware.

[StillTrying]

With "white cursor" I ment that I can see the result waveform in white, but I can't or don't know how to display information about that waveform.

I think you can only go by the grid/graticule divisions, (no math auto measurements).
Perhaps your math waveform is hiding the "Math scale:" bit. (Edit: It is a daft place for them to have put it.)

The waveforms are just my finger touching the end of the probe tips - both connected together.

[Neukyhm]

Exactly the same, I can see the white waveform but not any information about it.

[tautech]

Exactly the same, I can see the white waveform but not any information about it.

I'll have a fiddle with a unit and a real simple setup, say 2 different DC voltages and various Math functions with a view to reporting your findings (when confirmed) to the factory.
Probably tonight as I have a heap on today. Factory closes for the weekend at 10pm my time.

[tautech]

Simple Math test.

Both channels 1V/div and and all Ref levels (0V) set to -2V, Ch1, Ch2 and Math.
Math scaling set to the same as active channels: 1V/div.
Supplied voltages:
Ch1 5VDC, Ch2 2VDC.

Math Ch1 - Ch2.



The result for Math is as expected: 3 div above Ref so 3 V.
The highlighted function button allows the Multifunction control to adjust the Math scaling (V/div).
The uppermost function button allows the Multifunction control to adjust the Math trace Reference level.

There is no Math OSD measurements in the UI.

Should we ask Siglent if this can be implemented ?

[Brumby]

Congratulations on getting your scope up and running - but as has been said above, I would also suggest you spend some time doing that weird thing - RTFM  (Read The Flippin' Manual).


As for this:

I have also measure mains with a 2x 10Mohm resistors and I'm still alive!

I had little doubt you would survive.  The chances of something going wrong are low - but they're not zero.

Just do me this one little favour ... imagine what would happen if 5kV or 10kV were to hit your setup.  Then, take steps to contain the aftermath ... and lastly, whenever you use it, expect that to happen.

You may never have the event - but it is better to be prepared than have to deal with the consequences.


I know I'm going on about this a bit - but you can't count on getting second chances.  (I'll step away from the soapbox now.)

[tggzzz]

You may never have the event - but it is better to be prepared than have to deal with the consequences.

Just so. Engineers (i.e. not hackers or amateurs) consider how things fail to be as important as how they work.

We'll see if the OP appreciates that, but so far the evidence is against it.

[Neukyhm]

Should we ask Siglent if this can be implemented ?

That would be very nice. As I said, I thought that I would see the option to measure whatever math function you are using in "Measure" menu, I expected to find that option just where you select to measure CH1 or CH2, I mean, I expected it to also measure "Math".