# line voltage with oscilloscope

#### Volttrekkie

Joined Jul 27, 2017
63
There is this video on you tube where this guy says that you can measure the AC current at your breaker box with your oscilloscope if you connect the probe to either hot and the ground clip to neutral. He says just do not connect hot to hot and it will be ok. But other people are saying that you cannot do that. That it is dangerous. I am confused. Does anybody know for sure?

#### crutschow

Joined Mar 14, 2008
34,064
It can be dangerous if the hot and neutral are accidentally reversed.
Safer is to connect the ground clip to the safety ground terminal which is at the neutral potential.
That may add a little noise to the signal but that likely is not a problem for most purposes.

But that measures the voltage, not the current, which I assume is what you meant by "measure the AC current".

Joined Mar 10, 2018
4,057

#### MrAl

Joined Jun 17, 2014
11,278
Hello,

What kind of scope is this?

Some scopes can not do this even with a 10:1 probe because the voltage is still too high for the input.

I use an AC transformer myself so it is not only a lower voltage replica of the line voltage, it is also isolcated electrically from the line.

#### DickCappels

Joined Aug 21, 2008
10,126
Use probes rated for the maximum volts you will encounter.
Put one probe on Line and the other on neutral.
The scope ground clips are connected to earth ground.
Set the scope up to take a differential measurement "A-B" or "CH1 - CH2". Both channels must be the same number of volts per divisoin.

#### MrAl

Joined Jun 17, 2014
11,278
Use probes rated for the maximum volts you will encounter.
Put one probe on Line and the other on neutral.
The scope ground clips are connected to earth ground.
Set the scope up to take a differential measurement "A-B" or "CH1 - CH2". Both channels must be the same number of volts per divisoin.
Hi there,

Be aware that this wont work with some scopes because some can not take a larger input voltage. It really really depends on the scope. Some USB scopes can only take up to 6v on the input. They might be able to clip at 12v or even 20v, but i would never attempt 120vac with its 170v peak.

All i can say is check the manual for max input voltage allowance before trying anything.

#### ebp

Joined Feb 8, 2018
2,332
There are some reasonably decent 100:1 attenuator probes on the market at quite low cost. Most are rated for 1 kV or higher (MUST derate for frequency). Most are built much like ordinary scope probes but there are some, albeit more expensive, designed for better operator safety.

100:1 probes' typical tip capacitance is considerably lower than that of 10:1 probes, which sometimes makes them advantageous for certain low voltage work.

#### DickCappels

Joined Aug 21, 2008
10,126
I use a 100:1 probe for the Line voltage, which is 240 VAC here and some of the circuits I design. I see some decent looking ones on eBay for about US\$15

#### MrAl

Joined Jun 17, 2014
11,278
Hello again,

I have a 100:1 probe around somewhere too, have not used it in so long now.
I dont mind doing this with my CRT based scope, but with my USB scope i have second and third thoughts because i cant get over the idea of hooking one side of the line directly to the ground which no doubt is connected to the ground of the USB connector which of course is connected to the ground of the computer.
I know i can test the neutral of the outlet first to make sure it is really nearly at the same potential as the ground in the outlet, but i still dont like doing it.
The computer case is probably connected to ground of its outlet though so it might work, as long as the wiring in this older house is not too bad and there is no really significant voltage difference between grounds. I could try connecting right to the same outlet too i guess, which might work.
If anything serious goes wrong though i might loose the scope. I know someone who had this kind of accident not too long ago with a 200 dollar (USD) USB scope. Dont know what kind of probes they were using though. There was nothing apparently burnt inside the scope case, but it would not work at all anymore.

#### ebp

Joined Feb 8, 2018
2,332
Just what constitutes "ground" is always and forever an important concern with instrumentation. Using a isolated (typ. battery powered) meter with a suitable resistor of hundreds to a few thousand ohms and adequate power rating shunting the probes to check for voltage differences between presumptive grounds can be useful. The shunt resistor essentially allows an estimate of current that might flow. If the current looks low enough with the resistor, either a low value (say 10 ohms) could then be used as a shunt or the current measured directly with the meter. Starting with a direct current measurement is an invitation to a blown fuse in the meter, and some such fuses are rather expensive.

For anyone with a frequent need to scope AC mains with fairly high bandwidth, the best solution is one of the high-voltage differential probe boxes. They have input leads essentially the same as used for meters. They are inexpensive, as scope diff probes go, but still a few hundred dollars.

#### Volttrekkie

Joined Jul 27, 2017
63
OK. I see. Mine is a handheld. It says 10X: 600 working voltage. So, that setting would be OK? Up to 600 volts? But not the 1X:200 setting I assume.

#### MrAl

Joined Jun 17, 2014
11,278
Just what constitutes "ground" is always and forever an important concern with instrumentation. Using a isolated (typ. battery powered) meter with a suitable resistor of hundreds to a few thousand ohms and adequate power rating shunting the probes to check for voltage differences between presumptive grounds can be useful. The shunt resistor essentially allows an estimate of current that might flow. If the current looks low enough with the resistor, either a low value (say 10 ohms) could then be used as a shunt or the current measured directly with the meter. Starting with a direct current measurement is an invitation to a blown fuse in the meter, and some such fuses are rather expensive.

For anyone with a frequent need to scope AC mains with fairly high bandwidth, the best solution is one of the high-voltage differential probe boxes. They have input leads essentially the same as used for meters. They are inexpensive, as scope diff probes go, but still a few hundred dollars.
Hello there,

10 ohms isnt a good idea. I use 10k ohms. The reason is that if there is going to be a high current flow then 10 ohms wont do enough to protect the equipment. 10k is probably good enough, but you can start at 100k if you feel more comfortable or are working with 230vac.
You can also gradually decrease if you really feel the need. Down to 10k, 1k 100 ohms, then maybe that 10 ohms if you think you still need that.

If you get significant current flow though the 10k (or 100k) then you may want to void the test because that means there is low enough impedance to allow a damaging current flow. If on the other hand you see 100v before connecting the 10k and then 10v after connecting it, then you may be ok but a further test is a good idea too like with maybe 1k, then 100 ohms, accompanied by reduced voltage across that resistor.

In many cases you can even leave the 1k resistor in place to do the scope measurements. It connects the grounds, but because a passing test mark means there is little voltage across it, it wont affect the measurement too much anyway.

#### DickCappels

Joined Aug 21, 2008
10,126
(some text removed for clarity)

I have a 100:1 probe around somewhere too, have not used it in so long now.
I dont mind doing this with my CRT based scope, but with my USB scope i have second and third thoughts because i cant get over the idea of hooking one side of the line directly to the ground which no doubt is connected to the ground of the USB connector which of course is connected to the ground of the computer.
Please -Never connect a scope's ground clip to the mains, not even neutral because it is dangerous to you, your scope, and makes all of those little critters living in the soil between your lab/office/garage and the earth ground at the distribution transformer very uncomfortable. Seriously, don't do it.

If you have a two channel scope you can look at the power line differentially by subtracting one channel from the other.

In a pinch having only a single channel scope available, I once took too measurements -one of the Line and the other of the Neutral. The voltage should be in phases unless something is very wrong. The line voltage value can be subtracted from the neutral voltage value.

#### MrAl

Joined Jun 17, 2014
11,278
Please -Never connect a scope's ground clip to the mains, not even neutral because it is dangerous to you, your scope, and makes all of those little critters living in the soil between your lab/office/garage and the earth ground at the distribution transformer very uncomfortable. Seriously, don't do it.

If you have a two channel scope you can look at the power line differentially by subtracting one channel from the other.

In a pinch having only a single channel scope available, I once took too measurements -one of the Line and the other of the Neutral. The voltage should be in phases unless something is very wrong. The line voltage value can be subtracted from the neutral voltage value.
Hi there Dick,

I am not sure what you are saying here. On the one hand, you are saying not to connect the ground clip, then on the other hand you are saying that you measured the line voltage with your scope, so what did you use then a differential probe?

Not connecting the ground clip though is not always an option. I dont think new people coming into electronics should ever do it, so i agree, but seasoned electronics people know what they are doing or else they would never have gotten that far.
I have worked on systems up to around 30 kilowatts 3 phase 240vac systems and it was commonplace to use a single channel of a scope to measure any of the resulting voltages. It was necessary to measure various line voltages in order to predict behavior such as voltage regulation, current limit, THD, etc. In these cases the ground lead of the scope was connected to the neutral and the three line voltages measured in turn, one at at time, or else two of the three phase voltages were measured with two channels of the scope with the ground clip on neutral. This is the way it was always done. The scopes however were always the CRT based scopes as the USB scopes were not around yet, and digital scopes were just coming out some time later.

So perhaps you can specify some things to watch out for when doing this kind of things, such as what works and what doesnt work. For example the ground pin on the scope connector is either cut off or we used a three prong to two prong adapter and did not connect the ground terminal to the ground of the outlet that the scope was plugged into. On my own personal CRT scope i have the ground lead cut off permanently for example.

#### ebp

Joined Feb 8, 2018
2,332
By the time I've reduced the meter shunt resistance to 10 ohms I have already concluded that the current will be small. I certainly would never start with 10 ohms.

"If you get significant current flow though the 10k ..." then you can immediately conclude that there is a substantial voltage difference and it would be exceedingly foolish to directly connect the two nodes.

"Please -Never connect a scope's ground clip to the mains, not even neutral because it is dangerous to you, your scope, and makes all of those little critters living in the soil between your lab/office/garage and the earth ground at the distribution transformer very uncomfortable. Seriously, don't do it."

Where I live, electrical code requires a local ground and that ground is bonded to netural at the local distribution panel. BUT this is not universal and there is always the possibility of mis-wiring at a receptable.

You've never had anything to do with cathodic protection, have you? Or been anywhere where AC power is distributed by use of a single aerial metallic conductor and the return is earth ground? These things routinely put tens to hundreds of amperes through the dirt.

I admit to being astounded when I discovered it is common practice and legal to use building structure as return current for welders. That seems like an invitation for disaster.

"For example the ground pin on the scope connector is either cut off or we used a three prong to two prong adapter and did not connect the ground terminal to the ground of the outlet that the scope was plugged into."
Talk about an open invitation to disaster! That is appallingly unsafe and after half a warning a firing offense!

#### DickCappels

Joined Aug 21, 2008
10,126
@MrAl , Every two channel scope I have used has a way to display one channel subtracted or added to the other channe,, and that goes all the way back to the Tektronix 535 with a type 1A1 two channel plug-in. In measuring the AC Line, the ground clips go to earth ground.

As noted earlier one can make two measurements with respect to earth then find the difference.

Example:
Measure Line as 311 V P-P
Measure Neutral as 3.3V P-P
Voltage between Line and Neutral is 308V P-P

#### MrAl

Joined Jun 17, 2014
11,278
By the time I've reduced the meter shunt resistance to 10 ohms I have already concluded that the current will be small. I certainly would never start with 10 ohms.

"If you get significant current flow though the 10k ..." then you can immediately conclude that there is a substantial voltage difference and it would be exceedingly foolish to directly connect the two nodes.

"Please -Never connect a scope's ground clip to the mains, not even neutral because it is dangerous to you, your scope, and makes all of those little critters living in the soil between your lab/office/garage and the earth ground at the distribution transformer very uncomfortable. Seriously, don't do it."

Where I live, electrical code requires a local ground and that ground is bonded to netural at the local distribution panel. BUT this is not universal and there is always the possibility of mis-wiring at a receptable.

You've never had anything to do with cathodic protection, have you? Or been anywhere where AC power is distributed by use of a single aerial metallic conductor and the return is earth ground? These things routinely put tens to hundreds of amperes through the dirt.

I admit to being astounded when I discovered it is common practice and legal to use building structure as return current for welders. That seems like an invitation for disaster.

"For example the ground pin on the scope connector is either cut off or we used a three prong to two prong adapter and did not connect the ground terminal to the ground of the outlet that the scope was plugged into."
Talk about an open invitation to disaster! That is appallingly unsafe and after half a warning a firing offense!
"For example the ground pin on the scope connector is either cut off or we used a three prong to two prong adapter and did not connect the ground terminal to the ground of the outlet that the scope was plugged into."
Talk about an open invitation to disaster! That is appallingly unsafe and after half a warning a firing offense!
Hi,

Sorry but it sounds like you are talking through your hat. If you want to convince me then you'd have to convince all of the people i ever worked with in the past 40 years that worked on synthesized sine converters including LINE TIED solar applications as well as aircraft applications. That includes but not limited to Sandia Labs, Israeli Aircraft, and a host of other companies. You'd also have to change a host of manuals that tell how to measure output power and things like that.
BTW, you cant fire people that dont work for you especially when they work for another company you have no control over so saying anything about firing anyone is meaningless

This discussion isnt really limited to a scope really, a THD meter is another device often connected the same way.

#### Volttrekkie

Joined Jul 27, 2017
63
Hello there,

10 ohms isnt a good idea. I use 10k ohms. The reason is that if there is going to be a high current flow then 10 ohms wont do enough to protect the equipment. 10k is probably good enough, but you can start at 100k if you feel more comfortable or are working with 230vac.
You can also gradually decrease if you really feel the need. Down to 10k, 1k 100 ohms, then maybe that 10 ohms if you think you still need that.

If you get significant current flow though the 10k (or 100k) then you may want to void the test because that means there is low enough impedance to allow a damaging current flow. If on the other hand you see 100v before connecting the 10k and then 10v after connecting it, then you may be ok but a further test is a good idea too like with maybe 1k, then 100 ohms, accompanied by reduced voltage across that resistor.

In many cases you can even leave the 1k resistor in place to do the scope measurements. It connects the grounds, but because a passing test mark means there is little voltage across it, it wont affect the measurement too much anyway.