Scope´s ground

Hi,

I have a Tenma 72-8395 Scope at workshop. When I bought it, repairing some marine battery chargers I had problems tripping the earth leakage protection from the main electrical panel.

I found that the earth pin at 220Vac input plug is directly connected to output ground so when I tried to measure values at Vac I made a derivation to ground.
Finally I modified a cable to use only 220Vac without ground (only hot and neutral). The other day I felt small current at my skin touching a little bit the output ground, if I catch the ground connection I don´t feel anything but if I just touch it with a tiny part of my skin you fell something. Why?

I measured the ground output against 220Vac plug and to one point I get 217V, to the other 8V, but the earth cable is not connected to the scope...

There will probably a mains input filter on the scope. These cause some leakage to the earth wire on the scope. There is normally a capacitor from live an neutral to earth. The one between neutral and earth may be a larger value than the one between live and earth. Just as an example a 100 nF capacitor has a reactance of about 33K at 50 HZ The capacitors in the filter are probably about 4.7nF

Les

Most dual channel scopes have a provision for displaying a single trace that is (Channel1-Channel2). To use it, you have to have matched probes and set the vertical gain of both channels to the same Volts/Div.

This effectively makes a differential measurement using two probes so can keep the scope frame grounded through its line cord. You will have to look at the manual on your scope to find out what the maximum common-mode input voltage is limited to... (usually +-500V if using 10X probes).

Anybody that cuts off the third prong on the line cord of a scope (or uses a two-prong adapter) to isolate the frame of a scope is an idiot. I have fired a technician for that...

Les Jones said:

There will probably a mains input filter on the scope. These cause some leakage to the earth wire on the scope. There is normally a capacitor from live an neutral to earth. The one between neutral and earth may be a larger value than the one between live and earth. Just as an example a 100 nF capacitor has a reactance of about 33K at 50 HZ The capacitors in the filter are probably about 4.7nF

Les

Click to expand...

Thanks Les,

I can understand some leakage from one of the lines to ground, but I don´t understand why ground input is ground output at scope, is that normal? I think it should be isolated, if not any measure you do on mains AC is tripping the earth leakage protection.

Also I don´t understand why if there is not earth input (removed from the cable) when I measure against hot or neutral I still get 220V and 8V as if it was still there... I understand I should get 0V.

MikeMl said:

Most dual channel scopes have a provision for displaying a single trace that is (Channel1-Channel2). To use it, you have to have matched probes and set the vertical gain of both channels to the same Volts/Div.

This effectively makes a differential measurement using two probes so can keep the scope frame grounded through its line cord. You will have to look at the manual on your scope to find out what the maximum common-mode input voltage is limited to... (usually +-500V if using 10X probes).

Anybody that cuts off the third prong on the line cord of a scope (or uses a two-prong adapter) to isolate the frame of a scope is an idiot. I have fired a technician for that...

Click to expand...

Mike, I can understand you are not agree with what I did but I work alone, I did it under my responsability just to work with the scope at AC. There are a lot of things I don´t know yet about electronics but is my passion and now again my work. Due to country knowledge level, school, experience and a lot of different aspects your knowledge and basics about electronics can be higher so please respect. I´m here to help (if I can) and to learn (as much as possible), as I told you I work completely alone and sometimes is hard (no schematic support, no manufacturer support, only with my knowledge and my power to improve every day).

I upload the Tenma manual because I don´t how you do that:

**broken link removed** Operating Manual V1.0.pdf

You set scope to dual trace, invert channel B and look for a ADD mode switch.

2PAC Mafia said:

I can understand some leakage from one of the lines to ground, but I don´t understand why ground input is ground output at scope, is that normal? I think it should be isolated, if not any measure you do on mains AC is tripping the earth leakage protection.

Click to expand...

Yes that's normal, and that's how earthing works.

Officially, and for those who don't fully understand the implications, the device under test should be powered from a mains isolation transformer.

However, pretty well the entire service trade, and those who full understand the reasons for doing so, commonly disconnect the scope earth.

Earthing only makes things 'safer' under certain specific circumstances, in many other circumstances it makes it more dangerous - and pretty well all servicing falls in the 'more dangerous' category.

It's on often discussed subject on these forums - but unless you know EXACTLY why an item should be earthed or not, then leave it earthed (and the fact you've started this thread shows you don't understand it).

I would suggest you obtain a mains isolation transformer, it's a common requirement in service departments anyway.

Both of my scopes (An Advance OS240 and a Tektronix 2465B) have all the input grounds connected to earth. The reason the floating earth on the scope is at some potential between live and neutral is because the capacitors in the filter form a potential divider. IF the filter had been designed with two equal value capacitors. One between earth and neutral and one between live and neutral then The floating earth on the scope would read half the mains voltage. Filters often do not have equal value capacitors so this explains why the floating earth is not at half the mains voltage. I would expect the lower reading to be between neutral and the floating earth.

Edit.
To add to Nigel's post. Consider working on the primary side of a switched mode power supply. Your scope's earth would probably be connected to the negative of the bridge rectifier. This point would be at a lethal potential with respect to true earth. If you now touch the case of the scope at the same time as some item that is at true earth potential. (Which is connected to neutral at the substation transformer star point or where the mains cable enters your building.) you will receive a lethal shock. Even if you are thinking about this danger you only need to forget once.

Les.

Both of mine are unearthed, one doesnt have an earth on the power lead (philips), and the other I disconnected.
When I was daydreaming a while back I managed to destroy my laptop, I'm still not sure what happened, but I think the earth on my old 'scope was the cause.
I've also seen more than once wall warts that have a y rated cap from neutral to the o/p, including one where the mains in connector didnt have polarizing tabs so you could have it to the the live, the psu in your device probably has a cap to neutral.
As mentioned a isolation transformer is a really good idea, if like me you dont want to part with the cash, use 2 cheap transfomers back to back, then it doesnt really matter what the secondarys are, mine are control panel 110v transformers.

Mikebits said:

You set scope to dual trace, invert channel B and look for a ADD mode switch.

Click to expand...

You mean the math add function? Which channel I use to measure like that?

Nigel Goodwin said:

Yes that's normal, and that's how earthing works.

Officially, and for those who don't fully understand the implications, the device under test should be powered from a mains isolation transformer.

However, pretty well the entire service trade, and those who full understand the reasons for doing so, commonly disconnect the scope earth.

Earthing only makes things 'safer' under certain specific circumstances, in many other circumstances it makes it more dangerous - and pretty well all servicing falls in the 'more dangerous' category.

It's on often discussed subject on these forums - but unless you know EXACTLY why an item should be earthed or not, then leave it earthed (and the fact you've started this thread shows you don't understand it).

I would suggest you obtain a mains isolation transformer, it's a common requirement in service departments anyway.

Click to expand...

I have the mains isolator transformer, but it´s only 2A power so sometimes it didn´t supply me enough power to the units I was repairing. I guess because I put a serial bulb 100W so it was limiting the current input. Then I put 2 of them but I should look for a 400W load to put in serial.

Les Jones said:

Both of my scopes (An Advance OS240 and a Tektronix 2465B) have all the input grounds connected to earth. The reason the floating earth on the scope is at some potential between live and neutral is because the capacitors in the filter form a potential divider. IF the filter had been designed with two equal value capacitors. One between earth and neutral and one between live and neutral then The floating earth on the scope would read half the mains voltage. Filters often do not have equal value capacitors so this explains why the floating earth is not at half the mains voltage. I would expect the lower reading to be between neutral and the floating earth.

Edit.
To add to Nigel's post. Consider working on the primary side of a switched mode power supply. Your scope's earth would probably be connected to the negative of the bridge rectifier. This point would be at a lethal potential with respect to true earth. If you now touch the case of the scope at the same time as some item that is at true earth potential. (Which is connected to neutral at the substation transformer star point or where the mains cable enters your building.) you will receive a lethal shock. Even if you are thinking about this danger you only need to forget once.

Les.

Click to expand...

Thanks, in fact I already put the cable grounded back and this is the reason why I want to be safe and understand all the troubbles I find in my way.

dr pepper said:

Both of mine are unearthed, one doesnt have an earth on the power lead (philips), and the other I disconnected.
When I was daydreaming a while back I managed to destroy my laptop, I'm still not sure what happened, but I think the earth on my old 'scope was the cause.
I've also seen more than once wall warts that have a y rated cap from neutral to the o/p, including one where the mains in connector didnt have polarizing tabs so you could have it to the the live, the psu in your device probably has a cap to neutral.
As mentioned a isolation transformer is a really good idea, if like me you dont want to part with the cash, use 2 cheap transfomers back to back, then it doesnt really matter what the secondarys are, mine are control panel 110v transformers.

Click to expand...

So you had my same situation, do you know how to measure in the way the are explaining me with 2 channels?

Can you explain me how you did your own isolator transformer? You mean connect 220Vac to primary and secondary from that one to secondary from the other one, then you have 220Vac at primary?

Yes I know what mike means, the drawback is what do you do with the 'scope probe ground lead, unless the device under test has its ground earthed, which was the issue in the first place.

Yes your right, I put 240v ac into the primary of a 240 to 110 transformer, then stuffed that 110 secondary o/p into what would have been the 110v secondary o/p of another indentical transformer, which gave me back to 240v ac again at the second transformers primary.
Caution to be given here if you have a rcd or elcb then it wont protect you if its on the pimary side.

Can anybody explain in detail how to measure mains 220vac with the scope when earth input is connected to ground output?

2PAC Mafia said:

Can anybody explain in detail how to measure mains 220vac with the scope when earth input is connected to ground output?

Click to expand...

I thought I already told you.

1. Use a 10X probe on both Chan A and Chan B,

2. Check probe compensation on both channels. Compensate, if necessary.

3. Set both Channel A and Channel B to ~10V/div.

4. Set scope mode to A-B (Might require inverting Channel B and then setting the mode to A+B.

5. The trace is now showing you the difference between probe tip A and probe tip B. The scale will be 100V/div if you set the gain to 10V/div in step 3.

6. The only readings that make any sense are when both probes are connected to two points in the circuit. This way, the circuit can be supplied with an isolation transformer, or directly powered from the AC line... Using an iso. transformer is always safer...

Thanks Mike,

it will be the first time I use this method, I´m not sure if I understand point number 6, both probes should be connected at two points in the circuit, so in example, one to 220Vac and one to 5Vdc, then I have 220Vac + 5Vdc in the screen?

Is like this the scope using a floating ground?

ALL of my scopes, and most of my other test gear is grounded (with the exception of some OLD pieces).
NEVER float the ground on your scope. It's asking for trouble.
The DUT can be hooked to an isolation transformer so that you don't make a ground loop between the two.
Definitely a potential shock hazard. You feel a small electrical charge when you touch the probe,
just imagine if there was an internal problem and you had substantial voltage going to the frame.
It wouldn't trip a circuit breaker, but it will make your hair stand on end.

Everybody has it nailed right on.

it will be the first time I use this method, I´m not sure if I understand point number 6, both probes should be connected at two points in the circuit, so in example, one to 220Vac and one to 5Vdc, then I have 220Vac + 5Vdc in the screen?

Is like this the scope using a floating ground?

Click to expand...

Are you in an area where you have a 0-240 supply rather than a 120-0-120 supply? I think you are.

I didn't look at your manual YET, but pulling out the V/Div sometimes does an invert.

yep, scopes with the 3rd prong the inputs grounds and the third prong are one in the same. The scope usually has a metal case.

Meters could have different specs, but one I'm used to is say the front panel ground can' t be more than 30 V above real ground. This isn't for scopes.

Uploading is found under "More options" unless your account is messed up

Your manual link is messed up: here's the correct one http://www.restoretronic.com/descargas/72-8395 Operating Manual V1.0.pdf

Look at PDF page 24, the invert function. Inverting would be useful for display.

You have a math menu on PDF page 31, so you have to do it more complicated. A-B. or invert channel 2 and add which may make "MORE SENSE" when displayed as 3 traces. You can visualize add better. Maybe.

Troubleshooting SMPS power supplies require extreme care.



The A-B method is sometimes called pseudo differential. You need a reference somewhere. If it happens to be 8 VAC for whatever reason,so be it.

The scope measures A to the third prong and it measures B to the third prong. It inverts B and adds. It's not A-B, but rather A-R-B+R.

I know of a differential probe (non isolated) and an isolated differential amplifier. The latter is more expensive. The former has limitations.

Thanks for the explanation, I will test it, at the moment what I did is work with the earth again connected and modify my isolator transformer with a nearly 500W in series so I can use it without a low current limit. Yes, my area is 230Vac.

Here, https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CEYQFjABahUKEwjS0424nrjIAhWBVD4KHatOAFY&url=https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=1654001_CORCOM_PRODUCT_GUIDE_M_SERIES&DocType=Catalog+Section&DocLang=English&usg=AFQjCNGxs933X5FZJMvAlw7L8p0Hq1le0g&sig2=838NLtd4d5sDsgtQbUWX8w&bvm=bv.104819420,d.cWw

See the XM, ZX1 filter. There are two caps to ground on the right. An electrical model of the cap would have a resistor in parallel. That resistor is very high. Megohms or more. When you disconnect that ground, you have a voltage divider. The 0-240 system I haven't analyzed, but I've had grounds at ~1/2 the 120 V where the ground lifted on one of multiple devices that were plugged into an outlet strip.

I have a battery powered scope that I "float". It is double insulated. Made for the purpose of connected to the power line.
I also have "isolation" probes that are made to connect to the power line. They are much like using channel A-B but all in one box on one channel.
Ii is nice to have the right equipment.

Playing with ground don't pay I just got a job that I wanted better money. The guy I replaced was a nut He wired a house and almost killed the woman that lived here the water pipes was at 120 volts anything that was grounded stove ac was at 120 volt. If you touched the sink it was ground and the stove next to it you got hair raising shock.
It was crazy.
Test the tip with you dvm and see what you have turn the plug if it can turn not a 3 prog. I would power every thing i'm working on from the same outlet. Isolation transfomer Be safe you dont want the power using you as ground less then 70 mA can kill you.. Fixed some lights this guy installed I think he was trying to die. LOL

be80be said:

the water pipes was at 120 volts anything that was grounded stove ac was at 120 volt.

Click to expand...

Hey, he stole my electrical plumbing idea! Running the wires through the water pipes! LMAO
Just kidding. Electric is definitely NOT something to take lightly. I rewired a 480v 3 phase feed that powered some refrigerated trucks and installed a cut off switch one time.
It HAD to be hooked back up that night to plug the trucks in. Well the 240v leg was a direct connection to the meter with no breaker, and I couldn't get the electric company out soon enough to pull the meter, so I insulated some pliers really good and wore my rubber gloves (NOT the type I should have). It was all good, got my new wire in and was tightening the lug and my pliers slipped and shorted out. Blew 3/8 inch off one jaw, and shocked me down one arm and across my chest and out my other arm. There was a ditch there and I had to lean over it, against the metal building of course. I learned my lesson. Thankfully it didn't kill me. Just left my arms and chest muscles sore for a few days. Will not EVER do that again!
And ronsimpson, the battery powered scope is a great thing! Wish i had one sometimes.