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Oscilloscope with BNC ground = physical ground

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sync40

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Hi to everyone,
I am not new to electronics, in special working with lower than 24 volts DC, or with mains 127 VAC (always by using optocouplers to driving triacs and so), but now I a have a great doubt regarding my Tektronix 465 purchased yesterday.
In the past I have been using isolated hand-held scopes that could connect to just every point without any issues. However, I had never liked the limited pixel resolution, so I was looking for a big deal on an analog scope, and purchased the 100 MHz (almost 30 years old) Tektronix 465, just to know after reading the first chapter of its user manual, that the ground signal terminal (that is, the outer BNC probes from the two input channels, and of course the metal case) is tied permanently to the third terminal of a polarized mains plug (the ground or "earth" on the power cable). I know that this could be for safety reasons, but my main concern is that my house (and almost none in the neighbor has the third "earth" connector installed). Of course the appliances are connected through "3 connector outlets" but only the live and neutral are implemented; the earth circular connector is simply not connected.

So I am now in paranoia just checking all the voltages in the house and with the oscilloscope on and off, and I am very confused:

(remember that the third "earth" connector is not implemented in my installation):
1) the scope is plugged into the mains but with the power switch off; I put my DVM to read AC voltage between the shorter (live) mains jack and the scope case (that is the same that the BNC probes ground) and reads about 74 volts.

2) the same as (1) but the DVM between the larger (neutral) mains jack and the scope case, and reads about 37 volts.

3) the same as (1) but with the scope powered and it reads about 60 volts.

4) the same as (4) but with the scope powered and it reads about 50 volts.

I am planning to use my scope in 5 volt digital circuitry, always isolated by a wallmart transformer, or a linear power supply (transformer, bridge, regulator), and attach the BNC outer ground to my 5 volts ground, and the BNC center to the point that I want to measure (just the traditional way). Should I worry about the 60 volts present at the BNC outer ground when the scope is on, to induce overvoltages in my 5 volts circuit? I believe that no matter what, that 60 volts are just the reference for the scope, and if my 5 volts circuit is really 65 volts for the scope, that should not interfere with my circuit. Am I right?

Could I suffer from an AC shock hazard of the 60 volts in the BNC connectors, even by touching it?

Thank you very much for any help.
 

cesare

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You can supply the scope with an isolation transformer
 

raghuram_msc

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Dear Friend,

The problem you have at your hand is quite common with places without GND line.
My UPS, which was working seamlessly at mt earleir place stopped giving even 15-20sec of backup (the spec is 15min!!). The CPU outer body was giving out significant static shocks to us. Same was the case with almost all electric appliances. Even my TV antenna terminal was giving out solid shock..!!

I did not have a DMM (dig Multi-Meter) at that time..(iam sure I would have gotten similar results like you if i had one..!) Then we got proper earthing to our home and then everything was suddenly OK asif nothing was wrong in the first place.

Summarily, Even though ur DVM reads out some nasty figures I dont think there is anything of very serious concern, except some irritating moments whenever you touch body of your equipement. The expense we incurred was too less (<1000 Rs) and I would very severly suggest you get yur earthing proper.

Apart from this yuo are running acertain risk of damaging ( iam not using the word "blowing off".!!) ur Tektronix if at all there is a high volt power supply spike.
Remember a scope uses series of amplifiers...

All the best,
Sai
 

stefano.delfiore

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sync40 said:
Hi to everyone,
I am not new to electronics, in special working with lower than 24 volts DC, or with mains 127 VAC (always by using optocouplers to driving triacs and so), but now I a have a great doubt regarding my Tektronix 465 purchased yesterday.
In the past I have been using isolated hand-held scopes that could connect to just every point without any issues. However, I had never liked the limited pixel resolution, so I was looking for a big deal on an analog scope, and purchased the 100 MHz (almost 30 years old) Tektronix 465, just to know after reading the first chapter of its user manual, that the ground signal terminal (that is, the outer BNC probes from the two input channels, and of course the metal case) is tied permanently to the third terminal of a polarized mains plug (the ground or "earth" on the power cable). I know that this could be for safety reasons, but my main concern is that my house (and almost none in the neighbor has the third "earth" connector installed). Of course the appliances are connected through "3 connector outlets" but only the live and neutral are implemented; the earth circular connector is simply not connected.

So I am now in paranoia just checking all the voltages in the house and with the oscilloscope on and off, and I am very confused:

(remember that the third "earth" connector is not implemented in my installation):
1) the scope is plugged into the mains but with the power switch off; I put my DVM to read AC voltage between the shorter (live) mains jack and the scope case (that is the same that the BNC probes ground) and reads about 74 volts.

2) the same as (1) but the DVM between the larger (neutral) mains jack and the scope case, and reads about 37 volts.

3) the same as (1) but with the scope powered and it reads about 60 volts.

4) the same as (4) but with the scope powered and it reads about 50 volts.

I am planning to use my scope in 5 volt digital circuitry, always isolated by a wallmart transformer, or a linear power supply (transformer, bridge, regulator), and attach the BNC outer ground to my 5 volts ground, and the BNC center to the point that I want to measure (just the traditional way). Should I worry about the 60 volts present at the BNC outer ground when the scope is on, to induce overvoltages in my 5 volts circuit? I believe that no matter what, that 60 volts are just the reference for the scope, and if my 5 volts circuit is really 65 volts for the scope, that should not interfere with my circuit. Am I right?

Could I suffer from an AC shock hazard of the 60 volts in the BNC connectors, even by touching it?

Thank you very much for any help.

the voltage that your DVN reads is probaly due to the input emc filter of your oscilloscope. An emc filter has Y2 class capacitor connected across live and groud (earth) and across neutral and ground. This is not not a risk ( electrical hazard) for you but , this could produce strange behaviour of the circuit under test or sometime the failure, when you connect the the ground probe to it.
If you want to see your mains without risk you must use an insulation transformer (as advised) and the cable ground of your scope floating. Or you could use a differential probe.
Sorry I know, I am a little curious but this could be useful for me, can I ask you where do you live ? (nation state)

thank
stefano
 

jallem

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sync40 said:
........I know that this could be for safety reasons, but my main concern is that my house (and almost none in the neighbor has the third "earth" connector installed). Of course the appliances are connected through "3 connector outlets" but only the live and neutral are implemented; the earth circular connector is simply not connected.
In old systems the electricity companies usually provided the customer with
live(hot) and neutral as you said but at the same time was mandatory to use
a ground(earth) rod at home. This happens to my house, an old 1954 house.
This practice was then changed due to too much stray current and the hazards
associated with it.
This means even if you do not see the earth or ground from the supply company pole
the ground or earth should be connected at home using a copper ROD.
 

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