Proper grounding for measuring wall AC signal with oscilloscope
Proper grounding for measuring wall AC signal with oscilloscope
(OP)
Hi all,
I need to simultaneously record the AC signal from two separate wall 120v AC signals with a scope. I've got a metal enclosure with two step-down transformers (neutral and hot of each input go the transformer, and ground goes to the enclosure). The secondary of each transformer goes to a scope input.
I'm not sure the proper way to connect the grounds. My questions:
* The two inputs need to be able to be connected to different outlets, different circuit breakers, or possibly even different neighborhood transformers. Is it a good idea to connect only one of the ground connections (on my input wires) to my enclosure, or should they be connected together?
* On the output side of my transformers, (the input to the oscilloscope), the "+" signals on the oscilloscope probes will connect to one terminal on the secondary of each transformer. The oscilloscope input ground will be connected to the other terminal on the secondary of both transformers. Should this ground be connected to my enclosure ground and therefore to the ground in the wall plug(s), or should it float with respect to my wall ground?
* Alternately, I can leave the ground from the wall unconnected from my system, and ground my enclosure to only the oscilloscope input ground. Would there be any reason not to do this?
Thanks!
I need to simultaneously record the AC signal from two separate wall 120v AC signals with a scope. I've got a metal enclosure with two step-down transformers (neutral and hot of each input go the transformer, and ground goes to the enclosure). The secondary of each transformer goes to a scope input.
I'm not sure the proper way to connect the grounds. My questions:
* The two inputs need to be able to be connected to different outlets, different circuit breakers, or possibly even different neighborhood transformers. Is it a good idea to connect only one of the ground connections (on my input wires) to my enclosure, or should they be connected together?
* On the output side of my transformers, (the input to the oscilloscope), the "+" signals on the oscilloscope probes will connect to one terminal on the secondary of each transformer. The oscilloscope input ground will be connected to the other terminal on the secondary of both transformers. Should this ground be connected to my enclosure ground and therefore to the ground in the wall plug(s), or should it float with respect to my wall ground?
* Alternately, I can leave the ground from the wall unconnected from my system, and ground my enclosure to only the oscilloscope input ground. Would there be any reason not to do this?
Thanks!





RE: Proper grounding for measuring wall AC signal with oscilloscope
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Proper grounding for measuring wall AC signal with oscilloscope
It may be more helpful to all of us if you tell us why you want to record waveforms (that may well be distorted waveforms through a small transformer). Someone may be able to give you an answer directly or at least direct you to some method of measuring safely and accurately.
Bill
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"Why not the best?"
Jimmy Carter
RE: Proper grounding for measuring wall AC signal with oscilloscope
Please let me know what other info you'd like me to provide in order for you to make a recommendation.
To simplify the question:
* are there any issues with connecting the ground of a scope input (which should be internally connected to the ground of the scope power cable) to the ground pin of a plug in my neighbor's house, for example ?
* it is not unsafe to scope the secondary of a step-down transformer when the primary is connected to neutral and hot, correct?
thanks for any additional advice you can give.
RE: Proper grounding for measuring wall AC signal with oscilloscope
And, if that is so, not the Turtle type, which is rather tough to get a grip on, but those Echelon and similar signals in the 50 - 100 kHz band or thereabouts.
It is a bad idea to measure the complete waveform. You will have hundreds of volts at 50 or 60 Hz and very low kHz levels.
I use an isolation transformer on the signal (not power) side and put a blocking capacitor on the primary of that transformer. That way, I can get a solid waveform to analyze. See attached pdf.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Proper grounding for measuring wall AC signal with oscilloscope
You can safely obtain a measurement between the hot and neutral of the 120Vac mains at your house as long as:
1. The signal at the instrument input terminals is stepped down to within the specified input range.
2. The instrument is connected to the mains through an isolation transformer (this will prevent an accidental short circuit).
3. The isolation transformer is connected to a power bar with over-current protection.
Attention!
With respect to grounding you need to consider the following:
1. There is likely a potential difference (around a few hundred millivolts) between the ground at your house and your neighbors house. Connecting the two grounds (your and your neighbors) may introduce a ground current loop.
2. In some cases the potential difference between the two grounds can exceed several thousand volts (for example, a lighting bolt hits your neighbors house), a potentially fatal situation.
I recommend that you measure each location simultaneously with two separate instruments (one instrument located at each location). The data acquired can be synchronized through time stamping of the samples (or blocks of data) and then combined lated on. I doubt if your scope has this functionality. Check out ni.com, they might have the product for your application.
RE: Proper grounding for measuring wall AC signal with oscilloscope
Any possibility of putting an opto-isolator on each source?
RE: Proper grounding for measuring wall AC signal with oscilloscope
RE: Proper grounding for measuring wall AC signal with oscilloscope
Z
RE: Proper grounding for measuring wall AC signal with oscilloscope
Hooking the chassis ground of an oscilloscope to one outlet, and having the scope plugged in with a 3wire plug into another outlet could easily blow the scope, and if the house is miswired (more common than you would think) could be deadly. Even using a step down transformer might not save you, as there can be leakage currents thru the windings.
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting