| General > General Technical Chat |
| Hypothetical question about Isolated Supply, GFCI & Oscilloscope use |
| (1/2) > >> |
| bsodmike:
Hi all, Here are the assumptions as play: * We always float/isolate our DUT, never the other way around. * The mains breaker has GFCI already installed; this is how it came. Now, let's for a moment consider we have a mains supply, connected through an isolation transformer. The HOT lead at 240VAC runs through 2x 60W bulbs in parallel, acting as current limit devices for safety reasons of course. The oscilloscope is connected to mains-earth reference. The grounding lead on the probe is tied to mains-earth ground through the its power lead - for this demo it's left to flap in the breeze (safely!). We connect the probe tip-end onto the HOT (Live) wire coming off the Isolation supply. * Would the flow of AC current from Live through the Oscilloscope to ground cause the GFCI to trip? * Is it safe to use two probes, one connected to the Isolated HOT lead and the other on the Isolated Neutral lead; we can then use the MATH function to provide a computed signal of the mains voltage Of course, isn't the best approach here to use isolated differential probes for taking mains AC readings on an oscilloscope? Thanks! |
| bdunham7:
You should be more clear in your terms. Not that there are rigid definitions for terms like 'HOT', but you should be careful to be clear. HOT would typically mean a mains circuit with a significant potential to ground, or perhaps in some areas isolated from ground. NEUTRAL would mean a mains circuit current-carrying conductor with a potential near ground. Think of it as a return to ground, because that is how it is often wired. Depending on your situation, one or both of your incoming mains wires could both be HOT. ISOLATED means isolated and there is no hot or neutral unless you ground one side, either accidentally or deliberately. If I understand you correctly, you are using a normally connected scope to measure the outputs of an isolation transformer that is not otherwise connected--is that right? In that case, your single probe measurement will only get you noise, not a reliable reading. It could be anything, but you'll likely see your mains frequency, probably distorted at some voltage that I can't easily guess. The mains GFCI will not trip, unless perhaps you use a 1X probe with a 50R scope input and your isolation transformer leaks very badly or is under water. Two probes and MATH is the same principal as a differential probe and will work fine, only limited by how well your scope does the math. However, the point (one of them anyway) of the isolation transformer is that you now can somewhat arbitrarily select your ground point and measure from there. So if you simply grounded one side of your isolated output, you can now measure the other side directly. Of course, this defeats the touch safety feature of isolation. You can't have both at the same time. |
| David Hess:
--- Quote from: bsodmike on October 03, 2020, 02:32:59 pm ---Would the flow of AC current from Live through the Oscilloscope to ground cause the GFCI to trip? --- End quote --- No, because the isolation transformer on the DUT prevents any imbalance of hot and neutral currents. --- Quote ---Is it safe to use two probes, one connected to the Isolated HOT lead and the other on the Isolated Neutral lead; we can then use the MATH function to provide a computed signal of the mains voltage --- End quote --- Yes but ... since the DUT is isolated, its common mode voltage can drift around which may not provide the best results as it will subtract from the input voltage range of the probes. Since the DUT is isolated, the probe grounds can be connected to any single point on the DUT. The reason I use an isolation transformer is to that I can choose my own single point ground on the DUT, which is where the probe's ground clip goes. Without the isolation transformer, this point is chosen for you where ground and neutral are tied together which is typically back at the circuit breaker box. |
| nctnico:
--- Quote from: bsodmike on October 03, 2020, 02:32:59 pm ---Of course, isn't the best approach here to use isolated differential probes for taking mains AC readings on an oscilloscope? --- End quote --- Yes it is! Don't think about using isolation transformers. In order to use these safely you need your entire bench to be setup properly and you'll need training as well. With a differential probe you keep the potential danger area limited to the DUT itself so it is much easier to identify what is dangerous to touch and what is not. |
| bsodmike:
--- Quote from: bdunham7 on October 03, 2020, 03:09:36 pm ---You should be more clear in your terms. Not that there are rigid definitions for terms like 'HOT', but you should be careful to be clear. HOT would typically mean a mains circuit with a significant potential to ground, or perhaps in some areas isolated from ground. NEUTRAL would mean a mains circuit current-carrying conductor with a potential near ground. Think of it as a return to ground, because that is how it is often wired. Depending on your situation, one or both of your incoming mains wires could both be HOT. --- End quote --- Cheers for the heads up, I was using Live/Hot interchangeably, but that doesn't apply in the Isolated scenario. --- Quote from: bdunham7 on October 03, 2020, 03:09:36 pm ---ISOLATED means isolated and there is no hot or neutral unless you ground one side, either accidentally or deliberately. If I understand you correctly, you are using a normally connected scope to measure the outputs of an isolation transformer that is not otherwise connected--is that right? In that case, your single probe measurement will only get you noise, not a reliable reading. It could be anything, but you'll likely see your mains frequency, probably distorted at some voltage that I can't easily guess. The mains GFCI will not trip, unless perhaps you use a 1X probe with a 50R scope input and your isolation transformer leaks very badly or is under water. --- End quote --- Yes, exactly. Due to the galvanic isolation provided by the Isolation transformer, initially I thought that GFCI at the breaker box would not be affected, but later on I was like HMM... "What happens if I touch the probe's ground clip to one side of the Isolated output and the sensing-end of the probe to the other wire of the Isolated output"... of course at 10X probe impedance; thanks for the info on the 1X probe/50R scope input configuration.. interesting!! --- Quote from: bdunham7 on October 03, 2020, 03:09:36 pm ---Two probes and MATH is the same principal as a differential probe and will work fine, only limited by how well your scope does the math. However, the point (one of them anyway) of the isolation transformer is that you now can somewhat arbitrarily select your ground point and measure from there. So if you simply grounded one side of your isolated output, you can now measure the other side directly. Of course, this defeats the touch safety feature of isolation. You can't have both at the same time. --- End quote --- "Of course, this defeats the touch safety feature of isolation." -- Do you mean that the Oscilloscope frontend circuitry ground-points (especially the BNC connectors) are raised to a higher potential, and this could be a hazard or... ??? |
| Navigation |
| Message Index |
| Next page |