simultaneous use of isolation transformer and HV differential probes - Page 1
Forum section: electronics / beginners

author: JeanF
author_rank: Regular Contributor
timestamp: September 04, 2019, 04:57:57 pm
content: Hello,Recently I was reading various threads on this forum about differential probes and isolation transformers. I’ve also read the Tektronix reference documents that tggzzz and others have pointed out, and also the EEVBlog videos #279 and #932 on the matter. I have one or two questions, I’ve not been able to find the answers in the previous threads (I apologize if they are there indeed)I understand that powering the DUT through an isolation transformer is safer than powering it directly from the mains, provided that the DUT is really truly floating without any unwanted coupling to earth ground. One (and only one) accidental contact with any point in the circuit can happen without electric shock.Is the last sentence still true if one uses, in addition to the transformer, a general purpose HV differential probe ? Those which have a very high impedance between the 2 input terminals and between input terminals to earth ground, but are not galvanically isolated.I would assume that in case of a single accidental contact, the current going through the operator, through ground, and then back to the circuit via the probe’s high resistance path would be minute and/or negligible, but am I missing something here ? Also, I’ve read that some forum members are strongly against isolation transformers, for reasons that have been discussed.Personal preference put aside, is it considered bad practice to use an isolation transformer and a HV differential probe (not galvanically isolated, but high impedance) at the same time ? Should one use only the HV diff probe instead, and use the transformer as a doorstop as some mentioned ?While reading I noted that many of you have a very strong commitment to safety and I appreciate that. Rest assured I will not try anything silly, this is just a theoretical question. I’m not even at my bench. I am aware of the most essential safety principles, such as “if you don’t know how to do it safely, don’t do it”, “don’t probe live circuits unless you really need to”, “work with one hand only”, etc.Thank you

author: bob91343
author_rank: Super Contributor
timestamp: September 04, 2019, 05:29:40 pm
content: Not a problem. Isolation from the mains is a good idea. Probes are designed to disturb circuits as little as possible.Don't dream up scenarios that have no practical implementation.I use any probes I care to use, and I don't even isolate most of the time. If I'm working on gear that isn't isolated or if I am not even sure, I will isolate.This has no connection (pun not intended) with the use of probes. The best probes will not disturb a circuit enough toe contaminate a measurement, and isolation doesn't generally enter into the equiation. The 'ground' side of a probe is usually connected to the measuring device chassis, which is usually connected to the power line ground system. This is good. The device under test may also have similar grounding setups so there is no danger. If it doesn't, then isolation is called for and thus eliminates any hazard from that source.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2019, 09:41:06 pm
content: Quote from: JeanF on September 04, 2019, 04:57:57 pmIt is not that the DUT can be floating but that we can now choose where the coupling to the safety ground occurs.QuoteThe problem is that the isolated DUT will charge to the common mode voltage present at the differential probe which will be the average between the positive and negative inputs. If you were measuring high voltages, then the common mode voltage compared to Earth ground could become quite high. The differential probe does not care about this but you sure might if the capacitance between the DUT and Earth ground is high enough or the voltage that it charges to is high enough. Any RFI suppression capacitors going to ground my object also.So the isolation transformer allows attaching Earth ground to one point of the DUT without interference from neutral which is elevated from Earth ground by the neutral current.Note that the high voltage differential probe has *three* inputs. The other input is the "common" for the two high voltage inputs whether this is brought out as a separate input lead or not. Under normal circumstances, this common lead is connected to Earth ground through the oscilloscope and then back through the AC power connections to neutral and ground on the DUT unless it is isolated. This is more apparent when you consider an oscilloscope with two normal probes configured in add and invert mode where you now have a differential probe with two signal inputs and two ground leads.QuoteIn most cases, it is better to just use the high voltage differential probe. In some cases, also using the isolation transformer will help with the measurement by removing effects from the non-isolated ground and neutral.

author: JeanF
author_rank: Regular Contributor
timestamp: September 05, 2019, 03:13:08 pm
content: Thank you both for your answers.Interesting point about the parasitic capacitance to earth ground that could provide a path for an electric shock, I hadn't thought of it.QuoteSorry, I'm not sure I understand here. Do you mean the operator could receive a shock when touching the metal chassis of the DUT, because of leakage through the RFI suppression cap ? I can imagine that happening if the DUT has a metal chassis that is left unconnected.QuoteI'm a bit lost here. Could you please point me to a schematic that could help me understand? by "allows attaching Earth ground to one point of the DUT", are you meaning the ground clip of a standard oscilloscope probe? I thought that as we were using a HV differential probe, we would only connect the red and black leads to the points of interest in the circuit, but not connect the oscilloscope ground (which is tied to Earth ground) to anything.QuoteSo do you mean that if the DUT is connected to an isolation transformer, and that one is probing the circuit with a HV differential probe, one should always choose a point in the circuit and tie it to earth ground to prevent the circuit from floating? While the transformer should allow that, wouldn't that cancel the beneficial effect on safety of the transformer, in the first place?Thank you again

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 05, 2019, 03:18:44 pm
content: Also remember that not all *isolation* transformers are created equally.Some poorly designed and built units have significant inter-winding capacitance, allowing leakage currents in the order of milliamps. Though those may only give you a mild shock and/or don't destroy your scope, they will alter the readings.Unfortunately, good isolation transformers are expensive.Sometimes, if you have one of those lower quality transformers, using a diff probe is an added bonus.