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| Floating Scopes |
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| mtdoc:
From the OP: --- Quote from: dicky96 on June 21, 2017, 09:38:30 pm ---I've googled this and yes it had been discussed but I still don't understand why this is a big no no --- End quote --- And that is exactly the point. Any curious mind should want to know the why. I've just looked back through this thread, and other than one trolling type post, what I see is everyone agreeing that it is never a good idea to float a scope. No one has endorsed it or suggested that it be endorsed or that it is currently endorsed by any institution. Many have provided good concrete explanations about why it is dangerous. They have come at it from different angles but all reach the same conclusion. This is a good thing and how learning occurs :-+ |
| tronde:
There are many things to be said about how to measure live circuits safely. First of all: This is an internationl formum. Therefore we must consider the implications of that fact. From what I can see, it is one most important factor that has not been discussed in this thread, namely the mains power distribution system. It has only been briefely mentioned and from some comments I dare to say some should read up on it. Within Europe you will find mostly some kind of the TN-system with protective earth (PE) and neutral connected together at least one place in the building. You will also find the IT-system with only local floating PE and no ground on the source transformer. You can also find the TT-system with grounded source transformer and local floating PE. You can find homes with some rooms defined as "dry" and no PE in the wall outlet. In the US you can find the split phase system with grounded neutral and as I understand some other systems as well. I know that other systems also exist, and is in use in different countries. It can also be that the distribution system used in an industial estate or office can differ from what is normal in the country. This meaning of this is that what you will find in your lab or home is not necessarily the same as what the other person will find in his lab or home. If we fail to understand this, we can end up by giving some very wrong and dangerous advices. It is said that you should use an isolated differnetial probe to be safe. Soemtimes this is wrong. It is never wrong to use the probe, but the claim of being safe is sometimes wrong. If you are connected to a TN-system AND have polarised plugs and wall sockets you will know which wire is neutral and therefore close to the ground potential. If you test a product with a live chassis, the chassis will most likely be at the same potential as PE and you can touch that and your scope safely. Edit: Be aware that the wall socket known as "French" or "Type E" or CCE 7/5 is polarised by design, but the wiring of it is implemented differently in the countries that use it. This means that you need to test which wire is "live" and which is "neutral". Even if you live in a country that uses polarised sockets that is expected to be wired in a certan way, you should test the polarity to be sure. BUT if you don't have polarised plugs and sockets you will not know which wire is live or neutral, neither on your scope or product. If you manage to connect the chassis so it is on the live wire, you are in deep shit if you touch that and your scope even if you have an isolated differential probe. The probe will save your scope, yes, but not you. We also have the possibility of touching other parts of the circuit than the chassis. If we touch something that is "live" and the scope, no differential probe will save you. We must remember that the normal Schuko plug is used in many countries regardless of the distribution system and it is not polarised. Next thing is about isolating transformer for the device you test. In previous discussions on this forum some has said that an isolating transforer is a thing of the past and that is is better to rely on a Residual Current Breaker (RCB) that will trip if you get a shock. First of all: a RCB is not intended to be a safety device to protect you from doing silly things. Its main function is to be a last resort if everyting else fails. Then again we must understand that what you have can be something completely different from what is available to others. I understand that RCBs are differnet around the world, but in Europe we will most likely find a 30mA RCB known as "type A". We do also have a less known "type B". There is one most important difference between "type A" and "type B". Type A will not trip reliably if there is a steady DC component in the earth current. Since we quite often test equipent with rectifiers we will most likely not be protected by the RCB used in most of Europe as of today. If you think a "type B" is something you should have, you must consult the data sheet for that particular type first. If connected in the wrong place, it can block the function of other RCBs in the installation. I know that the reason for the suggestion of avoiding an isolation transformer is that you can still get a shock if you manage to connect youself in the return loop of the probe. This is true, but as we have seen a RCB can give a very false impression of safety too. The least unsafe will probably be to use a proper isolating transformer (one with a grounded static shield between primary and secondary) for the device you test and an isolating differential probe. If you manage to touch both "live" and "neutral", no transformer or RCB will save you. It seems like most people with a background in electronics are scared about mains voltage, and that it is somethiong really dangerous. Yes, it can be dangerous. Yes it can kill, but we are still talking about 240 volts and not kilovolts. Reinforced clearance distance is usually 4mm and creepage is 5mm and normal insulation is half of those values. It is not so that the voltage will jump out of the wall and bite your nose. What puzzles me is that people are afraid of 240V but consider 12V as safe. That is really ignorant. A charged car battery can cause a lot of danger too. Evaporating wires and metal splattering around for instance. What all this boils down to is that we really must switch on our brain before we start to work on any circuit. If the available energy is large enough a low voltage circuit can be really damaging to both you and your test equipment. If you don't fully understand what can go wrong, you should stay away from testing on that product regardless of the voltage. You will lnever be "safe" when you work on any kind of exposed enrgized circuit. You will only have different grades of "unsafe". Edit: added info regarding the "French" polarised socket. |
| Fungus:
I vote to leave that^ as the final post in this thread. It should now be locked. |
| nctnico:
--- Quote from: mtdoc on June 25, 2017, 10:44:46 pm ---There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe. --- End quote --- The danger in that is that you are going to explain how to do something which is dangerous 'safely' and very likely important bits are left out or interpreted wrong. Besides that taking measurements using floating mains takes acquiring discipline through training & mentoring which cannot be replaced by a few posts on a forum. |
| David Hess:
--- Quote from: Fungus on June 25, 2017, 10:29:13 pm ---* What problem is solved by floating a 'scope? and * When might I want to do it? --- End quote --- If you can put up with the high common mode capacitance, floating the oscilloscope provides higher common mode rejection and lower noise than using a high voltage differential probe but see below for better options. --- Quote ---The only correct answers are: a) Get a differential probe or b) Use AC coupling. --- End quote --- c) Float the device under test although this has many of the same problems as floating the oscilloscope. I have occasionally floated the DUT and used a differential amplifier to get higher common mode rejection ratio and lower noise than a high voltage differential amplifier will provide. d) Use an oscilloscope which has isolated inputs. e) Use a probe isolation amplifier. --- Quote from: alm on June 25, 2017, 10:09:32 am ---Back then in this thread referred to 1980s. By then differential amplifiers with excellent CMRR and bandwidth were available (Tek 7A13, 7A22, plus plugins from the 500 and 5000 series), as were high voltage probes (including the P6015 if you wanted to go crazy). Also 200-series battery powered isolated scopes (e.g. Tek 200 series that could float up to 700V on battery power) and A6902 isolated probes. --- End quote --- The 7A13 and 7A22 only have excellent common mode rejection when special adjustable attenuating probes are used and calibrated or the input attenuators are switched out and x1 probes are used. (1) Modern high voltage differential probes never have excellent common mode rejection but they are good enough for many applications including usually off-line power supply design. High voltage attenuating probes including the 7A13 and 7A22 when attenuation is used have another problem; they have high noise in direct proportion to their input attenuation. An oscilloscope with floating inputs or the A6902 probe isolation amplifier solves this problem. --- Quote ---I attached a snippet from the Tektronix 1982 catalog to see what they wrote back then about floating scopes. I find it high unlikely that the major scope manufacturers, Tektronix and HP (who together accounted for about 93% of the scope sales, at least in the US), were endorsing floating scopes in the 1980s. --- End quote --- Tektronix was still selling the A6901 Ground Isolation Monitor in 1991 although it only allows floating an oscilloscope or other test instrument to 40 volts. I have noticed before that where manufacturers bothered to specify it, the floating voltage specification is usually 40 to 50 volts and I wonder where that number comes from over such a long period of time. It is suspiciously close to the common definition of the maximum of "low voltage". (1) One reason I really like the 7A13 is that its cascode input amplifier operates on +/-50 volts allowing a +/-10 volt common mode input range on its most sensitive settings where no attenuation is used so noise is low. The only comparable modern instruments that I know of are the differential amplifiers made by Preamble who was bought by LeCroy and if you can afford one of those, then you can afford a modern DSO with isolated inputs. |
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