Basics of electrical safety in the design of electronic devices

Hi, Habr! 3r33434.  
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After the wave raised by my previous post , a rather noticeable number of people asked me (on facebook, in a personal, etc.), to which, in fact, pay attention, so that instead of a smart outlet on arduino not to get another 3-3310. the wash-euthanasic
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Basics of electrical safety in the design of electronic devices 3r33434.  
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The topic is large and complex, but I will try to highlight the main points - not least because of the errors that I saw in various real devices and projects, including those published on Habré. I will not be long and tedious to list GOSTs, but I will list quite basic things that need to be understood and observed in order not to kill at least yourself (if you plan not to kill others too, then after completing this article, do not be too lazy to go through the relevant GOSTs). 3r33434.  
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So, you are going to make a device that at least one of its ends is plugged into an outlet. 3r33434.  
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GOST IEC 60065-2013 referenced by the founding document - Technical Regulations of the Customs Union (ТР ТС) 004/2011 “On the safety of low-voltage equipment”. In particular, both documents indicate for power supply networks with a current voltage up to 300 V possible voltage transients up to 2500 V. According to similar documents, in principle, the whole world lives under the names GOST, IEC or UL 60950. [/i] 3r33434.  
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Label from IEC 60950. In general, when calculating user security, the standard recommends that all power mains be classified as category II. 3r33434.  
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An important point: the presence of isolation does not mean that current cannot flow between the primary and secondary circuits of the device. In some cases, to avoid such a current is impossible or unreasonable - for example, in pulsed power sources to reduce interference between the primary and secondary cells, there is a small capacitor. In this case, the device should be designed so that the leakage current between the primary and secondary bodies does not under any circumstances exceed the safe limit (3.5 mA for household fixed equipment, ???-??? mA for portable equipment; for medical equipment, norms, they are tougher 10-100 times depending on the type of equipment, 3-333132. here you can watch the presentation 3r-344? about the differences in requirements). 3r33434.  
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So, our minimum requirements are insulation with a strength of 2.5 kV between primary and secondary circuits with leakage current under normal conditions not exceeding 3.5 mA. 3r33434.  
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3r33417. How do we isolate it? 3r33418. 3r33434.  
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3r3303. All components connecting the primary and secondary circuits must be designed for an insulation voltage of at least 2.5 kV. In a pulsed power supply, this is usually a transformer, a feedback optocoupler, and a noise suppressor capacitor. 3r3304.  
3r3303. There should be no direct connections between the primary and secondary circuits. 3r3304.  
3r3303. Interference suppression capacitors connecting the primary and secondary circuits must be officially certified to a class not lower than Y2 (3r3157. Safety rated Y2 capacitors
) - such 3r-3292. and only such [/b] capacitors can be used in circuits where the failure of a capacitor is dangerous. Capacitors of class Y2 are marked in the current voltage of the AC network for which they are designed ("250VAC"), while they are guaranteed resistance to single pulses with voltage up to 5 kV. No other capacitors, including those marked 3 kV or higher, but not having a safety class, use 3r-3292 in such circuits. should not [/b] . A typical example is capacitors. Murata Series DE . For reinforced insulation (see below) class Y1 capacitors should be used. 3r3304.  
3r3303. When designing a printed circuit board, the gaps between conductors, parts and the device case must be designed for a breakdown voltage of at least 2.5 kV. 3r3304.  
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With the design of printed circuit boards begins, of course, the most interesting. 3r33434.  
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Firstly, the aforementioned IEC 60950 introduces four classes of insulation depending on its purpose and, accordingly, the required reliability (more precisely, the probability of failure to multiply the consequences of this failure): 3r3445.  
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3r3303. Functional - necessary for the functioning of the device itself, but not providing user protection. 3r3304.  
3r3303. Basic - providing an initial level of user protection, but not reliable enough to do without a second protective barrier. 3r3304.  
3r3303. Supplementary - the second protective barrier. It has the same strength as the Basic. 3r3304.  
3r3303. Reinforced is a high-strength insulation that can be used without a second protective barrier. It has twice the strength than Basic. 3r3304.  
3r3197. 3r33434.  
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Further, about the various options for the implementation of insulation is written quite a lot (IEC standards are paid, but we understand that 3r3202. In Yandex there will be all
?), Let's dwell on the requirements for printed circuit boards in home appliances. 3r33434.  
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To assess the dielectric capacities of various materials, IEC 60950 divides them into groups according to the CTI (Comparative Tracking Index) parameter - the higher the CTI, the better the insulating properties of the material: 3r3445.  
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3r3303. Group IIIb - 100 r3r3215.  
3r3303. Group IIIa - 175 r3r3218.  
3r3303. Group II - 400 r3r3221.  
3r3303. Group I - CTI> 600 3r3304.  
3r3306. 3r33434.  
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Ordinary FR4 glass fiber laminate has CTI = 17? that is, it belongs to group III, to the boundaries between subgroups IIIa and IIIb. 3r33434.  
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In addition, the dielectric properties of a material, a discharge in which can occur over its surface (a case of a printed circuit board), depend on the level of contamination of this surface, therefore IEC 60950 introduces several generalized pollution classes (in the standard more formalized definitions, below I attach them to operating conditions ): 3r33445.  
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3r3303. Level I - pollution that does not degrade the dielectric strength of the insulation. Applies only to equipment in clean rooms or in sealed enclosures that prevent ingress even household pollutants. 3r3304.  
3r3303. Level 2 - office or home environment, possible contaminants usually do not conduct current, but in isolated cases moisture may condense when it condenses. 3r3304.  
3r3303. Level 3 - industrial environment, agricultural enterprises, especially unheated premises. Contaminants can conduct current, as in the case of condensate, and without it. 3r3304.  
3r3303. Level 4 - use without protection from the external environment, regular exposure to water or snow. 3r3304.  
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I note that the desired level of protection can be implemented using adequate external enclosures - for example, a device with a level 2 can be used outdoors when using a sealed enclosure. 3r33434.  
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Finally, IEC 60950 uses two methods for measuring the distance that forms an insulating gap - clearance and creepage. 3r33434.  
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3r3303. Clearance - the shortest distance between the conductors. 3r3304.  
3r3303. Creepage - the distance between the conductors on the surface of the printed circuit board. 3r3304.  
3r3306. 3r33434.  
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For our case for a nominal voltage of 230 V ± 10%, it is necessary to focus on the requirements for insulation in networks up to 300 V rms voltage, up to 420 V amplitude and up to 2500 V emission during transients. 3r33434.  
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Depending on the type of isolation (we do not consider the functional, since we are talking about the security of the user) 3r33292. minimum required [/b] the distance on the printed circuit board will be:
 
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3r3303. Basic: 3.0 mm if the device has extra insulation
 
3r3303. Reinforced: 6.0 mm, if the device does not have additional insulation
 
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However, if we return to the above-mentioned Y-capacitor, we can easily notice that the maximum distance between its legs on the datasheet is 7.5 mm. 3r33434.  
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3r33333. 3r33434.  
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As it is easy to see, taking into account the contact pads, it will be difficult for us to get the distance between the conductors of 6.0 mm, if we don’t start to manually pull the legs apart. 3r33434.  
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Fortunately, there is a simple way out - as seen in the image above, the creepage can be increased by making a cutout in the PCB. The air has a higher dielectric strength than FR4 - for it, the breakdown voltage approaches 3 kV /mm, and in order to ensure safety, it is usually assumed to be 1-1.5 kV /mm. IEC 60950 requires for an air gap for circuits up to 300 V of width 2.0 mm for basic insulation and 4.0 mm for reinforced (if the production has a quality control program complying with the requirements of the standard, the width can be reduced to 1.5 mm and 3.0 mm, but now this is not our case). 3r33434.  
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That is, we can provide the desired insulation with 4 mm air or 6 mm PCB. 3r33434.  
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Due to the complexity of the issue, the standard does not consider a combination of of air and the printed circuit board, but in practice it is this combination that is used in most cases - in the board between the primary and secondary circuits, a cutout is made: 3r3445.  
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In this case, having made a cutout 2 mm wide and slightly longer than the width of earthen fields, we obtained a minimum creepage equal to ??? mm, which satisfies the requirement for reinforced insulation, and across the cutout, if we count "head" - 3.7 mm textolite and 2.0 mm of air, each of the values ​​corresponds to the requirement of a single layer of basic insulation, so that in total they can also be considered sufficient. 3r33434.  
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Here you can live with it. 3r33434.  
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I note that the correct design of the board does not relieve problems with the location of the components: between any conductive parts of the primary and secondary there must be at least the same 2 mm of air, and in the case of an ungrounded enclosure between it and the primary for reinforced insulation the standard requires 10 mm of air. 3r33434.  
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3r33417. Classic errors
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An obvious fatal mistake is, of course, a complete disregard for safety requirements and maintaining gaps between primary and secondary chains of scale 0.5-1 mm, according to the principle "when you first turned on, you didn't kill anyone - it means everything is in order." Here, for example, Typical amateur German documentary in which beautiful cuts of functional isolation between the conductors of the mains supply are milled, but there is a millimeter gap between the 230 V input and the secondary ground, on which the USB connector freely accessible to the user sits - to turn this design into 230 V is simply life-threatening. 3r33434.  
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In addition to guaranteed fatal errors, potentially fatal errors occur regularly. 3r33434.  
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First, inexperienced developers are intuitively perceived as high voltage between the two wires of the 230 V network, but not between the primary cell and the secondary, and they lay the cuts between them. This is not unreasonable if the lanes of the network go close to each other on the board, and this will be related to the provision of 3r33373. functional [/i] isolation, but not directly related to safety - in the end, in the standard circuitry between these wires you should have a varistor for a response voltage of about 430 V, so there will not be much more there. Moreover, if a high-voltage common-mode pulse arrives to you, then nothing particularly interesting will happen just between the wires of the network. 3r33434.  
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But between the primary and the secondary - even as it happens. 3r33434.  
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Secondly, the slot in the board by inexperienced developers is perceived as either something decorative, or as a silver bullet and a cure for all diseases at once. For example, the same Bitronix Lab laid out pictures of its USB interchange, which they promise to give out to all buyers of a dangerous set for free, and boasted that it was made with a margin of 5 kV: 3r33445.  
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For simplicity of calculations, I quickly outline it in DipTrace, the benefit of the names of the components is known, and the dimensions of the slots can be easily calculated from the image - 2 mm wide, the length does not exceed the width of the components. We do not know how the board is flooded with earth, but we will assume that the polygons do not go beyond the borders of the legs of the components. 3r33434.  
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Total: ADuM4160 - clearance 5.4 mm in PCB + 2 mm air, creepage ??? * 2 + 2 = ??? mm; AM2D - clearance ??? mm PCB + 2 mm air, creepage ??? mm. The values ​​are approximate, since the shape of the pads may differ, but ± 0.1 mm we are clearly not worried about here. 3r33434.  
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It is easy to see that the real parameters only barely exceed the requirements of IEC 60950 for a 300 V network with emissions up to 2500 V for the case of reinforced insulation - and as in the case of the Beatronics Lab we are talking essentially about medical equipment with direct contact with the human body it is advisable to design it under the highest possible level of protection. 3r33434.  
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The real guaranteed insulation strength of the whole structure will not exceed 3 kV. Claims about 5 kV with this board are inappropriate under any sauce - it is not designed for this level of protection. In this case, it was possible, even without increasing the dimensions of the printed circuit board, to move the insulator and DC /DC closer to each other and make a single slot under them, extending from the top and bottom beyond the limits of the component housings at least a millimeter. 3r33434.  
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I note that at high voltages - from 5 kV and above - the form of conductors also begins to play a role: the field strength and, accordingly, the probability of breakdown is higher on pointed parts. 3r33434.  
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3r33417. How to check an existing device? 3r33418. 3r33434.  
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Although laboratory tests according to GOST standards for most amateurs are very heavy, for small companies they are unpleasant because of their cost and duration, there are devices on sale that allow a rough assessment of the safety of devices - this is high voltage insulation resistance meter . 3r33434.  
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In fact, these are gigo-meters (with an upper limit of 10-20 GOhm), when measuring high voltage applied to the probes - 1000 V for low-cost models and 2500 V for those that are more expensive. 3r33434.  
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If you are engaged in the development of plug-in devices or you are interested in the safety of Chinese products, I highly recommend purchasing at least something like 3r33333. UT-502A
(in Chip-i-dip it is also there, but expensive). 3r33434.  
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If your device has sustained 10 seconds under the supplied voltage of 2500 V, then everything is not completely hopeless. Such tests are not grounds for considering the device complying with standards - as is easy to see, in general even a layer of functional insulation is already obliged to withstand such voltages, although the probability of its breakdown is considered too high to be used to protect the user. 3r33434.  
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Testing equipment with an impulse with a voltage of 5 kV would be more indicative, but, alas, such devices already cost a lot of money. 3r33434.  
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On the other hand, if even at 2.5 kV your device showed something different from the upper limit of the gigam, you now know 3r3447. what to do with him
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