What Oleg Artamonov is slightly wrong

After reading the note about the hypothetical “impartial tomorrow” , a young, growing, inquiring, growing engineering mind should be inflamed with righteous anger and, having pressed [with de-energized and cooled down] soldering iron with one hand to his chest and putting the other on a collective bible formed by a stack of worn semiconductor reference books products, sincerely, warmly, tearfully solemnly swear everything, always, everywhere to do the only and only right thing! But how is it right? The following note about the "basics of electrical safety" , designed to be for such young minds, if not guiding, so certainly explaining, it itself requires clarification, clarification and even ... some amendments.

This note does not pretend to be a guiding one, but is called upon very carefully, just a little (so that the revolutionary fire of a disturbed mind is not disappointed in contradictions with dry, harsh reality) to open the door to the pantry of traditional professional defaults.

Volt volt strife

Electric current is different qualitatively; distinguish at least: direct, alternating and pulsed currents. Quantitatively, the current is characterized, at a minimum, by the force measured in amperes and by the voltage measured in volts. When encountering an expression such as "several volts", it is always necessary to negotiate (negotiate) which volts exactly? That is, what exactly value (peak, acting, some other) is meant, since it is not always clearly or explicitly read from the context. Some current traditions in this area are given below.

VAC, Vac, Va.c. - this is the actual value (for a pure sinusoidal signal is equivalent to the rms value) [voltage] of an alternating current of industrial frequency (50 or 60 Hz).

VDC, Vdc, Vd.c. - so mean (usually average, without surges) the value of [voltage] constant (rectified) current.

V ~, Vrms - this is the mean square value [voltage] of alternating current of any frequency.

Vpeak, Vp, Vp-o - this is the amplitude (peak) value of [voltage] of any kind (constant, variable, impulse), taken (value) from any average value (for cases of alternating current, as a rule, zero).

Vp-p - this is the peak-to-peak amplitude (voltage) amplitude (also, as a rule, taken from any average value).

V =, Vavg, Vo - so denote (as a rule, averaged, smoothed) value [voltage] of direct (rectified) current.

Such a “context-independent” expression of values ​​allows quantities (characteristics) to speak directly on the spot for themselves, without taking away from them the focus of the reader's attention. Due to this, typical expressions (characteristics, conditions), such as, for example, U = 230 Vac, Umin = 20 Vrms @ 300 kHz, Umax = 46 Vp, firstly, look compact (“immediately in place”) and, in secondly, they reveal their own essence more broadly ("for themselves").

Unfortunately, the Russian-language approach does not provide such brevity and clarity, even the basic concepts of "alternating current" and "direct current" add up in the same abbreviation "p. t. ”, and even the expressions of the form“ 71 V peak value ”and“ 63 kV rms value ”present in GOST IEC 60950-1 cannot be compared with the corresponding“ 71 V peak ”and“ 63 kV rms "in the English original. Therefore, further, in order to be less confused, it is precisely the “English version” of semantization of the units of measure that is used.

IEC 60950-1

The publication IEC 60950-1 refers to the [historically inertial still] so-called “horizontal” safety standards, which (by definition) do not establish requirements for a specific type (type) of equipment. Such requirements, as a rule, are established either by the vertical standard (particular requirements) responsible for such a type (type) of equipment, or by technical specifications (specifications) if there is no state (or international) standard for this type (type) of equipment.

For example, IEEE 802.3 establishes the requirement that the Ethernet port requires basic insulation at a level of 1.5 kV AC (withstand voltage 1500 VAC) according to IEC 60950-1, thus determining the necessary (“vertical”, input) operational the characteristic that the referenced (“called up") horizontal standard "dereferences" (reveals, supplements, refines) with sufficient ("horizontal", output) design characteristics (requirements), such as air gap and creepage distance.

IEC 60950-1 is based on the “single failure” model, which means that any failure (in any part of the product) renders the whole product inoperative. In other words, everything is good in the product *, as long as everything is good in everything (there is not a single refusal), and in the product (more attentively) everything (!) Is not good **, if there is at least something in it not good. Yes, IEC 60950-1 does not consider (and in no way implies) fault tolerant systems.

Under the footnotes above, you can give the following comment: the standard in question is intended to ensure the safety of a person who deals only and with the product in a state of "all is well" (*); safety when working with a product that is already in the reverse state (**), or still in a state of direct transition (degradation during failure), is not included in the scope (regulation) of the standard.

With regard to the safety of electrical insulation, along with the determination of design characteristics according to exhaustively established external requirements (Appendix G), IEC 60950-1 allows (in the case of incomplete external requirements) the determination of such characteristics by an analytical method based on the calculation (or measurement on a prototype) of the corresponding peak working stresses. However, it is worth remembering that such an analysis also proceeds from the assumption that "everything is fine."

For example, the fact that in a PC to which, say, a USB to RS485 converter is connected, there is a power supply unit (node) connected to the primary circuits [power supply with a dangerous AC voltage], when analyzing operating voltages it will not even indicate (quite possible ) the need for galvanic isolation between these interfaces, since in the "all is well" state, the corresponding secondary circuits are considered either actually or equivalently drawn to the "ground" (protective ground). Such isolation here can only be an external requirement.

Regarding the analytical method mentioned, it is also worth adding that the possible result (among other things) is influenced by the characteristics of a particular design (circuit diagram, trace drawing) of the product (hence the presence of at least a prototype on which measurements can be made). For a comprehensive familiarization with the whole process, it is better to refer directly to the text of the standard itself. So what to read?

The English-language original IEC 60950-1 (and its replicas in the same language, such as UL 60950-1, EN 60950-1) are already initially set out somewhat frivolously (not always and not everywhere technically accurate, unambiguously interpret, exhaustively defined) and unpleasantly surprising the number of cross-references within itself. In general, this is far from easy reading, and far from cheap (the original is sold by the IEC at a price of about a thousand euros per document). On the other hand, the original is the original.

The Russian-language translation GOST IEC 60950-1 was developed in an attempt to obtain an “identical translation” (IDT) of the original, which it copes with on average, but does not transmit the entire completeness of the source text due to the presence of errors of varying degrees of rudeness in the translation. Regulatory subordinate to the original and, at the same time, sensitively inferior to the original in quality. In any case, it is recommended that you familiarize yourself with the topic for the first time.

Compared to the Russian-language translation, composed in the “Ponomaric” style, the original is much more convenient precisely for visual perception, not to mention the “fussy surfing” of the text itself. (It so happened that today it is English that has been left at the mercy of science and technology, therefore every engineer is now obliged to know it at the level of reading technical literature.) The original is simply strongly recommended for study by everyone with a professional (or other engineering) interest in the subject under discussion.

(See the "Quote" from GOST here. )

“Common stress” and “isolation level”

When developing an electrical appliance, the designer, as a rule, deals with two categories of electrical impacts, which (effects) will (may) be characteristic of such a device. These two categories are hereinafter referred to conditionally as “common stress” and “isolation level”, respectively, and are defined as follows.

“Common voltage” - implies that the signal corresponding to it is present on a powered (turned on, normally working) device for, as a rule, unlimited time. (In the case of a power supply signal, the device is powered from the same signal.)

“Isolation level” - is characterized by the test voltage (as a rule, multiple of the “nominal voltage”) and the exposure time[test voltage per device] (typically, tens of seconds, a typical value is one minute), as well as the fact that the test voltage is applied to the device only in a de-energized (!) state.

For combinations of various electrical circuits of the device, only “nominal voltage”, only “insulation level”, “nominal voltage” plus “insulation level”, neither one or the other, can be set [according to relevant requirements]. The values ​​themselves [established by the relevant requirements] of “common stresses” and “insulation levels” can vary both quantitatively and qualitatively.

For example, for a [hypothetical] “laptop” power supply unit that has five electrical circuits — two primary input power (L, N), two secondary output power (VCC, GND), one protective ground (PE) —on the ports and in the chassis of insulating material, the corresponding characteristics can be set as follows:

(a) “common voltage” between the L and N circuits - (100 ... 240) VAC; between VCC and GND circuits - (4,5 ... 5,0) VDC; between any other combinations of chains - not installed;

(b) “isolation level” between the PE circuit and the L, N circuits shorted together - 1.5 kVAC @ 60 s; between PE circuit and VCC, GND circuits shorted together - 0.5 kVAC @ 60 s; between the circuits L, N shorted together and the VCC, GND circuits shorted together - 4 kVAC @ 60 s; between any other circuit combinations - not installed.

Also, often to the "isolation level" (however, equally, as well as to the "common voltage") at the same time requirements are made regarding the effects of different types of current. For example, to the requirement of 5 VAC for alternating current, the requirements of 8 VDC and 8 Vpeak for direct and pulse currents can be added, respectively, and devices that withstand such influences are awarded (marked) with the corresponding “stars” (symbol C-2 according to GOST 23217), as [first three] on the KDPV.

Based on the set of characteristics required from the device, including “common stresses” and “insulation levels”, the designer then determines (calculated according to the accepted methodology, is selected experimentally during the tests, and specified in practical use) dependent design characteristics, such as air gaps ( [air] clearance) and the creepage distance), the order of which can be found in the IEC 60950-1, IEC 60065, IEC 62368-1, GOST R 53429 and other standards already mentioned, as well as (by the developer ) select e element base. Let us dwell on the latter in more detail.

If we agree that even for small-scale production, the list of used (purchased, custom-made) radio products calculated on the expected output volume should not contain nomenclature items with the quantity of the corresponding product less than tens of thousands of pieces, then [probably, the vast majority of enterprises in the Russian Federation can be considered as having [only] episodic production.

In such a situation, as a rule, the developer does not have the [economically justified] ability to “get what is needed” and uses components available on the market, that is, uses only “that [finished] that is,” thereby compensating for the loss in the case of device characteristics that are lower than those laid down in the technical conditions for the use of purchased products, and takes risks in the case of device characteristics that go beyond the technical conditions of use of purchased products.

Among the three - optical, capacitive, induction - popular methods of galvanic isolation of electrical signals, only in the case of the latter, the components required for this - winding products (transformers) - can be made [with acceptable, satisfactory quality] by the single-hand, non-serial method, for high-voltage capacitors and semiconductor optics, dispensing with mass production technology lines is no longer possible.

PCB cutouts

To increase the creepage distance between the conductive parts of the printed circuit board drawing by cutting out the material between them is a fairly common practice, which - in many (but, of course, not all) cases - upon closer examination gives a result that is more humanly encouraging than technically that Any guarantee.

This is due to the fact that the printed circuit board, as a rule, is electrically “full”, “valuable”, “working”, and so on, not by itself, but only with the installed electric and radio products (electronic components), so the electrical characteristics, including air gaps and creepage distances must be considered (analyzed, tested) in volume and in aggregate.

Some of these cases are illustrated below.

Case (1), when even in nominal (normal, initial, without taking into account time and operating effects) conditions, the cutout in the board either gives almost or nothing at all.

Case (2), when both nominally and in time and operating conditions, the cutout itself (without taking into account the "surrounding" wiring elements) can not only not improve, but also worsen the characteristics of the product.

Case (3), among other things, once again showing the importance of the cutout not by itself, but in conjunction with other structural elements, such as protective coatings (fillers) and auxiliary parts (barriers).

It is worth noting that cutouts in the printed circuit board are also made for other purposes that have nothing to do with the electrical safety of the product, for example, cutouts can relieve mechanical stresses that occur when mounting the printed circuit board in the general design of the product. On the other hand, the purpose of the cut-outs may remain less obvious in any perspective, except for the long-term, for example, if during testing and (or) long-term operation it becomes clear that the insulator (base) of the printed circuit board itself is destroyed due to moisture and [daily, seasonal] , other periodic] thermal cycling (heating-cooling).

Case (4), when there are two electrodes with a high electric potential between them, but there is no certainty (calculated, experimental, operational) that the circuit board will serve the specified period without a cutout, no.