Hardware Insights Forum

Basically, these represent the way I'd like things to be done, in preference to how they are done in the real world. Feel free to reference these in reviews and articles on this site.

LRS 001: Regarding PC PSU functionality

• Section 1: Reliability
  The unit must be designed for a service life of at least five years (43,830 hours) for the main supply, while delivering 80% of the rated power with 45°C at the intake, and ten years (87,660 hours) for the standby supply. At the end of this service life, the output ripple must remain within the allowed limits (120mV for +12V and −12V and 50mV for the lower voltages) at all power levels. (I realise that meeting this standard will pretty much rule out 85°C electrolytics even for the primary cap.)
  The unit must be able to deliver full rated power with 50°C at the intake (this is based on 40°C at the PC case's intake, plus a 10°C rise from there to the PSU), at the lowest input voltage. The unit is not required to last for the full service life under those conditions, but if it can be made to do so, that's great.
• Section 2: Output cabling
  Along with being of appropriate lengths, the output cables should have appropriate conductor sizes for the current draw and acceptable voltage drop.
  For the CPU and PCIe power cables, the total drop should be below 0.12V (1% of +12V) at the official load specification (for PCIe connectors) or 5A per wire (for the CPU connectors, unless the unit imposes a lower limit).
  Power cables for disk drives should be sized for a total drop below 0.24V (2% of +12V) with a worst-case spin-up current of 2.5A per drive. (In some cases, staggered spin-up may be needed to avoid excessive drops on cables shared between multiple drives.)
  For other cables, a reasonable maximum load should be used with a total drop of 0.12V for +12V, or 0.1V for the lower outputs. (Sharing of grounds between multiple outputs may complicate the maths.)
  In all cases, the wire gauge must be adequate to not exceed the rated maximum temperature of the insulation at an ambient temperature of 50°C. For low-current wires (−12V, Power Good, On/Off and the sense lines) the smallest gauge suitable may be determined by compatibility with the crimp terminals, rather than losses.

LRS 002: Regarding external PSUs
The labelling on these is presently quite inadequate, with no indication of the ripple or regulation. Any mention of efficiency is limited to just a few general "classes". So:

• Section 1: Labelling
  First of all, supplies designed only for the low range (100–127V~ nominal) must have a very prominent warning (on the unit itself, not in the manual which most people will never read) that they will be destroyed if connected to the high range (200–240V~ nom.). Supplies with a manual voltage selector (which are admittedly rare on external PSUs) must have an equally prominent warning to set it correctly.
  In addition to the nominal input voltage and frequency, maximum inrush current should be specified, if more than 1A or double the operating current (whichever is greater). Efficiency and power factor under full load should also be specified, as should standby (no load) draw and power factor.
  Along with nominal output voltages, the regulation (including the output lead's resistance) and (with the obvious exception of AC-output linear supplies) ripple (allowing for capacitor aging) should be specified. Switching supplies must have a complete list of protections (more on them below).
• Section 2: Design and functionality
  If the unit cannot be made compact enough to fit alongside other plugs of the type used in the region the unit is sold in (or if it wouldn't be economically viable to make a customised version for a small market), it should use an inlet from the IEC 60320 series. The mounting of this inlet must be rigid enough to withstand at least 1,000 plug/unplug cycles without failure. If the standby draw is greater than 1W (as with most linear supplies) or 2VA, I recommend including a switch on the unit itself. This switch should be rated to last for at least 10,000 switching cycles with the voltage and current (including inrush) applied.
  Due to the risk of strong static discharges, Y capacitors should not be used with Class II. Larger transformers may themselves have significant primary-to-secondary capacitance, which must be assessed.
  The lifespan must be at least ten years at 80% or lower load, or five years at full load, with an ambient temperature of 40°C. Linear supplies will likely last much longer (especially the AC-output types, which have no internal components besides the transformer itself).
  Switching supplies must have OVP and SCP, and OCP is also recommended. OTP is required for fan-cooled units; for passively cooled types it is optional, but if not implemented, a DO NOT COVER warning is then compulsory. If a fan is to be used, it should be temperature controlled to reduce noise and dust accumulation and prolong the lifespan of the bearing(s). 2BB fans (although allowed) may be a poor choice for this application due to limited shock resistance, and 1B+S configurations are prohibited; FDBs or similar are preferred, but ordinary sleeve bearings may be good enough for less demanding applications. A dust filter (that can be cleaned by the user) should be used.
  The output lead should be of a suitable length (I suggest 2m) for general usage, and I suggest calculating the conductor size for a drop of 1–2% of the output voltage (or a conductor temperature below 60°C at 40°C ambient, whichever is the primary issue). (Some chargers for cell phones or portable game systems use a lead with much higher resistance, and the device is designed to tolerate the poor regulation.)
  I recommend that the casing be held together with Torx screws, or failing that, plain old Phillips types.
  And one last thing – no half-wave primary rectifiers!

Any questions? I thought of making the fan-cooled external PSUs so that the dust filter can't be removed with the mains cord connected, and the cord can't be connected with the filter removed, but I guess that would involve a complicated mechanism.

On a semi-related topic, I was wondering about the feasibility of an external SMPS with a solid aluminium case (anodised black for maximum radiative cooling) acting as one large heatsink. Better cooling than the standard plastic-cased type with no fan noise – presumably.

I wish electronics were generally more reliable as well, and not built to fail, but nothing is impervious, nothing is perfect, nothing is immune, and nothing lasts forever, especially in this modern "hay day".... of planned obsolescence.

FWIW, I don't think 85*C capacitors are that bad. In a gross simplification, 85*C capacitors from good brands tend to employ either Ethylene Glycol or Boric Acid (or a mixture of both) as its organic solvent in the electrolytic solution (depending on the temperature range, IE -40C to +85C or -25C to +85C). Generally speaking, the H2O content in those capacitors is very low (Ethylene combines with water to produce Ethylene Glycol, essentially), but not quite as low as non-aqueous capacitors that generally (again, a gross simplification) use either GBL, DMA, or DMF (-55C to +105C temperature range). Those solutions have very good characteristics and are generally very stable over time (that is why they are superior to 85*C lytics), but aren't really capable of reaching the high voltages that EC or BA is, which is why you see even the non-aqueous capacitors, at their higher voltages, fall into the -40C or -25C category of temperature ranges rather than the -55C range of the lower voltage capacitors (stated in the datasheets). I'm more worried about the capacitors with very high H2O content necessary to improve the ionic conductivity of the electrolyte - the ultra low ESR lytics. That kind of electrolyte is bound to expand and dry up over time, especially with heat, and without the right oxidizers (which the good brands would have) nothing is there to avert aluminum hydration and the formation of hydrogen gas after the dielectric is consumed by the aggressiveness of the excessive H2O in the electrolyte. Smaller case sizes don't help this - this means thinner dielectrics and smaller foils (though highly etched foils do allow for more capacitance to be yielded per square centimeter).

The "85*C capacitors last 1/4th the time of 105*C capacitors" rule isn't really true, IMO. That kind of stuff tends to stem from the marketing department and not the engineering department. If 85*C capacitors really only lasted that long we'd be seeing a lot more failures from them, even in storage. They are rated up to 85*C because like double layer electrolytics rated up to 70*C, they are no longer stable over those temperatures. That's it. The "10*C doubles the life" rule does not take into account the eventual drying up of the liquid electrolyte and the eventual hardening of the rubber bungs as they decompose over time, and it does not take into account the wide variety of solvents and formulas used in many different lytics. That being said, I see where you are coming from - 105*C capacitors would be better for primary side filtering at higher temperatures than 85*C capacitors for the aforesaid reasons. I'm not suggesting that motherboards and power supplies use 85*C capacitors.

Can't say I disagree with anything else.

Wester547 wrote:FWIW, I don't think 85*C capacitors are that bad. In a gross simplification, 85*C capacitors from good brands tend to employ either Ethylene Glycol or Boric Acid (or a mixture of both) as its organic solvent in the electrolytic solution (depending on the temperature range, IE -40C to +85C or -25C to +85C). Generally speaking, the H2O content in those capacitors is very low (Ethylene combines with water to produce Ethylene Glycol, essentially), but not quite as low as non-aqueous capacitors that generally (again, a gross simplification) use either GBL, DMA, or DMF (-55C to +105C temperature range). Those solutions have very good characteristics and are generally very stable over time (that is why they are superior to 85*C lytics), but aren't really capable of reaching the high voltages that EC or BA is, which is why you see even the non-aqueous capacitors, at their higher voltages, fall into the -40C or -25C category of temperature ranges rather than the -55C range of the lower voltage capacitors (stated in the datasheets). I'm more worried about the capacitors with very high H2O content necessary to improve the ionic conductivity of the electrolyte - the ultra low ESR lytics. That kind of electrolyte is bound to expand and dry up over time, especially with heat, and without the right oxidizers (which the good brands would have) nothing is there to avert aluminum hydration and the formation of hydrogen gas after the dielectric is consumed by the aggressiveness of the excessive H2O in the electrolyte. Smaller case sizes don't help this - this means thinner dielectrics and smaller foils (though highly etched foils do allow for more capacitance to be yielded per square centimeter).

The "85*C capacitors last 1/4th the time of 105*C capacitors" rule isn't really true, IMO. That kind of stuff tends to stem from the marketing department and not the engineering department. If 85*C capacitors really only lasted that long we'd be seeing a lot more failures from them, even in storage. They are rated up to 85*C because like double layer electrolytics rated up to 70*C, they are no longer stable over those temperatures. That's it. The "10*C doubles the life" rule does not take into account the eventual drying up of the liquid electrolyte and the eventual hardening of the rubber bungs as they decompose over time, and it does not take into account the wide variety of solvents and formulas used in many different lytics. That being said, I see where you are coming from - 105*C capacitors would be better for primary side filtering at higher temperatures than 85*C capacitors for the aforesaid reasons. I'm not suggesting that motherboards and power supplies use 85*C capacitors.

Granted, my specified test conditions are a bit harsher than most normal usage (outside of industrial environments, that is). People in affluent countries will probably turn on the air conditioning before it reaches 40°C inside, although this may not be the case in Africa. (But my use of 40°C as the designated room temperature is also to allow for people who may place the unit near a heater in the winter, for example. Fortunately, heater usage and summer are normally mutually exclusive…) Of course, people in poor countries probably can't afford to replace stuff over and over, which is (IMO) all the more reason to make it to last. Additionally, using parts more tolerant of heat may enable cooling fans to be run slower and more quietly (not that I would advocate designing for ridiculous temperatures).

(Come to think of it, I think it would actually be a good thing if lower-wattage PSUs were to become available again. Personally, I'd take a high-quality 250-watter over a middle-of-the-road 350W or 400W; I would guess that a good 250W PSU would run my current rig just fine. Even 200W might work, if close to the limit. It would be nice if video card manufacturers would stop providing overspecified recommendations for the total PSU output, and if the "de-rating with age" nonsense would die out. Mind you, providing usable advice does seem to be something of an art.)

At least Chemi-Con says that their aqueous electrolytics (KZN, KZM, KZH, KZE, KYB, KYA, and KY) have a shelf life of 500 hours at 105°C, compared to 1,000 hours at 105°C for the non-aqueous types (LZA, LXZ, LXY, and LXV). They also say in their series table that the non-aqueous types are specified for "high reliability" applications, while the aqueous ones aren't (note that this is distinct from "long life", which all of those series are specified for). Panasonic also says that their aqueous series (FM and FR) are not suitable for automotive applications. But if Arrhenius' Law isn't the answer to capacitor lifespan determination, what is? (Given that time travel hasn't been invented yet – or has it? )

Besides, as you mentioned before, the endurance rating isn't how long it takes for the capacitor to fail – it's how long it can be subjected to the test conditions and still pass.

PS. I take that you meant to say "heyday". I don't think the stuff found on farms is really relevant to this topic.

LongRunner wrote:At least Chemi-Con says that their aqueous electrolytics (KZN, KZM, KZH, KZE, KYB, KYA, and KY) have a shelf life of 500 hours at 105°C, compared to 1,000 hours at 105°C for the non-aqueous types (LZA, LXZ, LXY, and LXV).

And KZJ as well. Except for KZG, though, which is still rated for 1,000 hours @ 105*C on the shelf... go figure!

Panasonic also says that their aqueous series (FM and FR) are not suitable for automotive applications. But if Arrhenius' Law isn't the answer to capacitor lifespan determination, what is? (Given that time travel hasn't been invented yet – or has it? )

I think bad capacitors are too unstable to follow that ruling is what I was saying. I would surely apply it to 85*C GP and 105*C GP/non-aqueous capacitors.

Besides, as you mentioned before, the endurance rating isn't how long it takes for the capacitor to fail – it's how long it can be subjected to the test conditions and still pass.

Yes, they aren't tested to failure in those tests. Those aren't published.

PS. I take that you meant to say "heyday". I don't think the stuff found on farms is really relevant to this topic.

Actually, my horrible pun was meant to be "hay day", because quite frankly, we are receding and devolving in this "technological age" of "gadgets", "gizmos", and "planned obsolescence"...

Wester547 wrote:Actually, my horrible pun was meant to be "hay day", because quite frankly, we are receding and devolving in this "technological age" of "gadgets", "gizmos", and "planned obsolescence"...

I'd say television sets nowadays are a stand-out example of that. Build quality and reliability aside, they don't care about accurate colours anymore, and apply that "edge enhancement" crap (here's an article on the effect as applied to DVDs) that probably results in a worse overall image. Many of them don't even have proper brightness controls. And why can't they provide a decent headphone output? It's all catering to the lowest common denominator, with Ultra High-Definition, among other marketing blitz (such as Sharp's Quattron which, if you think about it, can't actually work as claimed).

Also, I made two small edits to Section 2 of LRS 002. The first is that external PSUs that draw more than 2VA of apparent power (alternatively to, or along with, the 1W of real power originally stated) should also have the switch. (Besides the idle power used by the switcher itself, any X capacitors in the supply will draw a leading current. With the supply under higher load this is not noticeable, but it can become significant in a supply just sitting there. My calculations indicate that, at 230V 50Hz, a supply with a single 0.1μF X cap would just stay under that benchmark. Some very small supplies don't use X capacitors at all, instead making a pi filter on the DC side of the rectifier with a ferrite coil between two primary electrolytics.)

The second is simply a statement that half-wave primary rectifiers are not to be used (as is occasionally done in very small switchers). Really, subjecting the power grid to a net DC bias, when for just a marginally higher cost you can have a complete bridge (and less stress on the primary capacitor), isn't smart. Curiously, two diodes in series (one on each side of the input, usually) are used there, even though a single 1N4007 (1A, 1000PRV) would function. This would seem to be for redundancy, so that if one diode shorts, nothing else will be damaged. (A shorted diode in a bridge would just blow the fuse, provided the remaining diodes aren't too overstressed by the event, but if the sole diode in a non-redundant half-wave rectifier shorts, the circuitry following is finished, possibly spectacularly.)
EDIT: I've since found RFI reduction mentioned as another reason for the diode-on-each-side arrangement (this gives the SMPS primary some room to electrically "float" around the mains input, when the diodes are not conducting).

Those 85°C caps often are rated for higher current so with lower current they hold longer.

Anyway since there are even 10000 hours snap-in high-voltage caps for some time now, even I can get a box of them, I don't understand why I still see some 2000 hours 85 °C caps mostly, 2000@105 °C 420 V best (but 330uF than) in brand new power supplies which came from factory in november last year. Is for example EELXS421VSN391MR40S really THAT much more expensive?!

^ Apparently not, but the manufacturers probably reason that the lower rating is all that will be necessary, and in this case, that may well be true for most users (even the 85°C types usually hold up OK outside of the really hot-running PSUs). It could also be that they calculate that even a 2,000-hour @ 105°C cap would outlast the secondary capacitors anyway, given the relative temperatures and ripple currents, so they decide that there's then not much point in going for the LXS over KMR (for example).

The series with still higher endurance are bulkier; using your example of a 390μF 420V LXS (5,000 hours @ 105°C), which is 30mm diameter by 40mm tall, the LXM (7,000 hours @ 105°C) of the same value is 35mm diameter by 45mm tall, as is the TXH (10,000 hours @ 105°C) in 390μF 450V (TXH doesn't have 420V models), so those are only used where their endurance is really needed.

LRS 003: Regarding audio equipment
This one is especially tentative, in good part because my knowledge about it only reaches so far. I am mainly examining items in the middle of the signal chain, and coverage of digital components here is limited in scope. Professional-grade hardware is not my focus here either – I have more desire to set a reasonable standard for "consumer"-level stuff (a term that I of course use rather begrudgingly…).

• Section 1: General sound quality (noise floor, frequency response, etc.)
  The noise floor of the device (be it a mixer, preamplifier for something, or what have you) should be at least 80dB (and preferably 90dB) below the full level, in general (although this may be difficult to achieve with certain devices).
  Frequency response of the device should not deviate from flat by more than ±0.5dB throughout the audio band (20Hz to 20kHz), except of course when intentionally changed.
  There is not to be "severe" interference within the device that induces audible tones more than 10dB above the masking (white) noise into the circuit, or raises the device's overall noise product by more than 1dB. (For this purpose, the "audible" range is to be extended to 30kHz to be on the safe side.)
  Total harmonic distortion at 1kHz with normal levels is not to exceed 0.05%.
• Section 2a: Line-level inputs and outputs
  For a start, I'd like to cut the shit and go with a level of 0dBV (1V RMS, 2.83Vp-p) for unbalanced signals. I don't know how that −10dBV "consumer" level (according to Wikipedia) came about, and it gives mediocre SNR. The +4dBu "professional" level is functionally alright, but being in such an awkward unit… (They are in quotes as neither level is actually close to being universal for equipment aimed at either market.) (I also thought about levels of +3 or +4dBV to further improve SNR, but those are outside the range that typical PC audio codecs work with.)
  The outputs shall be able to drive a 3k resistive load at the full level without exceeding the above-specified distortion limit (and should also endure a 2k load resistance with distortion <1%), have an impedance no more than 300Ω (but preferably lower) to minimise loss of level (and possible HF roll-off, especially with longer signal leads), and the coupling capacitors used should have a value of 10μF minimum (although 22μF is preferable; going much beyond that is extravagant) to maintain good bass response. Similarly, input coupling capacitors should be chosen to give a −3dB high-pass point of 5Hz or less, down to perhaps 2Hz (although exceptions may be made for devices that aren't interested in the lowest frequencies anyway).
  
  The outputs must handle a parallel capacitance of 4.7nF without instability. Any output series inductor used for interference suppression must not be of such a value that results in response deviation of more than ±0.1dB from flat at 20kHz, with 2.2nF in conjunction with the 3k resistor. (Fortunately, that takes like 400-600μH to go out of specification - which is a bit excessive, to say the least.)
  
• Section 2b: Headphone outputs
  Again, we want to size the coupling caps for a −3dB frequency of <5Hz to not lose too much bass. Of course, the "correct" size depends on the sum of the amplifier's output impedance and the headphone's nominal impedance (well, you can take its DC resistance as being sufficient for the purpose here). I'll assume a nominal headphone impedance of 32Ω (the common "low" impedance, although a few go as low as 24Ω but that won't make matters too much worse), which gives us the following standard values:
  
  • With low-impedance output (no resistor) - 1000μF (1mF if you prefer)
  • With "standard" 120Ω output - 220μF
  • With an intermediate output impedance of 68Ω - 330μF
  If direct coupling is used (which of course is only possible where a negative supply is provided), the DC offset should be low enough not to dissipate more than the equivalent of 1μW in a load, assuming headphone impedance either equal to the value of the chosen source resistor, or of 32Ω with a low-impedance source (which implies an offset below 5.65mV). Any output series inductance must not result in a lower −3dB low-pass frequency (again with a 32Ω load) than 160kHz (with 32Ω headphones, the limit will be ≈32μH).
  
  The headphone output must also be able to drive an 18Ω resistive load to full level with THD <1%.
  
  The noise floor must also be reasonably low, of course. Taking an efficient modern headphone with sensitivity of 105dB SPL at 1mW, we want the equivalent of no more than 3.16pW of noise power in order to stay below 20dB SPL at idle, again with headphone impedance either equal to the source resistor, or of 32Ω with a low-impedance source (which implies less than −100dBV of noise through the audio band).
  
  As an aside, it would be nice if portable devices would add, if not a full graphic equaliser (as sound drivers on PCs often have), at least a pair of tone controls (bass and treble). (Of course, this would usually be implemented in software.)

Admittedly, I don't know if mixers intended for home use (by which I would mean something basic, using unbalanced signals, featuring maybe 3 to 6 inputs and with little in the way of adjustments besides level and perhaps pan controls) even really exist as a commercial product - not that they would be difficult (or expensive) to manufacture, if a market existed. (Even I could probably design a circuit for one, if I wanted to.)

Revision history:
Initial post 2015/12/25 08:54
r1 2015/12/26 - added limit on DC offset at direct-coupled headphone outputs, refined statement on line in/out coupling capacitor sizes
r2 2016/05/08 22:41 - reduced distortion limit from 0.1% to 0.05% (although that's still by no means exceptional), also added input/output stability requirements, inductance limits, and allowance for slight output over-loads before gross distortion (to be secure)

I am not assigning an LRS number to this one, as it's more expressly hypothetical than a (somewhat) real desire. I'm not sure where else it belongs, though (I briefly thought of starting a thread about the idea at the Electrical Contractor Network forum, but as I'm not a moderator there and the edit time is limited, it wouldn't work in quite the same fashion).

(Fictional) colour code
In the long history of electrical installations there have been many different colour codes, which to start with were chosen independently (and without knowledge of those used elsewhere). The associations of the colours made varying degrees of sense, but the bigger concern was that many of the different codes clashed with each other (not uncommonly in a lethal fashion, such as causing confusion between live conductors and protective earths), eventually urging the standards bodies (IEC etc.) to come up with an “acceptable” compromise (for flexible cords, and subsequently fixed installation). This turned out to be brown for active/line (with black, and later grey, used for extra lines and/or phases), (light) blue for neutral, and a combination of green and yellow for protective earth. While known just about everywhere (even in North America, where it competes about 50/50 with their traditional code in flexible cords), and making sense in the historical context, their actual connotations might as well be completely random.

If, on the other hand, I was to start from scratch, something like this might make more sense:

• Phase conductors would be bright, prominent hues (to convey a sense of danger, much like poisonous wild animals) - probably blue, yellow (although the yellow text is deliberately “dulled”, otherwise it would be illegible against a light background), and pink. (This is actually not far off from the code used for fixed wiring in the UK prior to 2006, which used red, yellow, and blue in that order. )
• The neutral would be just that - white or light grey.
• Protective earth would be brown, as that's what the earth looks like (although to be fair, grass is green, so that works in its own sense - at least for people who aren't colour-blind). Or maybe even brown with green stripes .

As a compromise that's compatible with IEC 60446 but a bit easier to tell the phases apart, I'd more realistically propose orange and pink for the second and third phases (single-phase colours remain the same).

Voltage
There seems no question that an RMS voltage from each phase to neutral/earth of 220-240 is just more efficient, overall, than North America's 120 or Japan's 100. (Historically dielectric stresses could have perhaps been an argument in favour of the lower levels, but with modern (robust) insulation materials this is a very minor point.)

There's been debate before about whether household 3-phase 231/400Y (nominal) service is a valuable asset or too dangerous.
Overall, I consider it to be reasonably safe, as long as the following conditions are observed:

• 3-phase branch circuits are provided only for dedicated high-power appliances (cookers, air compressors, etc.); each other branch circuit is limited to one phase.
• The outlets in any one room (kitchens and worksheds exempt) are all on the same phase, to minimise the risk of confusion.
• Circuits from different phases (or even separate circuits on the same phase) do not meet in the same wall/ceiling/junction box, except where suitably segregated.

DIY privileges?
This is a contentious issue. The dominant premises of the popular debate seem to be being keeping people out of trouble vs. freedom to do basic jobs themselves (with saving money probably the main motivation); cynics may even suggest that the anti-DIY arguments are really to protect electricians' profits.

Beyond the simplistic “safety” versus “liberty” talk, though, there are some larger socioeconomic factors at play.
Courtesy of an independent electrician in Estonia (via Wikimedia Commons) (below I've cleaned up the spelling and grammar, as best as I can)…

Dmitry G wrote:As usual, small companies try to save money on everything. There is one worker, who does the job of 10 specialists alone and the total price becomes as though only 3 persons worked. As the result, the quality of their work is too low… but lots of consumers have been seduced by low prices! However, they have no possibilities to employ fresh specialists or use them as slaves without salary. After working for some months without money, workers can't prove anything at the court or contractor directed money into complex schemas, where even economical police are helpless.

Big companies are working like zoo in big city. I have quite long experience of working there and what the problems are: nothing is coordinated. Big company is divided into small departments: designing, engineering, constructing, wiring, etc… and nothing is done in command. They take 2 weeks to do work, which could be done in one day! And after you have done a volume of work for 2000 EUR, they pay you only 350 EUR as it was specified in the contract.

Of course, there are companies with coordinated work in command… but they are not latex and can't employ everyone. As the number of those companies is limited, they employ only best of the best specialists with lots of experience.

Not unsurprisingly, then, the most qualified electricians are somewhat reluctant to just replace switches or sockets.
All I can say about that is, I can't blame them for it.

At any rate, attempting to stop homeowners from doing any electrical work whatsoever, is definitely wishful thinking - perhaps even dogmatic.
A compromise that has been suggested before as “acceptable” would be to limit it to replacing broken switches and sockets - although even this may work out to be too restrictive in some economic climates…

The instructions as included with most plugs, trailing sockets, etc. (at least in Australia/NZ), aren't sufficiently helpful to the average homeowner, in my view.
Only sometimes is it even expressed that wiring two plugs together is forbidden (much less adequately conveying the danger of doing so).

Power in the bathroom?
(Note: I am using the word bathroom here in the British sense, rather than American.)
There has been much debate before whether to allow a full-capacity outlet in bathrooms (for hair-dryers and similar devices) or to restrict it to a low-power (only for shavers, electric toothbrushes and similar) outlet provided from an isolating transformer (with of course resettable overload protection).
Particularly in the UK, the latter is standard practice - but the fatal (literally) loophole in that logic comes from the combination of real human behaviour, and extension cords (which, let's face it, no amount of warnings can stop people from using entirely).

More specifically: If someone really wants to use their hair-dryer in a bathroom with no suitable socket, they will just get an extension cord of sufficient length to run the hair-dryer from an outlet elsewhere in the home. If that outlet is not on an RCD of sufficient sensitivity, and especially if they commit the cardinal sin of using the hair-dryer while in the bathtub…

An alternative (and probably more realistic) suggested is to allow an outlet without needing an isolation transformer (which would be impractically huge and expensive to handle 2kW+), as long as it is protected by an RCD with 30mA (or stricter) sensitivity.
In North America their (modern) hair-dryers even include a leakage detector in the plug (sometimes resettable, other times a one-shot deal).

For lighting, the safest (current) option is of course LEDs running from an appropriate SELV (separated extra-low voltage) supply - although mains-voltage fittings might be considered OK on the condition that they are both of splash-resistant design, and on an RCD-protected circuit.
In the UK in particular, light switches were historically required to be either of the “pull-cord” ceiling-mounted type, or placed on the outside of the bathroom (my own home does neither though, and until 2010 didn't even have the lights on RCDs!).

Residual Current Devices
Must be A-type or better (F, B) for new installations. Circuits for fixed appliances with variable-speed motor drives require F or B types (F type is also recommended on general outlet circuits, especially where a washing machine will or may be plugged in); for three-phase electronic loads, type B is compulsory.

Arc-Fault Detection Devices
I'm in no rush to mandate these That said, the most worthwhile places to use them are probably on circuits dedicated to heavy resistive (or perhaps with PFC) loads, although the single-phase limitation kind of breaks that deal in Continental European installations . One of the "better" reasons I've seen given for more-general use is to cover for electric blankets with broken elements, but this can be (and apparently is in some models) better-implemented in the control unit.

I will add more sections as I think of them (I do have a local document along the same lines but more “formally” written).

Coloring here has changes at least four times in the last century alone, plus there are all those fabric-coated wires which have barely distinguishable plastic color beneath the fabric after several decades of use as it often degrades in many ways. (Solder-coated hard copper wires are the worst as I've learned just two weeks ago while replacing over half a century old cast-iron fuse boxes with new plastic breaker box, the insulating plastic stuck to the conductor surface and I had to scratch it away so the wire got clean enough to screw it to the terminals).

Latest version of PE is green-yellow, which I assume is that it's clearly visible as PE should be almost everywhere for safety grounding, so you can see it.

Hair dryers are popular killing machines in literature and TV, but there are some resonable doubts whether it would actually do anything in real world. Although the paths of god and electricity (esp. lightning) are often mysterious, almost definitelly not in plastic or ceramic bathtub (or shower), which is majority currently. And likely not even in steel bathtub as the conductive path through the resistor wire (and fan winding) is still wayyy more conductive than through (unsalted) water and grounding. Things get interesting if you add bathing salt.