[chbot] Why galvanic isolation is important

[Helmut Walle]

It is important to remember that this whole affair is as much a legal 
issue as there are technical safety risks in the design of switched-mode 
power supplies. One aspect of imported goods is that it often is 
somewhere between difficult and impossible to prosecute the (overseas) 
manufacturer if something goes wrong. This lack of ability to prosecute 
has facilitated the import of huge volumes of goods of various kinds 
(not only switched-mode...) that do not come anywhere near meeting the 
safety requirements of the countries they are being imported to. Product 
safety is not accidental. It needs to be intended and is achieved by 
following best-practice processes of hazard identification and risk 
analysis, followed by adequate mitigations, whatever the technology in 
question. As most of us here know, there are international standards 
that must be met to achieve certain certifications, which can be 
mandatory, depending on the kind of product and country of distribution 
to end users.

Now in the past the trouble has been that it was hard to get hold of 
(overseas) manufacturers, while the (local) retailers could not legally 
be held responsible, because they were not the manufacturer - and this 
unlucky situation has brought us here... In recognition of the problem, 
the straightforward consequence has been that laws have been changed in 
many countries in a way that makes it possible to prosecute retailers 
with pretty hefty penalties if something goes wrong, irrespectively of 
the cheap price of the product, and obviously irrespectively of the lack 
of the dealer's involvement in the manufacture of the product. The 
probability of killing someone is fortunately still pretty low, but we 
must acknowledge that the very few people who do come to harm do not 
find this very consoling. And this explains the level of the penalties: 
basically, this means that a retailer must see it as an existential 
risk, and they better work with their suppliers to ensure products are 
of adequate safety. And for anything used by consumers, like white-ware 
or consumer electronics, this means that the safety level must be very 
high, because there are millions of users who are mostly inexperienced 
and not all that aware of the risks in the first place.

It also helps little to say that "Killing oneself with 240v is actually 
quite difficult." - there are so many users of these products that even 
the slightest probability would still mean some would end up dead, and 
each one is one too many! Oh, and let's not forget sample bias: it is 
not even surprising that we find people stating how difficult it is to 
kill oneself... the ones who actually have succeeded are not around any 
more to report their success.

To bring this back on topic slightly, the whole risk management process 
is particularly interesting and demanding for mobile robots, most of all 
probably UAVs. Any updates on the risks associated with electric system 
failures and emergency shut-downs as they apply to mail delivery drones? 
Obviously airborne systems need to be as lightweight as possible for 
efficiency reasons, yet at the same time redundancy in power supply 
systems often comes at the expense of extra weight. I have to admit that 
I haven't looked at it very closely, but I am not sure whether these 
concerns have been addressed satisfactorily, and what the situation will 
be like once we see significant numbers of these in the sky above busy 
cities (which really can only be a matter of not all that long time).

A safe evening to everyone.

Kind regards,

Helmut.


On 30/06/14 17:22, Mark Beckett wrote:
[Some technical detail on the hazards and risks associated with 
switched-mode power supplies (or chargers in this case)...]