[chbot] Why galvanic isolation is important

[Mark Beckett]

Volker

I'm sure someone as intelligent as you can see the difference between a 
USB phone charger and an optically isolated SSR.


There is a picture here showing the outlet which would seem to explain 
why it occurred.
http://rt.com/news/168772-faulty-usb-electrocutes-woman/


This is a comment made here 
http://www.austech.info/electronics/92054-cheap-usb-devise-kills-woman.html  
(/including the  incorrect spelling/)

    The switched mode transformer in these things is tiny. The only
    thing inside them that is separating mains from you touching it is a
    couple of layers of insulating tape and not even caplan tape. It
    melts away easy with a soldering iron. There are no separate
    chambers like there was with the good old 50Hz transformers.

    The way these cheap chargers flood the market there is no
    Fairtrading or any other department that can keep up with this.

    Cheap or expensive, every switched mode PSU is a fire hazard.
    A voltage surge (indirect lightning) can short a diode and the large
    electro cap explodes on the AC mains voltage or they explode because
    they fail as electro caps commonly do.
    This happens instantaneously unlike a faulty old fashioned 50Hz
    transformer that would just get hot and activate the thermal circuit
    breaker or
    the hair thin copper wire on the mains side would melt at a weak
    spot and break the circuit. No fire.

We had some switchmode psu's used on good brand monitors that had some 
parts get hot enough to melt the case, and in most others the case 
mountings became fragile and the whole thing fell apart exposing the 
internals.





Yes there is a very remote chance that one day, the SSR may fail, but 
equally you may step onto the road and into a sink hole, be run over by 
the bus that had a COF hours before and then suddenly had no brakes, no 
steering and the driver had an unknown heart condition.

Do you check every appliance in your house daily, weekly, monthly or 
yearly as you are running the same risk as you have cited with every 
electrical appliance you touch.
Don't think that the fancy label on the outside is anything to rely on, 
since many of the 'known' brands contain the same parts used in cheaper 
brands.




Of course for the truly paranoid, you could add a fuse and varistors 
across the low voltage side and from each leg to earth, which would 
limit the voltage to  12/24/48 volts whichever you want.
This would result in the fuse blowing and rendering it safe.


Mark