Moments of my intimate contact with electricity:Computer casing

When i was a teenager, i had a desktop computer – after couple of years, i realized that every time i touch the metal casing – my arm, palm and fingers will feel numb, not much of a good feeling – but everything works – doesn’t seem like something has broken.. made me wondered for years.

Looking back – i got to say the cause must be due to the fact that the “Earth” wire (Green/Yellow) was broken – this cause voltage to build up on the metal chassis – and when i touched it, i was the “Earth” connection, and current actually flowed through my body to return to its source (for how this work see the other blog entry: http://electroniccircuitdesignsharing.blogspot.com/2013/03/3d-perspective-of-test-pen-and-how-it.html) .

My experience dated back in 1990’s, nowadays there are more appliances (computer, printer, tv, entertainment set…) being power up by 2-prong AC power (no earth connection), this is what being called appliance class II stuffs (A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).)

Below is a Class II symbol (from http://en.wikipedia.org/wiki/Appliance_classes)


So i guess it is getting very common for folks to experience such sensation. And it is supposed to be harmless

Links:
http://www.dell.com/support/troubleshooting/uk/en/ukbsdt1/KCS/KcsArticles/ArticleView?c=uk&l=en&s=gen&docid=DSN_9B1A222771A7495097F2C0C5AC42BB2C
http://en.wikipedia.org/wiki/Appliance_classes

Moments of my intimate contact with electricity: Touching Mosquito Racket Zapper net…

I knew it is a bad idea, that it is going to be painful… and yet the temptation was just too strong
Plus – no harm for doing that right? After all, people accidentally does it once a while…

So with the power on – and closing in with index finger – I finally get closed enough – for a spark to “shoot” into my finger – and yap, it was painful – just like being poked by a needle! And it never cease to impress me what electronic can do, with couple of AA battery, this thing can boost it up to kV for zapping!
So this is where the boost converter comes in, with a boost converter, we can easily step up any voltage to the desired level, and it is pretty common in electronics (camera flash, spark plug….) And who says electronic is boring??

Moments of my intimate contact with electricity:Telephone Line Voltage

I like wiring – since my childhood years – guess it is just the way i am.

One of my early experience as a teenager is to extend the telephone line – essentially i just strip the wires and twist the interconnect – with my bare hand. In the middle of doing that, muscle at my upper arm started twitching involuntary – it felt kind of funny – no pain but still… seeing part of your body behaving out of your control is kind of weird…

As it turns out, telephone line is typically at 40VDC, and when it rings it can go all the way up to 90VAC.. so i guess my twitching muscle act as the incoming call detector!!

Having your body as part of current path is never a good idea, few mili ampere of current is enough to mess up your heart beat under the right condition – so give electricity some respect is what we should do..

Side notes:

from http://en.wikipedia.org/wiki/High_voltage

IEC voltage range	AC	DC	defining risk
High voltage (supply system)	> 1000 Vrms	> 1500 V	electrical arcing
Low voltage (supply system)	50–1000 Vrms	120–1500 V	electrical shock
Extra-low voltage (supply system)	< 50 Vrms	< 120 V	low risk

< 120VDC is considered as low risk.