I've been reading the thread about the ABC Inventors show and the Protex device and a thought raised its ugly head so I thought I'd put pixel to screen and write.......
Just wondering from a safety point of view is a device with a 2 pin plug safer then one with a normal 3 pin plug?
And to that end why is it that 2 pin plugs on products have so proliferated the market?
-- John
I heard you on the wireless back in 52 Lying in Bed intent at tuning in on you If I was young it didn't stop you coming through Oh-a-oh
They took the credit for your second symphony. Rewritten by machine and new technology, and now I understand the problems you can see. Oh-a oh
Video killed the radio star Video killed the radio star In my mind and in my car We can't rewind we've gone too far Pictures came and broke your heart Put all the blame on VTRs....
** Unless it gets damaged or wet - a Class 2 appliance is safer.
Items that rely on their external metal parts being connected to supply earth become VERY UNSAFE when that earth is missing or the AC connections at the plug or socket are wired wrongly. In these circumstances the external metal work can easily become live at 240 volts and deliver a fatal shock !!
This CANNOT happen when the external metal case is not connected to the AC power lead and is NOT being relied upon to provide a safety barrier to users in the event of an internal electrical failure.
Also - the insulation that must be used and other detailed requirements for an item to comply with Class 2 make it safe in relation to the usual component failures and mishaps.
** It has taken a long time for this to become the case with most consumer electrical and electronics. A large part of the reason is that manufacturing is now concentrated in Asia with a world wide marketplace - in that world market it is easier to get appliances approved by the local safety authorities for sale ion each country if the appliance has been approved already ( ie in Asia) as meeting Class 2 requirements.
Whats normal about 3 pin ? A lot of the world has 2 pin for the vast majority of devices with only some getting three pin. (and nasty three to two adaptors commonly available).
In australia the sockets must be three pin, extension cords must be three pin, etc, so that a device that should have an earth wire can get it.
2 pin devices are "double insulated".
RCD's protect in most cases where "protex" does and a lot of cases where protex does not.
In fact, not adding the earth wire to the devices does ALLOW the use of RCD's! The reason is that connecting the chasis of many devices to earth will create a current through earth, and then that can trip the RCD.eg a tv where the high voltage transformer might leak a bit through the air.
The third wire on devices is only there to allow a short to earth trip an OVERLOAD circuit breaker or "blow the fuse". The third wire does little to protect people if the case of the device is broken open or the device is dropped into water... in fact it provides a greater possibility of a lethal circuit. SO the third wire is not put on where its no use. - plastic chassis or 'metal chassis not exposed' devices.
Yikes. What counts as a Class 2? My 2-pin DVD player had 250mA @ 230v flowing from the case, so we added an earth lead to it (Tired of getting shocked when changing connections at the back :)).
** Any appliance with the double square symbol or the words "Class 2" or the words " Double Insulated" on the back label.
** That would be 250 uA - not mA - or you would be dead long ago.
The tingles you feel are due to very brief current spikes when RF suppression caps of about 2000pF discharge into your skin at the moment of contact.
Russia does have a 220v 50hz supply that has a 2 pin mains setup in domestic situations. I have no idea if they have a system that has the neutral and earth joined like here - or if the mains is isolated, or some other strange system. The plugs can be inserted in either direction.
Appliances that in Australia always have an earth such as refrigerators and irons don't have earth there. Here are a couple of pics of a locally made domestic iron:
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Domestic Stoves however did have their own dedicated socket, the pins were thicker than an Australian plug that would tend to indicate a
15/20 amp rating, but I have no idea of the voltage or current used in their stove circuits and didnt measure it.
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Russian mains plug and GPO is on the right. on left is European plug for comparison. The pins are larger on euro plug, and even though there are some euro sockets (with the side earth strips) used there, their earth system is not connected or used with them (only 2 core fig
8 type wire is run to them) typically.
One of the domestic metering setups I saw that had circuit breakers? at top. or one is RCD maybe ? No idea.
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How devices like PC power supplies (that need an earth for the noise filtering) cope I have no idea.
Out of interest, regarding 2 pin appliances, double insulated and metal cases, during the 1980's I had a Japanese made Sharp VC 488 Hi-Fi VCR. It had substantial metal chassis throughout and a metal case top and bottom. All internal and external metal chassis parts were deliberately connected together via short lengths of wire and crimped eyelet terminals under the screws used.
This earth setup was quite deliberately connected back to the mains neutral via a high value resistor that was many Megs as I recall.
The cord and plug was 2 wire, but were double insulated, and the internal wires were coloured brown and blue
What was the purpose of all this ? it surely wouldnt provide any protection in case of mains getting shorted to the metalwork, and I fail to see what they were trying to achieve here - unless it was some form of shielding, or to minimise hum etc ?
** That *special safety rated resistor * is to leak away static charge that can accrue from the antenna connection on windy day.
** Basic electric shielding - same as all such units.
The case is directly connected to signal ground.
** Why did you imagine any electronics is built in a metal case ???
** The USSR is a place that rates human life as nearly worthless - the only lessons that can be learnt about safety practices from there is what
*NOT* to do.
AFAIK, no law there allows a person to sue another or the State for damages as a result of negligence.
Now, who was it who so accurately described the place as an "evil empire" ???
I said "A lot". You mean to say insignificant populations have 240 volts and the chasis of metal devices using neutral ???? You need to get out more. Vietnam for example, often has everything plugged into 2 pin 240 volt sockets.
beats me as to whether its safe to do that or not.
Thats a requirement as the element can burn out and create lethal circuit configurations "in normal use".
The chassis has to be earthed to ensure the fuse will blow if active touches chassis, or neutral touches chassis and theres significant power in the circuit so made.
I agree. RCD earth and no earth, doesnt do anything.
There's nothing that can be done for wet areas, except some sort of protex system.
You talk as if was am totally wrong.
Ok, so the third wire is not put even if it could be some use.
Thats just a more specific statement about a different case. perhaps I should have mentioned it, but well these things happen.
Just remove's the sting when you touch the case.
The vcr's that DONT have this give you a sting when you first touch them.
Most if not all of continental Europe has 220-230 V power with 2 pins. There are some devices like washing machines or fridges with 3 pin plugs but the construction is such that you can plug them into 2 pin power point. Most of the powerpoints in a household are 2 pin with a couple (eg. one in the kithen and one in a bathroom) having 3 pins.
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