7
\$\begingroup\$

Sorry if there is an answer for this kind of question somewhere. I can't find it anywhere.

Take a look at obvious toaster example: If you live in Europe and have non-polarized plug and a toaster (a bad one without double-pole turn off) - you have a 50% chance of touching the hot wire (via the heating elements or even bread if you're unlucky) if you plug it in the wrong way, so that the off switch turns off just the neutral wire. You will be shocked if you somehow connect your body to ground (through another device for example).

I understand that these conditions are not very likely to happen, but why do many countries still not even think about doing away with non-polarized sockets and adopt polarized? I'm sure even in Europe the light-bulb sockets are wired to be neutral-shell polarized, so why not wall sockets?

Sure it is not a cheap thing to do, but 10 years later it will be even more expensive since more stuff will be produced without a polarized plug.

\$\endgroup\$
  • \$\begingroup\$ Why does an all-plastic chassis USB charger need a polarized plug? \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 6 '14 at 2:18
  • 1
    \$\begingroup\$ This particular example doesn't need it. But there are thousand other electrical devices that are not all-plastic and even have opened wires(like toaster or light-bulb). \$\endgroup\$ – user50786 Aug 6 '14 at 2:22
  • \$\begingroup\$ Related: electronics.stackexchange.com/q/15228/2028 \$\endgroup\$ – JYelton Aug 6 '14 at 2:26
  • \$\begingroup\$ I have read that, still I wondering what stops Europe from moving to polarization. I mean, now everything is supposed to be as safe as possible, then why not sockets? Simply too expensive? Polarized plugs just look more finished, it's always nice to have extra-safety. I'm not trying to act like I'm overestimating the hazard, I don't really have a problem with safety, but just wondering... it seems to have logic for me, even if it will cost some good money(btw you can force people to buy sockets and plug-converters, they are cheap, no need for country to pay.) \$\endgroup\$ – user50786 Aug 6 '14 at 2:31
1
\$\begingroup\$

A couple of things.

A very small point, the UK is in Europe, and AFAICR we have had 3-pin plugs, 3-wire cables, since the late 50's.

My house was rewired in the 80's, and we have Earth Leakage Circuit Breakers (ELCB) on every circuit. So even if I stuck a fork into my toaster while grabbing a copper water pipe, I'd expect it to trip (I am not willing to back this up with evidence :-)

When I have visited continental Europe, I am pretty sure that I've seen the same ELCB technology in use.

I suggest that is even more effective than having an Earth connection; after all, if I touched the correct bit of wire in your toaster with my fork, without touching anything else, the Earth connection via a plug would do me no good. Further, unless the device had a metal case connected to Earth, I don't think I am much more likely to touch both Earth and live than just live alone.

I imagine the cost of rewiring all of the houses in Europe which have two-wire cabling t have three-wire would be very large. However, upgrading the distribution panel with ELCB is pretty simple (a drop in replacement in some cases for an old fashioned fused unit), and could be caused to happen more easily when electricity metres need replacing.

\$\endgroup\$
  • \$\begingroup\$ Well, yeah... damn, how could I forget about ELCB facepalm yeah, now it makes sense, thanks very much :) \$\endgroup\$ – user50786 Aug 6 '14 at 4:13
  • \$\begingroup\$ I wonder why this was down-voted without any feedback? That seems a tad unhelpful because I can't learn what I did wrong. I am willing to believe I am wrong in some way, but have no clue without feedback how. I thought the model was, as Uncle Ben said "With great power comes great responsibility". :-( \$\endgroup\$ – gbulmer Aug 8 '14 at 15:30
3
\$\begingroup\$

In the US, there is a "symbol" of a square inside of a square that stands for "Double Insulated". You can find these on mains powered devices that have a chassis that is mechanically designed to keep power from reaching any conductors on the outer housing (The part the user touches). Cell phone adapters are not polarized because the connector that you attach to the phone is isolated by a transformer. This keeps the mains voltage away from the user. Also besides safety, when using a bridge rectifier to convert the AC to DC, the bridge doesn't care which side of it gets the "hot" line and which gets the "neutral" line.

\$\endgroup\$
  • \$\begingroup\$ I'm not talking about double insulated devices and especially DC powered. I'm talking about Class 0 and Class 1 devices, that can possibly have exposed wires and mostly not DC powered through transformer. \$\endgroup\$ – user50786 Aug 6 '14 at 2:47
  • \$\begingroup\$ Those devices should be using polarized plugs. And yes, chassis grounds should be used in such devices. Depending on the country, devices like those wouldn't pass safety testing and would not be certified "safe". It would be more important to require grounding pins on these devices and to also require GFCI type outlets. There is an expense involved and many people would have to pay to have a third conductor run in their walls to provide a "ground" wire. Even many areas America, if you have a house with 2 wire, non-polarized outlets, you are not required to upgrade unless you sell the house. \$\endgroup\$ – Enemy Of the State Machine Aug 6 '14 at 3:09
  • \$\begingroup\$ There's a reason class 0 devices are prohibited in a lot of countries. The advantage of not having a polarized plug is that a device can be plugged in each way which would be a pain for thinks like chargers an other 'not symmetrical' plug devices. \$\endgroup\$ – RJR Aug 6 '14 at 3:30
  • \$\begingroup\$ Can you rely on transformer isolation in switch mode power supplies? \$\endgroup\$ – UpTheCreek Feb 9 '17 at 12:21
1
\$\begingroup\$

On any device, according to EU regulation, you shall never be able to touch either phase or neutral wire, even by touching heating elements (don't try it : it's insulated but it's hot). The package of your toaster should (and, at least in my country, shall) be wired to the ground. neutral and ground aren't connected anymore in modern (less than 30 years ?) electrical systems (not before the circuit breakers, at least), to allow the use of ground fault circuit interrupter.

On old electric systems, you may have no ground connectors on wall plugs, in that case, the package of your devices is just floating, it is neither connected to neutral nor to the phase. In that case, you should modernize your electric system, ground connection is mandatory since the seventies (in western Europe).

\$\endgroup\$
  • 1
    \$\begingroup\$ The primary purpose of polarized plugs is to ensure that the neutral wire not be disconnected without the hot wire also being disconnected. It's possible to construct double-pole fuses that can ensure that, but they're a lot more complicated than single-pole fuses. How do non-polarized devices handle fusing? \$\endgroup\$ – supercat Jan 11 '17 at 22:53
  • \$\begingroup\$ Uh ? There are two kinds of fuses on domestic electric systems: one which protects against overcurrent (if you've more than 16 Amps going either in or out the plug (it's alternating, anyway), it shuts down the power. The other is differential: if the current going in one wire isn't the same coming in the other (with as little as a 15mA difference), it shuts the power down, as the current may be returning to the ground through a human body (which probably doesn't like that). Polarity is irrelevant for AC electric system protection. \$\endgroup\$ – Jacen Jan 13 '17 at 18:16
  • 1
    \$\begingroup\$ Unless all each mains-power circuit is protected by what in the US would be called a ground fault circuit interrupter (GFCI) that disconnects power when a current imbalance is detected, having an overcurrent fuse disconnect only the neutral wire could create a hazardous condition since parts of the device which should be at neutral/ground potential would instead be at mains potential. \$\endgroup\$ – supercat Jan 13 '17 at 18:57
  • \$\begingroup\$ I'm not a english native speaker. The device I called "differential circuit breaker" is a GFCI. In my country, every domestic circuit have a GFCI, but it seems that few country require this. en.wikipedia.org/wiki/Residual-current_device \$\endgroup\$ – Jacen Jan 17 '17 at 10:35
  • \$\begingroup\$ If the power source is protected by a DCB/RCD/GFCI, one could minimize both electrical hazards and interference by having roughly equal and opposite voltages on two interchangeable AC power leads. If the power source is not protected in such fashion, however, and one power supply lead is disconnected, it's important to minimize the voltage on the other lead. \$\endgroup\$ – supercat Jan 17 '17 at 19:36
0
\$\begingroup\$

THREE methods are important for human safety:

1.socket and plug polarity.In the event of insulation breakdown of line conductor, It prevents exposed metal chassis of loads exposing the person to fatal currents. human body can tolerate upto 65vac for 10s OR 250vac for 100ms.

2.metal chassis attached to earth conductor with resistance below 1 ohm. this helps in case polarity fails. It provides low resistance path to earth as compared to resistance of 500 ohms of human dry skin. and also if the prospective earth fault loop current is below the instantaneous trip voltage rating of the circuit breaker, it trips within 120 seconds as per IEC 60898.[which is not protective for human body].

  1. IN WET AREAS, HUMAN SKIN RESISTANCE IS CONSIDERABLY WEAKENED , and so even a good earthing wont do its job, OR in conditions where good earthing is not possible or cannot be checked routinely. So in such conditions RCDs[ ELCBs/RCCD/GFCI/DCB] ,as per IEC 60479-1,IEC 60364,IEC 61008 & IEC 61009 SHOULD BE USED, which protect from fatal shocks within approx 500ms for a 20ma current.
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy