Why are the Edison screws allowed to be used? They seem unsafe. Why are they designed that way?

They require a complete grounding system to ground devices for the rare situation where the hot wires somehow touches the metal case of the device

While they let you use a lamp fixture which has an exposed hot contact when you remove the lamp, and nothing is there to protect your finger from touching it.

I would expect this socket to have a different design which will cover the contact from accidental touch.

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closed as primarily opinion-based by Dmitry Grigoryev, PeterJ, Richard Crowley, RedGrittyBrick, Dave Tweed Jun 24 '16 at 22:07

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Forget Edison bases, what about the standard Nema 1-15 or 5-15 outlets? Completely dangerous. \$\endgroup\$ – Passerby Jun 21 '16 at 22:31
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    \$\begingroup\$ @Passerby Sounds like the problem is that all electrical systems in US are silly dangerous. The split-delta windings is silly dangerous too... \$\endgroup\$ – Aron Jun 22 '16 at 1:59
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    \$\begingroup\$ In my experience they're a lot easier to change than bayonet, and the fitted bulb doesn't end up wonky / off-centre. \$\endgroup\$ – OrangeDog Jun 22 '16 at 9:04
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    \$\begingroup\$ Why are they designed that way? They were designed earlier than 1910 near the birth of the electric light bulb. They had bigger things to worry about in those days than sticking your finger in the socket accidently - things like getting electricity to the home so they didn't have to use gas or kerosene lighting with open flames and the possibility of accumulating gas which could explode when you tried to light them.. \$\endgroup\$ – davidbak Jun 22 '16 at 17:46
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    \$\begingroup\$ @dbanet -- LOLwut? the neutral in 99.9999% of US wiring is continuous all the way back to the service entrance -- and we somehow aren't shocking ourselves into oblivion over here! \$\endgroup\$ – ThreePhaseEel Jun 23 '16 at 0:26

Here's your opportunity. The market's looking for that right now so build a better mousetrap.

The USDOE and California CEC want to murder the Edison base to finally stop people from using incandescent bulbs, and enable fixture designs that don't have to worry so much about dissipating heat. They mandated GU24 in 2008, which solves some of your concerns. Take a look at how that's going 8 years later. LOL.

There are several flaws in the GU24 that you should address in your new design.

  • Ease of installing "blind" when you just can't see the socket or it's deep in a recess.
  • Equipment Grounding Conductor.
  • 3-way lamp support.
  • Or since dinosaurs called and want their dual-filament bulbs back... how about a standard for a signal pin and protocol to command the bulb to "dim". In track lighting, the signal line could be bussed to each outlet and controlled by a single dimmer.
  • Multi-voltage, either standardize that all bulbs must be multi-voltage, or have different keying for 120V, 220-240V and 277V.

Good luck!

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    \$\begingroup\$ Sorry for stupid comment, but where is 277 V in common use? \$\endgroup\$ – AndrejaKo Jun 22 '16 at 13:00
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    \$\begingroup\$ @AndrejaKo 277V is single phase of 480V 3-phase. In USA factory and workshop equipment often runs off 3-phase 480V, so 277V is the lowest voltage you can easily get in such system. Edison-screw 277V bulbs are quite easy to get over the internet. \$\endgroup\$ – Agent_L Jun 22 '16 at 14:06
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    \$\begingroup\$ Given that LEDs in well design fittings, last a long as most fittings, I expect we will move away from replaceable bulbs. \$\endgroup\$ – Ian Ringrose Jun 22 '16 at 18:02
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    \$\begingroup\$ @ThreePhaseEel Well, that's half of it then... now we just need a plug. Unrelated, there's now a requirement to put a connector between fluorescent ballasts and their line-voltage supply source. There is no standard as to the shape. Some are 3-pin, and now I know why. \$\endgroup\$ – Harper Jun 23 '16 at 0:39
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    \$\begingroup\$ @AndrejaKo Yes, it's one "wye" leg of 480V 3-phase. It's mainly used on sodium/mercury/halide lighting and fluorescent ballasts. With MWBC, it lets you put a lot of light in a ceiling cheaply: 3 lighting circuits on 4 wires (conduit is ground), 9 circuits per conduit without de-rating (neutrals don't count in MWBC). 40KW on twelve 12AWG wires. Not bad. \$\endgroup\$ – Harper Jun 23 '16 at 0:50

The Edison lamp base design dates predates all twentieth-century safety regulations (because it predates the twentieth century altogether). Having light bulbs screw in and out is not great in environments where vibration is a problem, and replacing a bulb which has just burned out while in use may require use of a cloth to hold it, but it is generally advantageous to have a lamp which can be inserted or removed by handling the bulb rather than having to mess with the holder. Making a metal thread assembly that can mate reliably with a metal socket is easier than trying to mold threads into glass, and having one contact in the center of the socket is easier than trying to have two concentric contacts.

While I doubt the Edison base would be approved by any safety agency if it were being introduced as an entirely new product, it has been effectively "grandfathered in" because it has been used for a long time, people are familiar with it. A "safer" design that people aren't familiar with might lead to more accidents than the century-old design which, for all its imperfections, is well understood.

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    \$\begingroup\$ gotta love "(because it predates the twentieth century altogether)" !! \$\endgroup\$ – placeholder Jun 21 '16 at 22:58
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    \$\begingroup\$ Not only is is "well understood," but everyone has lots of practice with it. Plus, if the fact that this construct isn't known for being dangerous in daily use is any indication, it would seem that it's been refined into a reliable design over the years (something like how lithium batteries have been made fairly safe despite being extraordinary dangerous by nature). \$\endgroup\$ – jpmc26 Jun 22 '16 at 8:49

Nobody answered why were they designed that way?

I recall it was for practicality and cost of manufacturing.

I heard about it indirectly, on a TV show about Tesla. He was contracted to provide lights as Edison's competitor, but could not use Edison's patented sealing/base method and had to use his own which lacked the advantages.

Trying to find more about it, I think the point works like this: you seal the glass capsule and shape it like a bottle top easily enough. The fitting needs to slip over that, and be simple and cheap to make. The contacts along the outside surface of the simplest cap shape is the simplest. Naturally the contacts will be co-axial, and you need at minimum 2 metal areas separated by insulation. Making one of those parts be the thread as well saves components and joints. It also becomes easy to connect the emerging wires to the cap without any alignment.

In short, it's the simplest possible fitting to manufacture.

From the link above, the author quotes Alan Makkos,

The familiar screw-in base was a whole other can of worms. Edison’s first bulbs slipped into their sockets without a way to secure them, until he was struck by a utilitarian design while having lunch in his workshop. “Edison saw a can of kerosene on a shelf, and said, ‘Oh, the lid for that kerosene would make a dandy screw base for a light bulb.’ So they got the can down, cut the lid off with a band saw, and made a light-bulb socket out of it,” says Jenkins. “By the time they were in production in 1885, they had reduced the size of the base quite a bit, and it looked a lot more like modern bulbs.”

  • \$\begingroup\$ Has the metal base ever sealed the enclosure? Making air air-tight seal between metal and glass that won't leak under changing temperature conditions is hard. By my understanding, a sealed glass bulb is constructed with two wires passing through the glass; those wires are then attached to the base. \$\endgroup\$ – supercat Jun 24 '16 at 22:35

Today's sockets for incandescent or CFL lamps (and now some LED lamps) come with a black and a white wire. The white wire is neutral and is connected to the outer "threaded" part. The black (hot) wire goes to the center contact. I have checked this myself with an ohmmeter to prove it.

On the old 'Edison' sockets you have 2 brown wires. The light brown wire with ridges is the neutral, and connects to the threaded part of the socket, allowing a bulb to be replaced with no shock hazard. The other darker brown wire is the 'hot' wire to the center connector of the socket, coming from the light switch.

The socket is wired this way so that whilst screwing in a bulb the neutral makes contact first, and the neutral line should have just a few volts on it at most, depending on if it is a 'shared' circuit with other loads. All neutrals are grounded at the main breaker panel to keep the voltage low (<10vac) on the neutral wires even with active loads throughout a house, apartment, etc.

Touching the threaded part should not cause a shock - ever! If it does then the wires to the socket may have been reversed, either at the socket or the light switch for that socket. Even if it is a socket for a 3-way bulb, the threaded metal part is connected to neutral.

NOTE 1: A high wattage device such as a space heater far from the breaker panel could have 10vac on the neutral connection at the AC outlet, but 10vac is not a shock hazard even if you found a way to test if it is a shock hazard.

NOTE 2: Without a bulb screwed in and the power turned on, there is a 220vac shock hazard present. For European light sockets, it is 4 times the energy of American sockets running at 120vac if someone touches the center contact. Most light fixtures are above the reach of children, but not desk-top lamps. Best solution is to always keep a bulb screwed in-don't leave light sockets open.

For information on what constitutes a shock hazard in terms of neutral voltage, I found this article here.

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    \$\begingroup\$ This doesn't answer the question. \$\endgroup\$ – user57709 Jun 21 '16 at 21:57
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    \$\begingroup\$ Yes, I'm not talking about the netural connection, I'm talking about the hot contact at the bottom , people can accidently touch it while replacing a bulb or leaving a socket without a bulb. \$\endgroup\$ – Ronen Festinger Jun 21 '16 at 23:42
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    \$\begingroup\$ @Sparky256 backstabbing? What? Lets calm down here. Your score on this question is currently +1 and -1, so 0. If someone partially up voted you, I'm guessing it means someone undid a down vote as far as I can see on the timeline. +10 -2 is the rep score, as down votes cost you 2 points but up votes give you 10. You're still 8 in the green. The reason someone down voted you is wholly up to them, and they don't have to comment or explain why. That's how SE is designed, and it's just a matter of fact here, you either grow a thicker skin or let it get to you. Make that +2 / -1 votes. \$\endgroup\$ – Passerby Jun 21 '16 at 23:42
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    \$\begingroup\$ While I agree that the answer as-is doesn't cover the center contact risk, the reason for it is pretty obvious in terms of history. I'm upvoting this because of the useful and rare information about stray voltage levels on neutral -- this is seldom discussed or understood even among electricians. \$\endgroup\$ – stevegt Jun 22 '16 at 3:37
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    \$\begingroup\$ "For 220vac european sockets (and some other countries) there is no neutral" - this sounds strange - could you elaborate? (I'm in a 220vAC European country - GB - and we certainly have a neutral.) \$\endgroup\$ – peterG Jun 22 '16 at 20:38

I suppose the real answer is because we don't live under a completely tyrannical government. People have been living with this type of socket for over a hundred years. This design has a lot of hours on it, and people understand how to use it, and are generally aware of the risks. Any competing design would have to convince, not only the government officials responsible for building codes, but enough of the population so that those government officials don't get replaced come next election. There is a market, and when people realize a clear benefit to any given solution, they will switch. Although you have pointed out some flaws in the legacy design, you must realize that each user does not attach the same weight to those flaws as you do. There is also a potential well that a new design must overcome, where it's benefit must outweigh the additional cost or inconvenience of using the new design.

I think you are looking at if from the perspective that it is the place of the government to only allow those behaviors this citizens which it deems fit. The United States System of Government, is based upon limiting the power of the government relative to the citizens, so that there must be a compelling reason for the government to act and restrict the freedom of it's citizens. Looking at if from that perspective, lets evaluate the apparent flaws in the prevailing design:

  1. Flaw: They require a complete grounding system to ground devices for the rare situation where the hot wires somehow touches the metal case of the device Mitigation: This is a one time cost, upon fixture installation. Magnitude of the cost is not that great, as it involves one extra wire in the cable.
  2. Flaw: While they let you use a lamp fixture which has an exposed hot contact when you remove the lamp, and nothing is there to protect your finger from touching it. Mitigation: Sparky's answer and user training do a great deal to eliminate this apparent flaw. It may appear that this is a dangerous design, but the facts don't really back it up. The Consumer Products Safety Commission evaluated electrocution deaths over a 7 year period from the last decade. They found only 25, 3-4 per year, were attributable to lighting equipment (https://www.cpsc.gov//PageFiles/108404/2008electrocutions.pdf). This does not mean that these 3-4 deaths would be prevented by a different fixture design, only it puts an upper limit on the risk from this design. My guess is that no one is killed by this design, because even it is wired incorrectly, it just gives you a small shock. Clearly though the risk is quite small.
  3. Flaw: I would expect this socket to have a different design which will cover the contact from accidental touch. Mitigation: It is only really an issue while the circuit is energize without a bulb present, so users have a sufficient work around. Again, the realized risk in this situation is diminishingly small.
  • \$\begingroup\$ "They're allowed to be used, because that's what we've always used." In essence. \$\endgroup\$ – Bitrex Jun 23 '16 at 18:07
  • \$\begingroup\$ I think a better way of putting it is that they are allowed to be used because there is not a compelling reason to disallow them. \$\endgroup\$ – Mauser Jun 23 '16 at 18:09
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    \$\begingroup\$ I think the risk could be easily lowered from the beginning if it was designed so that the base of the bulb was a narrow rod and the the base of the socket was insulated with a small hole in the center which the rod will fit into. \$\endgroup\$ – Ronen Festinger Jun 23 '16 at 18:17
  • \$\begingroup\$ Perhaps, take a look at my updated answer. We are talking about a very small risk here, so there does not seem to be much incentive to fix it. \$\endgroup\$ – Mauser Jun 23 '16 at 18:33

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