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I have a plug for a wireless router that ends in two prongs (no ground). Is it safe for me to plug this into an extension cord that has the ground prong, which would be left empty? I believe it is but want to double check.

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    Generally speaking I believe plugs were designed to mechanically allow things which are safe, and prevent things which are not. Therefore you cannot plug a 3-prong into a 2-prong, but the other way is safe & allowed. – UuDdLrLrSs Mar 23 at 15:11
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    @UuDdLrLrSs True, but a 2-to-3 prong adapter ( a "cheater plug") is unsafe in certain configurations even though it's a perfect mechanical fit, so "if it fits it's safe" isn't a perfect rule of thumb. – Nuclear Wang Mar 23 at 16:35
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    @NuclearWang I think "rules of thumb" are imperfect by definition – UuDdLrLrSs Mar 23 at 16:37
  • @UuDdLrLrSs except for the rule that no (human) person has more than two thumbs! – Dai Mar 24 at 11:36
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    This question needs to qualify that it is talking about US plugs. For the UK, this would not be safe nor easy. The socket protector would have to be levered back manually & a 2-pin plug would then have to be forced into a socket not only the wrong shape but also the wrong size. You can buy 2 - 3-pin semi-permanent adaptors for pence. – Tetsujin Mar 25 at 9:49
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Yes you can safely plug the appliance into a 3-wire cord and plug set, as long as that is the factory installed cord and plug on the router.

Appliances that are double insulated do not require an equipment grounding conductor to earth because the live electrical parts are specially separated/insulated from the case in a way that prevents any single failure from electrically charging parts that the user can touch.

There should be the words "double insulated" on the chassis, and/or this symbol:

enter image description here

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    Just to put this more in layman's terms, grounds on plugs are so that the outside case and buttons don't shock you. You (generally) only need them if the case is made of metal. If your router's case is plastic it doesn't need to be grounded. – SaSSafraS1232 Mar 24 at 16:23
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    @SaSSafraS1232 You’re leaning hard on that “generally”. Some plastic cases include metal screws. The best thing to understand here is that if the equipment required grounding it would have a way to ground it like a ground pin. An extension cord with or with out a ground hole doesn’t change that any more than an outlet with a ground hole would. In other words, the requirement comes from the plug side. Not the socket side. – candied_orange Mar 24 at 18:22
  • @candied_orange you're really over-complicating this, but metal screws are fine as long as they're not coming into contact with the electronics. Most consumer electronics stuff that doesn't have a ground on the plug is assembled with metal screws that go from one side of the case to the other and don't contact the electronics. The electronics are then mounted with internal screws into the plastic that are not accessible from the outside of the case. – SaSSafraS1232 Mar 24 at 20:16
  • @SaSSafraS1232 safety measures are about when things go wrong. Not when they go right. This question isn’t about how to build them. It’s about avoiding circumventing them. – candied_orange Mar 24 at 20:18
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If the power cord on the router is one of the modern type like this ...

enter image description here

Picture Source

... with one blade wider than the other then you have a polarized plug with the neutral routed through that wider prong. You want to make sure to properly orient the plug at the extension cord so that the wider blade goes into the wider slot.

Note that there are lower quality extension cords where it is possible to force in the plug wrong way around because the molded plastic will just stretch. This is one reason that you now see cords that have hard plastic around the holes that is then overmolded with the more flexible material.

enter image description here

Picture Source

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    Tim Allen had a stand up bit about tools and Sears. He says he bought a grinder explicitly to grind the wide prong down so it would fit both ways. Lol. – 19565 Mar 23 at 17:20
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    Outlet appears thoroughly horrified about Tim Allen's plan. – A. I. Breveleri Mar 23 at 17:30
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    TIL, "This is one reason that you now see cords that have hard plastic around the holes that is then overmolded with the more flexible material." – Doktor J Mar 23 at 23:44
  • @19565 - Wait does that work? A friend wants to know. – DMoore Mar 25 at 16:45
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    @DMoore should do - but it means live and neutral are swapped. So in a simple case like a desk lamp, the bulb's holder-socket is on the live wire with the switch on the Neutral side. Thus with the switch OFF and the plug flipped over, the exposed terminals under the lamp/bulb are live and a shock-risk when changing the lamp. – Criggie Mar 25 at 18:32
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In general, yes; in terms of grounding this is no different from plugging such a cord directly into an outlet.

Where you need to be careful is that your extension cord is rated for the power that your device is going to draw, but this has nothing to do with two-prong vs. three-prong plugs. (Also, grounded extension cords are typically rated for more current than devices with two-prong plugs typically draw, and even then, I think almost all cords are rated for at least 1A, which means you will likely only need to worry about devices that draw more than 1A / 100W. Edit: looks like you're unlikely to find less than 10A, so up to ~1000W should be safe, but if you're paranoid, stick to 500W unless you know the rating.)

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  • You can buy extension cables only good to 1A? That sounds horribly dangerous. Is this US again? Everything here has to be good to 13A, UK. – Tetsujin Mar 25 at 15:21
  • Yes, US. I actually don't know what a typical "cheap" extension cord can handle, just pretty sure it's at least 1A. It might well be 10A. (Actually, poking around on Amazon for cheap cords, looks like 13A may be the minimum here also. I know the heavier, 3-prong cords have different — from each other — ratings though.) – Matthew Mar 25 at 16:15
  • For example, here is an 18 AWG, 10A cord, and here is a 12 AWG, 15A cord. Both 10' long. – Matthew Mar 25 at 16:20
  • In the UK there is only a single standard for consumer mains. Everything must be rated 13A, even if fused for less, the underlying circuitry must be 13A. tbh I have no idea what these different AWG things mean, there's no real equivalent over here. Basically, things here are designed so you don't have to read the small print before plugging anyhting in. – Tetsujin Mar 25 at 16:21
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    Yeah, my 2kW router doubles as a room heater :-)). It's glowing in the dark! it needs to be 3 feet away from combustible surfaces! It heats my porridge in the other room with its microwaves! – Peter - Reinstate Monica Mar 26 at 13:36

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