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My local fire department, @Barrie_Fire, recently tweeted (then subsequently deleted) this:

Don’t even THINK about using a 2-prong plug in a 3-hole slot! Use only the required number of slots in an outlet or power strip.

Below was a picture of a burned-out grounded extension cord.

I'm hesitant to argue with anyone in the business of keeping our food, shelter, clothing and loved ones from combining with oxygen, but this seemed quite strange; I can't think of any possible way this could be a fire hazard.

The NEMA 5-15 wall receptacles in Canada are grounded by default, for reference.

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    \$\begingroup\$ Note to Barrie Fire Dept: don't believe everything you see on YouTube! (apparently their warning was based on misinformation in or misunderstanding of a YouTube video) \$\endgroup\$ – Peter Bennett Sep 8 '18 at 23:10
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    \$\begingroup\$ FYI, the tweet's been removed now. An archive or screenshot might still be available somewhere, if it matters now. \$\endgroup\$ – Xen2050 Sep 9 '18 at 2:39
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    \$\begingroup\$ @JonathonReinhart I'm hesitant to link to the tweet now; this seems to have taken off to some degree, and I don't wan't the fire department to get flack :) \$\endgroup\$ – 0xDBFB7 Sep 9 '18 at 21:25
  • \$\begingroup\$ In the US, neutral and ground are connected on the panel. Fire department usually do not consist of electricians. \$\endgroup\$ – Sebastian Sep 10 '18 at 11:55
  • \$\begingroup\$ The original tweet has been deleted, most likely because they acknowledged the inaccuracy: they sent a corrective tweet twitter.com/Barrie_Fire/status/1038898725391925248 \$\endgroup\$ – dim Sep 10 '18 at 12:00
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The Fire Dept is wrong - it is perfectly normal to plug a device with a 2-pin plug into a 3-hole socket.

Breaking the ground pin off a 3-pin plug, then plugging that into a 2-hole or 3-hole socket may produce an electrical hazard - possibility of a shock.

If a high-current load, like an electric heater, was plugged into that burned outlet, and the contacts made poor contact, that would cause the overheating and resulting fire, whether the heater had a 2 or 3 pin plug.

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    \$\begingroup\$ Think of all those two-prong phone chargers..... \$\endgroup\$ – Solar Mike Sep 8 '18 at 21:32
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    \$\begingroup\$ So you're saying this should be fine then? pbs.twimg.com/media/C6lEnAiW0AE_ufl.jpg :P \$\endgroup\$ – 0xDBFB7 Sep 8 '18 at 21:48
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    \$\begingroup\$ @0xDBFB7 by the standards of US home electrical systems, what's the problem? (The OP's question is non-existent in the UK, since you can't plug anything into a wall socket unless it has an earth pin which pushes a mechanical shutter out of the way to allow the other pins to connect. "Unearthed" appliances still have 3-pin plugs, but the unconnected "earth" pin is just a piece of molded insulating plastic, to operate the safety shutter.) \$\endgroup\$ – alephzero Sep 9 '18 at 0:49
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    \$\begingroup\$ @alephzero Yeah, type G and Schuko plugs are really nice. I kind of wonder if it's worth it though; electrocutions are really quite rare even with NEMA 5-15. "you can't plug anything into a wall socket unless it has an earth pin" Don't tell me what I can't do! Give me a big enough screwdriver and a place to rest it and I can open any shutter :P \$\endgroup\$ – 0xDBFB7 Sep 9 '18 at 1:39
  • \$\begingroup\$ @0xDBFB7 Type G plugs like to lay on their back... if you've ever stepped on one you'll come to appreciate that NEMA plugs aren't without their own niceties. \$\endgroup\$ – J... Sep 9 '18 at 14:42
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The statement by "Barrie" is nonsense, many devices don't have a ground connection and thus only have a 2-pin mains plug. Such devices are "double insulated" and have the 2-squares logo and possibly some text like:

enter image description here

The damage of that power strip was very likely caused by a short circuit and/or overload. There is no ground/earth connection required for that to happen. Likewise it is unlikely that a ground/earth connection would have prevented that damage from happening.

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    \$\begingroup\$ That sort of damage is characteristic of a bad connection that resulted in an slow runaway failure. Poor connection->heats up-> heat further degrades connection->heat increases, etc. A properly operating circuit breaker will usually catch a simple overload, but a bad connection can cause a fire even when the actual power being consumed is quite low. \$\endgroup\$ – ajb Sep 8 '18 at 21:38
  • \$\begingroup\$ What statement are you talking about? I'm not seeing any reference to "Barrie" in the question \$\endgroup\$ – Ferrybig Sep 10 '18 at 8:41
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    \$\begingroup\$ @Ferrybig First line: My local fire department, @ Barrie _Fire, recently tweeted.... \$\endgroup\$ – Bimpelrekkie Sep 10 '18 at 15:47
  • \$\begingroup\$ @Bimpelrekkie DId you consider that, when I posted my comment, the question never contained that name? You sound a bit rude by highlighting part of the question that wasn't there in the first place \$\endgroup\$ – Ferrybig Sep 10 '18 at 17:02
  • \$\begingroup\$ @Ferrybig And I think it is a bit rude suggesting that I should consider what version of the question you read (and how would I know) as there are many: electronics.stackexchange.com/posts/395046/revisions I just remember that there was a "Barrie" mentioned when I wrote my answer. Anyway, I don't understand what the big deal is about "Barrie" being mentioned or not. \$\endgroup\$ – Bimpelrekkie Sep 10 '18 at 18:36
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Connection a 2-prong device into a 3-prong socket is OK. Properly designed 2-prong devices are isolated and don't need protective earth.

What is dangerous is plugging a 3-prong device into a 2-prong socket, or using a 2-prong extension cable with a 3-prong device. That will cut the protective earth wire on an appliance which needs it, exposing the user to electric shock upon failure inside the appliance.

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