Why is the AC phase hotter than neutral when unplugged?

[girts]

I was vacuuming my house, the vacuum cleaner is 1600w rated I remember, then i unplugged the cord from wall socket and I observed something interesting , and to be honest I remember I have seen this before with my washing machine and elsewhere, when I take out the plug from the wall socket I notice that one lead is noticeably warmer than the other, I tried this multiple times and everytime the result is the same, now I haven't yet measured but I assume the hotter one is the phase and the colder one is neutral.
Anyway maybe I have forgotten something but at least in DC current is the same in a given loop at every point of the loop and the current through the loop is given by voltage and load resistance (assuming power supply has enough power to hold certain voltage under certain load resistance)

Ok in an AC circuit this rule is the same right ? current is the same both in phase and in neutral? at least it should be right?
although I realize that since we are talking about an inductive load (motor) factors like impedance and reactive power come into play, but can anyone please explain why there is this difference in heat between the wires?

 

[berkeman]

I was vacuuming my house, the vacuum cleaner is 1600w rated I remember, then i unplugged the cord from wall socket and I observed something interesting , and to be honest I remember I have seen this before with my washing machine and elsewhere, when I take out the plug from the wall socket I notice that one lead is noticeably warmer than the other, I tried this multiple times and everytime the result is the same, now I haven't yet measured but I assume the hotter one is the phase and the colder one is neutral.
Anyway maybe I have forgotten something but at least in DC current is the same in a given loop at every point of the loop and the current through the loop is given by voltage and load resistance (assuming power supply has enough power to hold certain voltage under certain load resistance)

Ok in an AC circuit this rule is the same right ? current is the same both in phase and in neutral? at least it should be right?
although I realize that since we are talking about an inductive load (motor) factors like impedance and reactive power come into play, but can anyone please explain why there is this difference in heat between the wires?

It sounds like there is a loose connection in the wall socket on one of the connections, and that is heating up and transferring the heat to one prong of the power cord. You are correct that the current is continuous and the same in both the Hot and Neutral wires and connections.

Maybe start by using a simple circuit tester on all of the sockets in your home, and then consider having an electrician do an inspection.

https://images-na.ssl-images-amazon.com/images/I/41JHnPCTf6L._SL500_AC_SS350_.jpg

 

[jocarren]

You are correct that the current is continuous and the same in both the Hot and Neutral wires and connections.

Careful there, current in a home outlet is alternating not continous. You are right though, it's probably a loose connection.

 

[berkeman]

Careful there, current in a home outlet is alternating not continous.

LOL. I didn't say it was DC; the term "continuous" can be applied to a constant amplitude AC signal as well.

 

[sophiecentaur]

This hot pin is common in cheap or old 13A plugs. When the internal fuse (in the Live side, of course) makes a bad contact with the holder it can generate quite a few Watts of heat. But any terminal that's not screwed in tightly can also provide enough resistance. It isn't and never was a problem in good quality plugs.

 

[Tom.G]

The hot(ter) spot occurs at a point of high(er) resistance. This could be the connection from the cord to a prong in the plug, between a prong and the contact in the socket (wall outlet), or, less likely, the wiring between the socket and the building wiring.

Overall, it's fairly common, with an older socket being more likely to develop the problem. With age and use, the springy contacts in the socket loose their grip on the plug prongs, causing a high(er) contact resistance and raising the temperature. The higher temperature oxidizes the plug and the socket contacts, and anneals the socket contacts (makes them less springy), which then raises the temperature even more. Fortunately, this tends to be somewhat self limiting. For motorized devices, before the resistance gets high enough to cause dangerous temperatures, the device will usually stop working. Then the operator looks into the situation.

A mild to moderate temperature rise is normal up to the rating of the plug or socket (usually 15 or 20 Amps in the USA). If you can't hold your finger on an overheated contact, (that's above 135°F, 57°C) it's time to take action. Frequently, just polishing oxidized plug prongs will correct the problem, or at least improve it for a while!

Cheers,
Tom

 

[Baluncore]

when I take out the plug from the wall socket

Can we assume it only happens on the one wall socket ?
If only one outlet, then it is a dirty/burnt/oxidised connector or a bad cable termination to the connector terminal in the wall.

 

[sophiecentaur]

@girts From your use of the term "lead", and not 'cord', would I be right to conclude you may be UK based? The rest of the world mostly do no use fused plugs / ring main circuits so the replies here may reflect this.
I am not picking on you when I make this comment : Country of residence is one of the few facts that are really relevant in members' profiles when it comes to their contributions to EE discussions. I think PF should encourage members to include it. It is hardly a high risk piece of info for individuals to publicise.

 

[jim hardy]

Is this one of those 'fused plugs' as @sophiecentaur mentioned ?

My first thought was in US the neutral blade is wider than the live blade ... but it's so much bigger than the wire in the cord i dismissed that idea.
So...
Think how a fuse works
Its element is heated by current .
So long as heat flows out at about the same rate it's produced, the element won't heat up very much. So its resistance doesn't go up by much.
It's a balance.
So below the fuses's rated current it just conducts heat into the environment , and it does that largely along the wires affixed to its ends because copper is a better conductor of heat than plastic insulation.
As current approaches the fuse rating the amount of heat conducted away warms those wires enough for you to feel.

When current reaches or exceeds the fuse's rating, heat conduction away can no longer keep up with heat production inside so the element starts really heating up .
That raises its resistance, which further raises the rate of heat production(I²R) and thermal runaway commences. Then the element melts.

So what you observe is i believe to be expected in high current appliance cords that have a fused plug and operate near that fuse's rating.

I've never been across the pond. What do you guys over there think ?

Good for you @girts .
Using one's everyday experience to figure out basics of physics really helps his thinking process.
And it makes Francis Bacon smile.

old jim

 

[sophiecentaur]

There are different styles of fuse holder; some consist of a semi-cylindrical 'leaf' spring which grips around most of the fuse end, spreading the contact over a decent area. Others have a sort of fork shape with two notches which grip the fuse end cap. These rely on a good deep contact with a small area. Afaik, plug fuses are never held in with a screw. A bit of corrosion in the plug (condensation against a damp cold wall etc) can cause a bit of contact resistance and this builds up if the temperature rises and causes thermal runaway. The Neutral wire is in good thermal contact with the chunky rectangular plug pin (which is also in contact with the metal in the socket socket) but the live wire is only in contact with a small piece of metal, holding one end of the fuse. That end is the place where the temperature can build up. A good quality plug never seems to have a problem but there are imported versions that only work for ever if operating with less than the 13A (3kW) max rating.
The ring main system is very good in many respects (and cheap) and there are seldom any problems. The plugs are bulky, though.

 

[girts]

ok, so thanks for the replies first of all, secondly I guess I should have said a bit more in my original post to spare you the speculation.

Now I do not live in the UK, I do live in eastern Europe, Latvia to be exact, we mostly use standard European style or maybe I can say international style (if there is one) sockets and plugs,
https://www.worldstandards.eu/electricity/plugs-and-sockets/

look at this webpage , the sockets we use here are the types C, E,F, they are mostly interchangeable , if one has the wall socket with wider diameter holes one can stick any type of the mentioned inside such a socket, and both grounded plugs or plugs without ground go inside.
back in the day we here had very simple two wire sockets and plugs, there was no ground connection whatsoever, I still don't have ground connections in my sockets as I haven't changed them, I only added such feature to those sockets that deal with things like water heater, or washing machine. I have dry wooden floors and mostly all new electrical stuff is double insulated so thee is no risk of running it ungrounded if one knows what he is doing.

Now the most likely explanation is Baluncore's and Tom.G oxidization as that is what I think myself could be the case, or simply sometimes the pressure against the pinks from the socket is maybe not enough and that too can cause a difference in resistance, given the device used has enough power the current going through that surface area is high enough and so a 1 ohm difference makes some heat.Well I've tried multiple devices in various sockets and almost in all of them one pin gets hotter than the other, I assume the obvious explanation would be that almost no wall sockets are made with such precision that they would have equal strength springs and quality brass,
I even tried my water heater kettle in my room where I have a selfmade high quality 6 socket extension which uses a 2.5mm square copper cable soldered to the brass rails that are inside the socket, still even though the pins were noticeably colder in these sockets , atleast in one of the six that I've tried one pin got hotter than the other.

So I assume the message to take away from this is that simple AC house electrical connections do matter, because even if a copper soldered brass connector that is almost new and fresh gives some heat in the pin then old and bit loose or worn out plugs and sockets and connections can get really hot if one uses higher power appliances, this might be especially true for US or other countries that use lower AC voltage as the current is nearly double there.
There have been many cases over the years where a big house fire in many cases with fatalities was said to been caused by a electrical heater, now would it be fair to assume that unless the heater was put directly under bed or other flammable material that the cause for the fire could have been such a bad connections getting hot and causing the wire insulation to combust or other nearby materials, because in many cases you here people saying "oh it was a short circuit" but I think that is misleading because with a short circuit there are either one of two cases, either the house has good protection and fuses and so the short results in an immediate electrical outage or the fuses have nails put inside them but then the short would cause a rather quick overheat of the wire insulation which would lead to a fire rather quickly , but such a fire that starts quickly would be noticed by the people in the house usually.
In fact in many cases you read that this process was going on for much longer, say a heater was switched on and people went to sleep and then sometime in the night the wire started so we could assume the process was gradual, sounds to me like a gradual heating of a bad connection.

Well that's just me ranting a bit about what I thought while doing this.

 

[CWatters]

Some makes of sockets are better quality than others. Some wear out faster. How old are those in your house?

 

[Baluncore]

When a plug is inserted or removed, oxide is wiped from the contacting surfaces. If the plug is left inserted and not removed, then moisture gets into the contact gap and corrosion begins to increase contact resistance. This is more of a problem with higher power appliances such as electric kettles, hot plates or heaters. I expect power plugs and sockets to gradually wear out with use, as contact material is lost.

Where a switch is provided on the outlet there is often no incentive to remove the plug. The switch contacts then work well because they get exercised. I do not like switches on outlets because plug contacts are more likely to corrode. “Have you switched it off and on again?” has interesting implications.
Develop the practice of switching off an outlet, then removing the plug each day when you finish using an appliance. When next needed, plug it back in, then switch it on. That keeps all contact surfaces clean and low resistance.

I used to fix some faulty equipment in chemistry laboratories by pulling the power plug, blowing on the contacts and plugging it back in again. That was effective at cleaning the highly predictable contact corrosion film caused by the chemical environment.
It could also be quite entertaining if I knew I was being watched, for then I spoke into the plug like a microphone “testing testing, 1, 2, 3” then holding the plug to my ear and listening intently. I then chastised the equipment, (not the plug), repeated the audio test via the plug again before pronouncing it "well disciplined and now OK". I then plugged it back in and the equipment would predictably come to life again.

 

[jim hardy]

Great and not-boring anecdote, Mr @Baluncore !

 

[sophiecentaur]

Great and not-boring anecdote, Mr @Baluncore !

I could imagine that those Chemists would repeat the fault just to have been given a repeat performance.

 

[dlgoff]

I could imagine that those Chemists would repeat the fault just to have been given a repeat performance.

bold by me
Would make a good Youtube video. Hint, hint.

 

[Baluncore]

I could imagine that those Chemists would repeat the fault just to have been given a repeat performance.

The very elegance of the situation was that the fault could never be demonstrated or repeated. Any activity prevented demonstration.
It was noticing the subtle flicker of an indicator lamp when the power plug was first pulled that generated all the confidence needed for a successful repeat performance.

Would make a good Youtube video. Hint, hint.

I agree, but unfortunately, nothing could beat Basil Fawlty chastising his car.

 

[jim hardy]

It was noticing the subtle flicker of an indicator lamp when the power plug was first pulled that generated all the confidence needed for a successful repeat performance.

i can tell you're a good troubleshooter. Little things mean a lot...