17

The house I recently acquired had some wiring done in the basement by the previous owner. I've identified 3 outlets (all on the same circuit) where the live and neutral wires are reversed. I've switched off the breaker to that circuit so people don't use those until I've remedied the wiring. The few things that were plugged into those sockets worked fine. Obviously lamps and fans will have no issues, but I'm concerned. What sorts of appliances will malfunction or worse, potentially harm someone, if plugged into an outlet that is wired backwards?

Additional Info:

I apologize for not clarifying my locale and its specifics. Apparently, it does matter.

North American outlets are as shown here:enter image description here

Many appliances have two equal sized vertical prongs and no ground prong so they can be reversed. Many appliances with a ground prong obviously go in one way only.

But there are also many appliances with two vertical prongs and no ground prong, but one vertical prong is slightly wider than the other and that is supposed to go into the taller slot (on the left in this diagram). Wiring codes, require that side of the plug to be wired to neutral, while the shorter side is to be wired to hot. It seems sensible to me to assume that one prong is wider for a purpose. It is those appliances in particular that I am concerned about. If one prong is intended to be hot, surely it would be a big deal if it is not?

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    I think this is sufficient grounds to have a professional electrician check your house. There may be more errors. If you found these errors with one of the cheap triple neon lamp test modules, note that some of them are quite unreliable. They perform the advertized tests adequately, but due to an error in the circuit logic, they can show all three lights (= correct wiring) when active is wired to neutral and neutral and earth are wired to active!!! – replete Jan 24 '17 at 7:59
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    You are focusing on the wrong thing. Pretty much everything will work OK, but it creates a hazardous condition. That is why it is a no-no. – mkeith Jan 24 '17 at 8:03
  • @mkeith is that meant for me or the OP? – replete Jan 24 '17 at 8:05
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    My house has a lamp that is switched by touching (grounding) a metal part, if we reverse the plug the lamp doesn't work any more. Though that is specifically of how it's switched using something fancy. – ratchet freak Jan 24 '17 at 11:32
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    See addition to my answer re effect with single pole switch and reversed P&N. – Russell McMahon Jan 24 '17 at 13:28
18

What sorts of appliances will malfunction or worse, potentially harm someone, if plugged into an outlet that is wired backwards?

  • Badly designed (and thus uncertified) ones.

  • Wrongly wired ones

In the real world, people do die from this, alas.

______________________

Modern appliances are EITHER double insulated or have a safety ground.

Double insulated is more a level of insulation than two physical layers of insulation. DI means that the appliance is never electrically unsafe under regulatory acceptable conditions of use. If you eg stand in a puddle while holding it and pour water into it, or use it while in a bath or shower, you may exceed the regulatory conditions of use envelope.

A safety ground is an arrangement that connects accessible or modestly insulated metal parts of the appliance to system (in this case "usually" house) ground. If an electrical path develops between supply "Phase" and the earthed metal parts, ground current will flow and curcuit protection will operate (fuse or circuit breaker). Systems with resdiual current tripping (named variously ELCB/GFI/RCD) devices can detect very small currents to ground (<= 20 mA) and discnnect the circuit in <= 1/2 a mains cycle.

The mains feed consists of two wires - usually termed Phase and Neutral .
Neutral is usually held at ABOUT ground potential but is NOT connected to the ground lead within the appliance. Phase and Neutral can usually be reversed at the appliance feed point without causing immediate hazard. In a DI appliance there is minimal effect. In a grounded appliance there SHOULD be minimal effect.

If neutral and ground are connected within the appliance it will still usually operate but large metal parts connected to ground will now be at neutral potential. This may affect the ability for house fuses or breakers to operate correctly and WILL trip residual current devices.

Any appliance which has the earthed metal parts connected to Neutral (which they should not have) will have the metal parts placed at phase potential if feed PN are reversed. If the metal parts are a toaster or electric jug or vacuum cleaner or ... body then this may be a very very very bad idea indeed.

If an appliance cord is wired incorrectly, strange things may happen, or it nothing obvious may happen but trhe appliance may become letally dangerous. Long long ago I was asked to investigate a situation where there was a house with an external sleepout. Most appliances worked OK in the house or the sleepiy but a casette recorder would only work in the hpuse but not in the sleepiut. Investigation revealed that

  • the sleepout had P & N reversed, and

  • the tape recorder has N & E reversed.
    Usually this would be a relatively benign fault.

In this case most appliances saw N-P rather than P-N, E was OK so they worked OK.
However the tape recorder saw N-E on what should be P-N so did not work.
s has Phase on it's earth pin and the whole body was live at 230 VAC. What saved people was probably that the sleepout had no major grounded metal and, as the TR did not work when plugged in, people spent little time touching it.
So close !


Switched conductors:

Swapping phase and neutral may result in the appliance "internals' being live when it is turned off.

I'm in NZ. The mains supply is 230 VAC, 50 Hz. I note informally only that it seems that with 230 VAC people seem to care somewhat more about shock hazards that with 110 VAC. I've seen practices in other countries which would be utterly unallowed on NZ.

An important aspect of swapping phase and neutral is when the appliance has a single pole switch. This can only 'break' ne of the two mains leads and it is invariably designed to disconnect the phase lead. If P & N are swapped the Neutral lead is disconnected and the appliance will not operate BUT the Phase lead is connected to the equipment internal circuitry. The chances of an accident are significantly increased.

Some equipment is required by law to disconnect bot leads when switched off. An excellent example is an electric toaster where, if P & N were reversed, a single pole switch would cause the element assembly to be at phase potential. Some years ago a batch of Chinese manufactured toasters that allowed a single leadswitching fault to occur were imported to NZ. The fitted two pole switch could have one of its two "fingers" trapped against an insulating mount of not properly assembled. As the toaster still switched off thermally if either lead was broken this fault was missed by the manufacturer and importer. About 5% to 10% of the toasters has phase on the element when turned off. This was brought to my attention when an adult nephew did what I would not have done in my wildest dreams (hopefully). He picked up an "off" but plugged in toaster to move it by inserting fingers down the two slots to grip the middle bar !!!!!!!!!!!!!!!!!!!!!!!!!! . Shocking!. The importer was somewhat cagey. I lashed up a capacitance based tester that could tell if either witch pole was stuck closed. In such a case the element to body capacitance was far higher than when both poles were opened - plug toaster 3 pin plug into the capacitance tester and voila! A trip through a few showrooms revealed the extent of the problem and an appliance recall resulted.


Caravans, campers ... supplied power via mains lead:

In many countries Caravans require a neutral earth link in their in-van switchboard. This is safe if fuses or breakers are installed in the caravan and will work with fuses or breakers in the feed switchboard. RCDs in the feed switchboard will trip if this link is in place. Removing the link stops remote RCDs tripping but then the caravan MUST have RCDs as well.

SO - If P&N are reversed on a caravan with a van N-E link then the van will now have a P-E link and will blow supply fuses. BUT if the supply ground is faulty the van chassi and metal parts will be at Phase relative to ground. Properly wired appliances will work OK.
Stepping in and out of the van WITHOUT providing a ground to caravan metal bridge with your body will not cause a problem.
But if you bridge van metal to ground with your body you will receive a full mains shock. You may die and people have died in just this situation.

Removing the in-van N-G link will prevent this shock hazard but then the fusing may not work correctly and eg fire may result and [all together] you may die. Or your family.


Added August 2019 - ht Ray Butterworth

"Auto-Transformers":

Ray Butterworth notes another hazard:

Auto-transformers have a single 230 VAC winding with a tap at 115 VAC.
Apply 230 VAC relative to ground across the whole winding and you get 115 VAC relative to ground at the tap.

BUT - Reverse phase and neutral inputs so that you get

  • 230 VAC live at the "ground" input and

  • ground or neutral at the "live" input.

This results in 110 VAC on top of a "120 VAC pedestal".
ie

  • What should be output ground is now at 230 VAC and

  • What should be 115 VAC relative to ground is still about 115 VAC relative to ground (so measuring the voltage from this point to ground does not reveal the problem!).

Death can easily happen.

Ray says:

  • "One additional appliance that can be directly dangerous is a step-down transformer (e.g. plug into 230V line and produce 115V output). These often use an auto-transformer to split the voltage. If plugged in correctly, the output will be neutral and 115V. If plugged in backward, the output will be 115V and 230V. Most things plugged into it will still work correctly with the 115V difference, but there is potential for a major shock or fire there. And some high-tech devices rely on neutral being at approximately ground level; they will break."
  • So are you saying it is important for me to fix these, even though all appliances should operate normally? – Octopus Jan 24 '17 at 7:54
  • @Octopus See my now completed answer. Risk is small. Risk is non zero. People have died. Remediation is (probably easy). – Russell McMahon Jan 24 '17 at 8:01
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    I'm intrigued by the N-E link thing within caravans - in my country (UK), this would definitely not work (it would trip any supply which you connected the caravan to anyway, as you say). Where is this done, and what's the benefit? – user3186 Jan 24 '17 at 8:42
  • @WillDean I've got (built) a UK camper van, and I agree with you. On all UK sites I've stayeds at recently there's been an RCD for the 1--4 units fed off box of hookups. My socket tester lives in the van and I check for L-N reversal when I hook up. I haven't had any issues in the UK for years, but in France it's not uncommon to have to swap the polarity if the hookup cable for the duration of the stay. – Chris H Jan 24 '17 at 9:03
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    @WillDean partly habit -- I'm used to polarised mains. But I had some logic glitches on my fancy microwave that went away when I sorted the polarity (could equally have been coincidence with brownouts/bad earth contact). But I've worked with enough kit where small-signal noise is an issue and filtering/screening isn't always symmetric. For example a double-insulated SMPS may have a copper foil screen connected to N, and L is more subject to switching interference. – Chris H Jan 24 '17 at 9:29
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Since even the neutral line cannot be guaranteed to actually be neutral (at ground potential) all mains connected must be equally well isolated from live as from mains. Devices must therefore also be able to withstand live mains voltage on the neutral connection.

So in my opinion, no devices should be affected.

In my country (continental Europe) the mains plug is symmetric so there is a 50 % chance of getting live-mains or mains-live. I mean, here all devices must be able to cope with both situations. As far as I know, it's only the UK (and perhaps Australia ?) which have asymmetric plugs where live and neutral have designated pins.

  • I can confirm that NZ/Australia plugs are asymmetric (even the non-grounded ones) – user253751 Jan 24 '17 at 8:44
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    Thanks, they look like ( / \ ) while here the EU ones like ( o o ) – FakeMoustache Jan 24 '17 at 8:51
  • With the wiring reversed wouldn't an appliance still be supplied with power even when the fuse has blown (since this is usually on the live connection) thus creating a potentially dangerous situation? – Robin Elvin Jan 24 '17 at 9:33
  • @RobinElvin If the fuse is in the house's fusebox: no, as there it must be on the live side. If it is in the plug (only UK mains plugs have this as far as I know) then also no because the device must be isolated anyway. Isolated for both live and neutral connections. You're assuming that touching the neutral would always be safe, so that you could safely connect neutral to the metal case of an appliance. Well, you cannot as there is no guarantee that the neutral is actually at ground level. So a metal case must be grounded. Ground must be 0 V always. – FakeMoustache Jan 24 '17 at 10:04
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    You are right that if the fuse in the plug blows and the fuse was in the neutral line then the appliance does become (internally) connected to live mains. But that is always is assumed to be the case. All mains connections must be properly isolated anyway. An isolating case (plastic) around an appliance provides this. – FakeMoustache Jan 24 '17 at 10:08
13

In the case of a lamp with an "Edison base" bulb (the common screw-in type A bulb), the socket shell of the lamp is intended to be connected to Neutral, because while the bulb is being inserted, or if it is not fully inserted, fingers could contact the shell of the bulb. The shell cannot be grounded, since it is part of the circuit, so there is no point in providing a ground pin, yet it would be an unavoidable hazard to have the shell be live.

This is the best reason for having the unequal sized prongs. It is a case that has nothing to do with how the device functions, and everything to do with safety and human factors. Simple, obvious, unavoidable.

  • Very nice explanation, which I up-voted, and likely applies in some countries. But notice that in Europe (except UK), we very commonly have lamps with screw-in type A bulbs and symmetrical two-pronged plugs. I wonder if there is even a rule about if the switch on these lamps is to be on the shell or center contact. – fgrieu Jan 24 '17 at 18:19
5

As others have said already, most devices will be fine.

The only ones that may suffer are communication devices such as LAN-over-main or some old Intercom systems that use the mains to transmit the data.

These devices are mostly gone now that WiFi is widely spread (although you can still buy them) but I remember installing an intercom-over-mains system several years ago at my grandma's house and it didn't work because the live and neutral were switched in one of the sockets.

2

One hazard of reversed poles is that turning off a switch to a fixture does not cut the power to the fixture, it only breaks the ground. I know. I replaced a ceiling fan yesterday and the neutral was hot. I should say the neutral was not neutral. So I can affirm that the one appliance that malfunctions is the technician working on wires with only the switch turned off, not the breaker.

This one is still breathing, however.

2

All equipment which relies on live detection should not work if live and neutral are switched. Simply rotating the plug 180 degrees would remedy that of-course, but that's only possible in certain regions (grounded USA-type and British plugs can't be rotated for example).

The only equipment like that which comes to mind are some types of car chargers, but there are probably more. Basically all equipment that needs to know which wire is the live one and which is the neutral (it's sometimes required for communication purposes to know which plane on your PCB is connected to neutral) one has two choices:

  • measure and decide
  • assume and verify

A 'measure and decide' device will attempt to measure which wire is live and switch it's internal electronics to handle it.

An 'assume and verify' device will assume the live is where it's supposed to be and verifies this. If the verification fails, the device should fail to function.

Most electronics can't be bothered which is which, so the chance you actually got something at home that's affected by your 'reversed' wiring is slim.

The chances of any device causing anything harmful should be zero, assuming your devices are in decent shape and properly designed.

Of-course, all bets are off if you plug-in something built by a person who had no idea what he was doing.

  • "Rotating the plug" -- you must have a very specific plug in mind, as not even all US plugs are unpolarised – Chris H Jan 24 '17 at 9:04
  • @ChrisH Good catch, I'm used to the European system where we got our own set of troubles. – Mast Jan 24 '17 at 10:45
  • Apologies for assuming American when I saw reversible plugs, especially as I've come across them in Europe too. – Chris H Jan 24 '17 at 12:32

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