There are some regions of the world such as North America, where polarized receptacles are used, while other parts of world such as Europe do not use polarized receptacles. The specific purpose of using a polarized receptacle is to ensure neutral of the connected appliance is connected to the neutral of supply, and phase (hot) of the appliance is connected with supply phase. By convention, neutral is the wire that is grounded (earthed) at the source. Contrary to what many think, when phase and neutral are reversed, the connected appliance will more than likely continue to work as designed. However, reversing the polarity can have some safety implications when single-pole breakers or fuses are used to disconnect the circuit. Reverse polarity can also be troublesome in control and automation applications.
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Many modern devices using switched mode power supply such as laptops, cell phone chargers and many other electronic devices do not care about polarity. Output of switched mode power supplies such as cell phone chargers is DC, which is electrically isolated from the AC side. Many chargers are made of plastic bodies and there is no need for ground (earth) wire thus making a typical charger a two-pin non-polarized device. These devices are also double insulated.
There are however some devices especially household appliances and control and automation circuits where applying correct polarity will lead to a more electrically safe device.
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In North America, NEMA has standards covering plugs, receptacles (sockets), and wall plates used in residential, commercial, and industrial buildings. These are covered in ANSI/NEMA WD 6.
With a polarized receptacle, a polarized appliance plug can only be connected in one way meaning the receptacle is pin-polarized. This ensures that the phase and neutral lead of the receptacle are connected to the designated phase and neutral lead of appliance. Polarized plugs will not fit in to non-polarized receptacle whereas all polarized and non-polarized plugs will fit in a polarized receptacle.
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A non-polarized three pin plug when inserted into a polarized receptacle will always have left pin at neutral potential, right pin at phase (hot) potential and third pin at ground when looking in to the pin (see figure 1).
While the polarized plugs ensure that it can be inserted one way, wiring mistakes inside the receptacle (socket) can lead to accidental swapping (reversal) of hot (phase) and neutral leads. To alleviate this concern, receptacles have markings (electrical-polarized) indicating neutral, phase and ground terminals. Commonly phase (hot) has brass colored screw, neutral is silver color and ground is green colored.
Risks associated with reversal of phase and neutral
When phase and neutral are swapped, designated neutral point of the appliance will be energized at phase voltage. An example will be the threaded section of an incandescent lamp. Typically, outer metal portion of the lamp will be at neutral potential. If phase and neutral are swapped, then a person trying to screw or unscrew a lamp while energized can experience shock hazard if their fingers make accidental contact with the outer portion of lamp.
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Some appliances will have built in overload or short circuit protection on the designated phase lead. Once overload protection trips, appliance should not have any voltage inside. If phase and neutral leads are swapped upstream, only the neutral circuit will be opened by overload while the phase wire remains energized. This can be a shock hazard if an unsuspecting person opens the device and starts troubleshooting. Built in overload protection exists for many common household appliances such as kitchen garbage disposers, space heaters etc.
Another application where reversal of polarity can cause safety hazards is control and automation circuits.
Under normal circumstances, the above ‘defective’ circuit will work fine. When switch is opened, power relay is deenergized and motor is OFF. Now consider an accidental short circuit to ground on the neutral wire at location shown in figure 6. Short circuit will complete the circuit and pull power relay causing motor to start. This can be dangerous depending on the specific circumstances.
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How to check for polarity in a receptacle?
Socket testers or receptacle testers are available that can check for polarity, ground (earth) integrity and optionally test GFCI (or RCD) in a receptacle. Examples of socket testers are Fluke ST120+, Klein Tools ET310 etc.
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Socket testers can check for conditions such as: Open ground, Open neutral, Open hot, Hot/neutral reversed, Hot/ground reversed.
Socket testers however will not detect if ground/neutral is reversed. If circuit has open ground in addition to hot/neutral reversal, tester will detect as open ground only.
Another quick but not perfect method to check for polarity will be to use a standard multimeter and measure voltage between phase, neutral and ground in a polarized receptacle. In a 120VAC receptacle, multimeter should read 120Vac between phase (hot) and neutral, 0Vac between neutral and ground and 120Vac between phase and ground.
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Summary
Polarized receptacles ensure that when a polarized plug is inserted, phase and neutral of the source is connected to designated phase and neutral of appliance. Reverse polarity occurs when phase and neutral leads are accidentally swapped in receptacle or in the appliance cord. Polarized plugs will not fit in to non-polarized receptacle whereas all polarized and non-polarized plugs will fit in polarized receptacle. Many electronic devices such as laptops and cell phone chargers do not care about polarity and hence those devices will typically have non-polarized plugs.
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When phase and neutral are reversed and neutral is switched, designated neutral of device such as incandescent lamp outer screw can remain energized at phase potential even if the lamp is turned off. This may pose a safety risk. Also, appliances with built-in overload protection will have phase potential inside even after overload trips. Control and automation circuit can malfunction when phases are swapped and a wiring fault occurs.
Polarity can be checked in household receptacles using a polarity socket tester or a multimeter.