Why do some US electrical plugs have a “ground” and many do not?


I don’t know much about how plugs or electricity works, obviously, but I was taught that one side is the “positive”, one side is the “negative”, and the bottom (seemingly quite optional) is the “ground”. It’s odd to me that so few plugs use the “ground”, so it made me curious why it exists, and why it’s optional. Are there any safety benefits to having a “ground”, or safety concerns with not having one? Thank you!

In: Engineering

The alternating current coming out of a power socket is like a dog zooming back and forth between the positive and negative plugs represented by opposite sides of a backyard. Normally, this is okay if there are no other things in sight of the doggy.

But the doggy has a huge prey drive that represents how much electricity loves to flow from high potential to ground. If a squirrel comes within eye contact of the dog, the fence and other people could try and stop it, but it will run through them as eager as possible. With just two sockets, electricity can easily jump through objects (like people) if there is a path to ground.

Adding a ground prevents this by providing a direct ground to go through if the electricity leaks in any way. Often, electronics enclosures are grounded. This is like putting a tube for the running path of the dog so it never gets distracted enough to run out.

A ground has other advantages such as providing a fixed reference for signals in case certain circuits are noisy.

In the two-prong plugs, one is hot and the other is ground. In the three-prong plugs, one is hot and two are ground.

In the event of a failure on the wire, before the outlet, the extra ground can save lives and prevent fires. This usually means a break in the wire.

If the hot wire fails, generally there’s no problem, as electricity won’t flow to or through the outlet, stopping where the hot wire has failed. Any thing connected to that outlet turns off.

If the ground wire fails in a two-prong outlet, the hot may still deliver electricity, which will find another way to ground, on the plug side of the outlet. This can include going through whatever is plugged-in (say a lamp) and through something else it is touching, like a table or person. Yes, there needs to be conductivity, so not all situations result in trouble this way.

If a ground wire fails in a three-prong outlet, the electricity should continue to flow through the other ground.

These tend to be most dangerous if electricity is flowing at the moment of a failure, after which a fuse or circuit-breaker should provide protection. If the failures occur before electricity flows, the additional conductivity of the object being powered matters.

Consider a movie trap scenario. The bad guy compromises the wiring, and leaves a radio on a shelf. The victim switches on the radio, and because of the construction of the radio, perhaps contact and construction with the shelf, or a puddle of water the person may be in (it is a movie trap). For at least some moments, the radio will try to flow electricity, but since the ground wire is cut, it has to go through the trap, either the shelf or person.

The scenario is only a bare stretch, and does happen without malicious configuration.

Positive and negative doesn’t really apply here, because they switch back and forth 60 times per second. You have hot and neutral. Hot is connected to one of the phases, neutral is the same potential as ground, but only connected to ground in the main breaker box. stuff that uses a lot of power will often have two hot wires with 240v between them, and the neutral wire is halfway in between. If you have an appliance with a metal case, you connect the ground wire to the case, so that in the event of a failure somewhere, the outside of the device shorts that back to ground and trips the breaker, instead of sitting there at mains voltage waiting for someone to touch it.

Two prong plugs are used for double insulated appliances, usually items with plastic shells that won’t conduct electricity if there’s a failure, but it can also apply to metal items so long as a single fault won’t cause an electrocution hazard.

Two prong outlets were installed in ungrounded systems-no actual connection to ground on the property at all.

Three prong outlets were installed in grounded systems-the neutral is physically connected to the earth at one single point at the main disconnect.

The grounded system is safer because it bonds all metal in the system together and it also bonds w metal pipes in homes and with metal cases on equipment that plugs in.

If a voltage is accidentally placed on a metal pipe (for example) in an ungrounded system, the voltage stays there until it is removed or it may even push a current through your body if you touch that metal pipe.

If a voltage is accidentally placed on a metal pipe in a grounded system, a current will immediately flow through that metal pipe and everything bonded to it back to the main service. Since the impedance is purposely very low through all these bonded metals, the current will quickly grow large. The circuit breaker for this branch circuit will then trip very quickly because this current demand is large. In a properly bonded system, this would happen very quickly and the risk of shock and fire goes down tremendously.

Things which are ungrounded are double insulated. They have near zero possibility of having a failure which causes any exposed metal parts to have dangerous voltage on them. A large appliance normally has a metal case and it will be grounded for protection.