Why Electrical Circuits are Grounded

July 29, 2010 | by Fred (email) |

outletLast week we discussed why outlets and plugs are polarized (it’s a safety feature… if you missed that article, it’s worth a quick read). This week I want to take a minute to describe why modern day electric circuits and appliances are grounded.

Most of the electric circuits in your house have three wires going to them, a black wire (normally hot, or energized), a white wire (normally neutral), and a bare copper wire (always ground).  The larger left slot in an outlet is connected to the neutral wire; the smaller right slot is connected to the hot wire, and the circular slot beneath them is ground.

In the electrical panel for your home, all of the neutral and ground wires are connected together on a single bus.  That’s right, neutral and ground are equivalently grounded. So why is it necessary to have two wires going to each appliance?

The reason is safety.  The bare copper ground wire is connected the metal box of an appliance.  The ground wire ensures there is a path for electricity to get back to the breaker box if there’s a short circuit in the appliance that bridges the hot wire onto electric-conducting parts of the containing box.

Why Grounding is Necessary in Electrical Circuits

If you didn’t have this safety feature, bad things could ensue.  Consider this situation: Let’s say the hot wire comes loose in your washing machine and makes contact with the metal box that houses the appliance.  The box is sitting on rubber feet so there’s no exit path for the current through the ground (rubber is an electrical insulator).

Since the electricity has nowhere to go, the box remains charged.  You unsuspectingly touch the washing machine and the current suddenly finds an exit point–through your body and out your bare feet to the floor.  You get a terrible shock, or worse, you get badly burned or even killed.

Now, consider the same scenario except that the box is grounded. When the initial short circuit occurs, a rapid amount of current flows through the ground back to the electrical panel. Since there’s almost no resistance across the ground, the circuit overloads and the breaker trips almost instantly, sparing you a hair raising experience.

It Doesn’t Take Much to Kill You

While we write about this elsewhere, it’s worth noting here that you will likely be dead before a breaker trips because too much current has flowed through your body. It takes well under 1 amp of current to cause cardiac arrest… and most household circuits are governed at a whopping 15 amps!

This is why GFCI protection is so important on circuits that are more likely to be shorted by humans (such as those in kitchens and baths). This type of protection detects when current passes to ground rather than back through the neutral wire and shuts off the circuit.

The combination of grounding (for all appliances), and GFCI circuits (where humans are likely to bridge a circuit to ground by accident) create a pretty sound safety net. More recently, arc fault protection has also been introduced into the code to protect against current arcing across wires such as in situations where a staple was driven too tightly across the insulation and damages the wires inside.

2 Responses
  1. David S says:

    One thing to note is that in subpanel boxes (like out in a shed or another outbuilding) do not connect neutral and ground together at the panel but infact are seperated busses with ground going to the grounding rod at the building and neutral traveling back to the main panel..

  2. Fred says:

    David S, Excellent addition! I had forgotten to include that.

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