Do GFCIs and RCDs need a Ground Wire to Work?

Before we begin, we should really do a quick review of the definition of “ground” or “earth” in electrical terms and what it really means to us. If you think back to your days in school, you may have heard the term “earth or mass” thrown about when discussing grounding and earthing. Ringing any bells?

A very common misconception about Ground-Fault Circuit Interrupters (GFCI) or Residual Current Devices (RCD) depending on what part of the world you call home, is that they do not need a ground or earth conductor to operate. This is true in some regards, but it is not entirely true, and understanding why is important for human safety.

When we are talking about GFCIs and RCDs we need to think about that term “Mass”. After all, will a GFCI work in an airplane? How about a car? What about the International Space Station? I’ll bet you they will definitely work… However, I can also set up a ungrounded GFCI in a plastic box with plastic conduit that won’t work. Why?

So, how does a GFCI or RCD work in the first place? Well, there are several methods that can be used, but the most common method deals with the monitoring of electromagnetic fields. When we install electrical wires in raceways, we have some very strict rules about ensuring those wires are paired to either the corresponding neutral wire, or its corresponding phase and neutral wires in another conduit. When you turn the circuit on and energize the light bulb, you better have a fire extinguisher nearby because the metal conduit will start to glow red-hot like an element on the stove!

This is because the white neutral wire has a magnetic field that is 180 degrees out of phase from the black hot wire. When the black and white wires are side-by-side they cross cancel out each other’s magnetic fields. When the black wire is in metal conduit without the white wire, its magnetic field is not canceled out and will induce the metal conduit, heating it up. This is exactly what happens on your electric stove top; we intentionally create this lack-of-magnetic-field-cross-cancellation to cause the elements to heat.

Now, this may be a bit of a long-winded explanation, my apologies. However, it is important to understand that when the black and white wires in a properly designed circuit are next to each other they will cancel out each other’s electric fields. This is what the GFCI and RCD are relying on, a magnetic field that is virtually zero because of the cross-cancellation between the hot and neutral wires.

Inside the GFCI, a simple small Rogowski coil has been looped around both the hot and neutral wires. In a normal circuit, the amperage in the hot and neutral will be equal and thus the magnetic fields generated by the two wires will be equally cross canceled, resulting in zero current on the Rogowski coil. However, should an electrical fault occur, some portion of the current in the circuit would travel back to the source transformer on a fault-current path (either copper wire or on steel conduit), and not on the neutral wire. When this occurs, the neutral wire will be out of balance with the hot wire. In other words, the hot wire will have more amperage on it than the white wire has, thus the white wire will NOT be able to cross-cancel out all of the black wire’s magnetic field. Thus, the Rogowski coil will pick up some current (it commonly only takes 5 to 30 mA to switch to kill the circuit, thereby stopping the fault.

Note, that the key to the GFCI be able to work, is that during an electrical fault, some portion of the current must come off the neutral wire, in order for the Rogowski coil to detect the differential. So where can that current go? There are actually three options: