Electronic – How does earthing work when earth is dry


Earthing is meant to provide reliable contact of an electric appliance to earth so that if there's an insulation fault current goes into earth instead of through a person's body. This requires earthing to be made of thick conductors driven deep into earth.

Here's how good earthing was described in one domestic pump manual (I'm pretty sure that it correlates well with local building codes): three steel pipes each at least one inch in diameter and twenty feet (six meters) length must be driven into earth vertically in a triangle pattern with at least two feet distance between each two pipes. The top of each pipe must be at least two feet below the ground surface. A common steel rod must be welded to all three and the equipment being earthed must be connected to that rod. Welding spots must be painted to protect them from corrosion.

Now that's plenty of metal and looks impressive. But how does it guarantee a low resistance path for insulation fault currents? What happens if earth is dry and not conductive enough?

Best Answer

When the earth is dry. sometimes, earthing works badly.

A pathological case for earthing is things on top of mountains.

The mountaintops tend to be pretty dry. People like installing observatories atop mountains. In a past life I worked with observatories.

The earthing rods are laid out in a ring around the observatory with a buried cable linking them. The rods have tops high in the air; point is to get some lightning protection, and an earth that works at all. (Lightning likes high, metallic things like observatories.)

The air is very dry on top of mountains. ESD is normally prevented by earthing.

Standard protocol was for staff to urinate on the earth spikes instead of using the porta-potty (whenever possible without upsetting tourists).

To make it harder, many sites share the mountaintop with radio transmitters. All that radiated RF is hard to screen when there is no functional earth to connect to. (Radio guys, you have the same problems but you caused mine dammit!)

A co-worker had similar problems earthing in a past life with sand dunes and earthing. Getting the septic tanks to drain on to the dunes can help.