Earthing and Bonding

What do you think the difference is?

What an excellent topic for a Google search.

This is not an easy topic, although you will see comments from people who will trivialise this very important field.

I recently bought a single phase 240V AC 3KW pure sine wave inverter from an Australian company that designed DC to AC inverters. The inverter was to go into a Van. I asked a question about how do I "ground"the inverter. It was really interesting talking to an engineer who realised that this issue of bounding ,grounding, earthing, shielding is like a black art. He told me of a situation in an industrial area which had multiphase power systems, where a company was using heaps of power and yet they were using only machinery that used modest amounts of power.

This engineer was brought in and after some considerable effort they discovered 100 amps of current was flowing through one of the gutters in the building. I for one cannot explain this easily but the as you know a metal shed is made up of various metal pieces bolted together. The trouble is how do you guarantee that the various metal sections are all electrically connected to each other, so they are all at some common potential (well roughly anyway). This is where the process of using nice thick cable and good terminals to make sure all separate pieces are electrically connected together. This is what I would call "bonding"

Earthing or grounding is another issue. that is more to do with lightening protection and atmospheric electricity. The problem with the so called ground or earth, is that many people think that large amounts of "earth" make for a good conductor. If you have dry sandy soils then these areas are not going to be good conductors.There could be massive potential differences between Ground stakes.

Once again this issue of earthing can become a black art and is still an area of on going research. When I was in the Country Fire Service we had a communication site in Adelaide known as dont laugh "Bulls Knob". It got constantly struck by lightening damaging equipment in the radio shack, until one day we walked in and the whole room was blackened by exploded radio equipment.

A company known as CRITEC came along and their solution was first creating a known point where you want lightening to strike. It involved triaxial cable with a thing called a "Dynasphere" bit like a world war one German helmet right at the top.

The tower required massive cooper earth mats and known fixed point that became the ground point, the coax cables from the tower had to enter the building via a massive metal plate with energy diverters . The system is like catching a cricket ball, the tower absorbs say 90% of the energy from the direct strike, then the building plate absorbs 90% of the 10% that is left over, then as the cables come into where the cabinets are smaller diverters are on the cabinets, what is important is that the cabinet ground gets connected to the building diverter plate and then the diverter plate goes directly to the substantial earth at the bottom of the tower. This 90% rule is of course a very rough rule.

In over 30 years the site has not suffered any damage due to lightening. I hope this little story shows that Earthing , bonding, grounding is far from trivial stuff. It is complex and expensive and is still not well understood because of its statistical nature linked with atmospheric electrical effects

Earthing provides minimum resistance path to fault current. If the live conductor touches to the electrical panel body or the live conductor touches to the stator body of the motor due to insulation failure, the fault current should find minimum path so that its protection device can get trip.

The body of the electrical equipment is earthed from the two distinct points to divert the fault current to earth for protection of the equipment as well as for protection of human. The process is called the earthing of the equipment.

The metallic items which are not designed to carry electricity in the room or building are intentionally connected to earth to avoid the possibility of electric shock. This is known as bonding.

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