Thank you for your question regarding the Neutral-to-Earth Bond in the main panel. The short answer to your question is that the neutral-to-earth bond is needed to properly operate the circuit breakers. Over Current Protection Devices (OCPD) such as circuit breakers and fuses actually require a short and intense INCREASE in electrical current (a short) in order to detect the fault and cut the circuit off. Without a sharp and drastic increase in electrical flow, a fault could go on without triggering a circuit breaker to stop the flow. This actually occurs quite often and can be measured easily by checking the amount of current flowing on your earthing conductor. It should be less than 1-amp in most cases. If the current flowing on a earthed conductor is higher than an amp, and you are not in a high-voltage (600V+) environment, it typically indicates an erroneous neutral-to-earth bond somewhere in the system.
To visualize the reason why the neutral-to-earth bond is required, you must consider the entire electrical circuit from a 120-volt outlet all the way back to the utility transformer hanging out on the pole:
- In a properly designed circuit, if a fault were to occur on the 120-volt outlet between the hot-wire and the earth, the current will flow through earthing wire back to the main panel, where it will move to the neutral wire via the neutral-to-earth bond, up to the utility transformer, back down the hot wire to the circuit breaker, tripping the breaker.
- In an faulty designed circuit, if a fault were to occur on the 120-volt outlet between the hot-wire and the earth, the current will flow through the earthing wire back to the main panel, where because it does not have a neutral-to-earth bond, the current will be forced through the earthing rod, into and across the earth, and up the utility earthing rod and in to the utility transformer, back down the hot wire to the circuit breaker. The resistance of the earth is almost always to great to allow sufficient current flow to trip the breaker, and you end up with a steady-state earth fault, that never trips the breaker, and this is a hazardous situation indeed. You cannot use the earth as a conductor.
Another issue that can occur, is that multiple (and illegal) neutral-to-earth bonds can exist in the system (only one bond is allowed in the main panel). When this occurs, both the earthing wire and the neutral become current-carrying conductors, which effectively means that you have two (2) neutral wires running in parallel. This divides the current and places electrical energy on to the chassis of all metallic objects within the system. Another hazardous situation.
Also, Arc-flash energy exposure can also go up if you don’t have a solid neutral to earth connection because of the inverse-time curves of circuit breakers.
This subject can be a very difficult concept to understand and the improper application of neutral-to-earth connections can have very serious and life-threatening consequences. If you have any doubts at all, we highly recommend getting a licensed electrician to assist you. We hope this information helps.
The Engineering Team at E&S Earthing Solutions
Photo Credit: http://www.flickr.com/photos/deltamike/2637796725/sizes/o/in/photostream/