Portable Generator - To Ground or Not to Ground

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Portable Generator - To Ground or Not to Ground

It is not always clear as to when to (or when not to) ground (connect to the earth) a portable electrical generator. There are many factors that influence the decision. It may surprise you to know that sometimes you should NOT supply an additional grounding rod to the generator; sometimes it really doesn’t matter; and sometimes you should ground your generator. So it is important for you to know the difference so that you can have your system in a safe operating condition at all times.

Portable generators are used to generate electricity on a much smaller scale then the utility companies. For that reason, they are ideal for temporary, remote or emergency electrical service. Depending on how the generator is being used, it will need to be properly wired for the application. It is nearly impossible to address every application in this forum.

This article was intended to be a general overview of when to ground a generator without diminishing the fact that electricity can be very dangerous and requires a knowledgeable expert (a licensed or qualified electrician) to  understand and appreciate all the details associated with each application.

Proper grounding and bonding is important to help prevent shocks and electrocutions. You can find information on grounding and bonding in the National Electric Code (NEC) at www.nfpa.org. Your local electrical codes are probably based on the NEC, but they might have some additional requirements that you should be familiar with.

In my mind, there are two different ways to use a generator. The first is as an isolated system in which the generator is powering aspecific piece of equipment like concrete saw through the receptacles on the generator. I call this the cord and plug system. The other method is to use the generator as a means to supply power, much as the utility company would supply power, at a remote location or for emergency power. Typical elements receiving power in this case would be a complete job site, a trailer, a structure or a house. I call it a power source system.

If you are using the cord and plug system, typically you do not need to supply an earthgrounding rod with the generator. There are no other sources of electricity, lightening surges, or fault currents that you would need to contend with. The housing/frame of the generator will serve as an adequate ground for the fault currents generated by that particular generator. You will still need to have a grounding conductor running from the piece of equipment to the generator and the grounded pin in the receptacle, the neutral element will have to be properly bonded to the grounding source at the generator, the generator housing or frame. In addition, all of the non-current-carrying metal parts of the generator and the vehicle or trailer that it is mounted to must be bonded to the grounding source, the generator housing/frame. You can supply a grounding rod if you want for the cord and plug system, but it is not necessary.

Using a generator as a power source system to provide temporary, remote or emergency power for a house, trailer or complete job site is a much more complicated scenario in terms of whether or not to drive a new grounding rod. Either way, the generator used this way must have the proper earth grounding, for example, a grounding rod. The generator will need an additional earth-grounding rod if it is a separately derived system. If it is not a separately derived system, there is a good chance you should not have another earth grounding rod. This is where it gets complicated, and this is where you must use a licensed or qualified electrician.

To overly simplify the reason why, you must first understand that the power source system is wired to a circuit breaker panel by means of a transfer switch. The transfer switch is simply a means to transfer power from the utility power source to the generator. If the transfer switch only switches the hot, then the generator is not a separately derived system and must be bonded to the grounding rod of the utility power at the transfer switch since it can be unsafe to have a potential difference between the two grounding points.

If the transfer switch switches both the hot and grounded (neutral) conductors, then it is considered a separately derived system and must have a proper grounding rod in place for the fault currents such as lightening, line surges, unintentional contact with other voltage sources or back feeding currents.

“To ground or not to ground, that is the question.” Keep in mind, that this article is strictly a guide as to the simple concept ofwhen or when not to ground a portable electrical generator. For practical applications, please remember that electricity can be very dangerous and requires a knowledgeable expert such as a licensed or qualified electrician to safely wire and configure the system.