I understand why we ground for nearby lightning strikes and surges. The other main reason we ground is to establish zero potential to neutral and ground, to the Earth. Is there more nuance to the purpose of establishing zero potential?
Grounding electrode system
- Thread starter Pinnie
- Start date
Zero potential is a way of saying that there is no difference of potential between two points. Current only flows across a difference of potential, so if you can make all points equal, no current will flow. Calling it "zero" is just for reference. Voltages are relative, so we need some point to reference against. Since the earth is the largest conductor around, we connect one conductor in the system to it and call it "zero" and measure everything else against that. Connecting the frames of all of our equipment together and to the earth establishes a local zone of equal potential so there is no difference in voltage between points that could allow a hazardous current to flow.
That’s what I thought thank you. My next question is about separately derived systems. Why do some manufacturers require ground rods and some don’t at generators?
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
Another stated reason for grounding is to reduce the potential (voltage) to ground.
Probably because they don't know what grounding even means. They know that buildings are grounded so they think their generator should be grounded. It never made much sense to me to ground a portable generator. Or to bond the neutral inside it either.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
Your question has false presumptions. It isn't a "manufacturer" that requires ground rods, it is the function of the generator and the applicable codes. Top that off, Engineers and owners may require ground rods from the case to ground even when code doesn't, similar to a job requirement to drive a ground rod at each site light pole.
When a generator is a separately derived system code requires the neutral and ground to be bonded together and for a grounding electrode system to be connected to it. While engineers MAY specify a separately derived system for no good reason, I have been told that the only time a generator is required to be a separately derived system is when the main breaker of the utility system is ground fault protected, which the code requires in conditions stated in NEC 210.13.
Does bonding the neutral cease it to be a separately derived system?
Okay gotcha
Yes this is what I’ve heard
so it’s undesirable for it to be so?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Technically it's the switching of the neutral that will determine if it's a SDS or not.
Gotcha. Is the purpose of tying the SDS’ GES to the premises GES only to ensure zero potential betweeen the two ?
Fred B
Senior Member
- Location
- Upstate, NY
- Occupation
- Electrician
No in an SDS you Ground the neutral point at the source just like a POCO does to derive the neutral. Think about the proper term for a neutral: "Grounded Conductor" and it is not the same as the grounding conductor. The grounded conductor has nothing to do with a zero potential in fact most times you may have some varying voltage between the grounded and grounding conductor as what you would see is the voltage drop of the utility side as loads change on the utility system.
As the name implies an SDS is a separately derived system, you are deriving another "service" source
Thank you for your response.
That’s good to know that makes sense. When grounding the neutral point of a SDS, do you tie the grounding electrode system of the SDS to the grounding electrode system of your service(s) in your building/house?
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
The code requires the service grounding system and any SDS grounding systems to be tied together. If you closely review article 250, you will find that with only a couple exceptions, grounding a service is the same as grounding a transformer for example, if I install a transformer outside and it feeds an overcurrent device on the outside, it doesn't have to have a grounding conductor and the neutral and ground get bonded together at the overcurrent device.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Technically the SDS and the service are just required to be connected to the same grounding electrode system at the building or structure.
A good way to think about it is that a grounding electrode system belongs to its building or structure, not to any particular electrical system.
If you have a service or SDS at a building or structure, and that service or SDS requires grounding, you must connect that service or SDS to the GES. If the GES doesn't exist yet, you must install it. If the GES already exists or is going to be installed for another service or SDS, and you have an additional service or SDS, you connect it to the same GES.
If the service or SDS is at a separate building or structure, that building or structure gets its own GES.
A good way to think about it is that a grounding electrode system belongs to its building or structure, not to any particular electrical system.
If you have a service or SDS at a building or structure, and that service or SDS requires grounding, you must connect that service or SDS to the GES. If the GES doesn't exist yet, you must install it. If the GES already exists or is going to be installed for another service or SDS, and you have an additional service or SDS, you connect it to the same GES.
If the service or SDS is at a separate building or structure, that building or structure gets its own GES.
Wow, that was an amazing explanation. Thank you.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
I would just like to point out that separate buildings from the same service create more nuance(complication) in regards to bonding the neutral to the ground. There are so many if-then points in 250, so be cautious when grounding and bonding.
I have recently seen three significant sized jobs, 800A or more that the Engineer specified grounding and bonding incorrectly. Usually in the second means of disconnect, not the first.