Generator Earthing

To cover there own backsides.

The risk is a piece of metalwork becomes live, and is not grounded to the same point as the supply thats making it live.

sQuick IEng MIIE (elec)

Thanks. however I am still a little confused why an earth stake is necessary. How can the connection of a wire from ground to the frame of the generator guarantee that the peice of metalwork you refer to is connected to frame as well?

Sorry for appearing dumb.

When ever you create electricity there must be a path to earth. With out the earth ground you run the risk that you circuit breaker will not trip. Ever notice that every electrical service in your neighbor hood has an earth ground? If the utility could get away from using them do you not think that they would quit spending the money? Not earthing the neutral can cause voltage imbalances to be created. A lot of people just rely on the frame of the genset to be in contact with the earth... close enough. It takes no time to drive an minimum 8 foot ground rod flush with the earth and connect it to the generators frame to the lug provided. (usually)

Make it safe and make your SO's safe. It only takes about 5 milliamps to kill. Less in some conditions.

I worked for a university and the city folks set 10 20 kw generators on tires for temp Christmas lighting not one was grounded. I shut them off and locked them out. The mayor and the president of the university were in the electric shop the next morning. I explained the safety issues for the public and the mayor immediately called his people and they installed the required ground rods. Christmas lights were dark one night. No big deal compared with making the headlines all over the world. Personally I do not want to be on TV.

| When ever you create electricity there must be a path to earth. With out the | earth ground you run the risk that you circuit breaker will not trip. Ever | notice that every electrical service in your neighbor hood has an earth | ground? If the utility could get away from using them do you not think that | they would quit spending the money? Not earthing the neutral can cause | voltage imbalances to be created. A lot of people just rely on the frame of | the genset to be in contact with the earth... close enough. It takes no time | to drive an minimum 8 foot ground rod flush with the earth and connect it to | the generators frame to the lug provided. (usually) | | Make it safe and make your SO's safe. It only takes about 5 milliamps to | kill. Less in some conditions. | | I worked for a university and the city folks set 10 20 kw generators on | tires for temp Christmas lighting not one was grounded. I shut them off and | locked them out. The mayor and the president of the university were in the | electric shop the next morning. I explained the safety issues for the public | and the mayor immediately called his people and they installed the required | ground rods. Christmas lights were dark one night. No big deal compared | with making the headlines all over the world. Personally I do not want to | be on TV.

At how many points is ground needed? Can this be safely done with a single solid ground at the transfer switch on a permanently installed generator?

Electrical grounding is always done at a single point at the point of service or generator. It get a little more complicated when you are working with a transfer switch. Assuming single phase residential USA. Transfer switches come in 2 pole solid neutral and 3 pole where even the neutral is switched. I prefer the 3 pole switches because then the neutral is directed to the source and is not shared with the other source. Which can be a bugger to figure out some times.

The ground rod for the generator can be driven local to the generator. Then you will carry the ground to the other source and tie it together. You will be creating a supplementary ground system. When either source has a fault the path will take it back to the ground. Tripping the breaker. If you do not carry the ground you can have a situation where is can become dangerous. Try to locate the Soars Book on Grounding at the library,

Might be a better picture than my words. I just moved, and still have not unpacked my books.

Section 230 of the NEC is written for grade 22. I have submitted several times to the NFPA to change the wording so it is not so hard for people to understand. They have nixed it each time. Oh well I like tilting at windmills.

Thanks SQLit,

I can see that a 1phase unit needs an earth stake now. Is that because one end of the winding is connected to the frame of the genset and the other end is the active?

If this is the case do I need to use an earth stake with a 3 phase gen set connected in Star if the earth and neutral are internally bonded ie earth -neutral link.

Is the star point on the 3 ph genny always at 0 potential?

| Electrical grounding is always done at a single point at the point of | service or generator. It get a little more complicated when you are working | with a transfer switch. Assuming single phase residential USA. Transfer | switches come in 2 pole solid neutral and 3 pole where even the neutral is | switched. I prefer the 3 pole switches because then the neutral is directed | to the source and is not shared with the other source. Which can be a | bugger to figure out some times. | | The ground rod for the generator can be driven local to the generator. Then | you will carry the ground to the other source and tie it together. You will | be creating a supplementary ground system. When either source has a fault | the path will take it back to the ground. Tripping the breaker. If you do | not carry the ground you can have a situation where is can become dangerous. | Try to locate the Soars Book on Grounding at the library, |

| | Might be a better picture than my words. I just moved, and still have not | unpacked my books. | | Section 230 of the NEC is written for grade 22. I have submitted several | times to the NFPA to change the wording so it is not so hard for people to | understand. They have nixed it each time. Oh well I like tilting at | windmills.

So should the service be grounded at the pole transformer, or at the entrance to the building being served? If there is a transformer inside the building handling all the power, and the neutral on the secondary is grounded, does it have to be grounded to the service neutral, or can it be independently grounded right there at the building entrace?

I disagree with what you say here. In many ways, systems would be safer if they weren't grounded. You'd have to touch two wires to get shocked instead of just one. The reason utility wires are grounded is because of lightning and the possibility of a very high voltage wire falling onto a lower voltage one. A breaker won't trip on an earth ground fault at normal voltages because the resistance of the earth is too high -- you must bond the grounding system to the neutral in order to trip a breaker through the equipment grounding wire. However, once a system is grounded, then you need to keep grounding everything connected to it or else you could still be shocked. An isolated generator generally is immune from most of these risks. A bunch of long extension cords or christmas lights could be susceptable to lightning, but that is all. An equipment grounding (really bonding) wire that goes back to the generator neutral would also be very helpful if metal equipment is plugged in.

Weren't these portable generators (even though they are big they were temporary weren't they)? Aren't these exempt from ground electrode requirements?

I suppose you want me to ground my car too? And the 12V to 120V inverter I plug into it?

-- Mark Kent, WA

You sure it is not the green wire that carries the fault current to circuit breaker which makes it trip and not the earth connection.

Basically the earthing of every service entrance allows the electric company to have a nice low impedance path back to their ground reference. They provide a ground about every 1/4 mile on the pole line. They actually use every one of the service entrance grounds to help provide a lower earth impedance path.

What amazes me if earthing was so important; and you can't get a earth ground rod resistance of less than 25 ohms you drive a 2nd rod; if it is still higher than 25 ohms its ok to stop there according to the NEC. So I could drive two rods 6 feet apart, end up with 100 ohms to ground and that would be fine with the NEC and inspectors.

I doubt unless the voltage is high enough like over 600 volts any fault current is going to overcome earth resistance to blow a breaker.

Section 250.4(A)(5) says the Earth is not an effective Fault Current Path.

I work on a floating bridge, there are no ground rods there, though concrete is a pretty fair ground. The transformer secondary X0 is bonded to the equipment grounding conductor in the P1 panels. Otherwise the earth ground floats I have no trouble blowing breakers on the bridge. Now if I remove the equipment grounding conductor I'd bet i would have a terrible time blowing the breaker.

Gary K8IZ Washington State Resident Registered Linux User # 312991

What breaker are you referring to? If you are talking about the facility breaker, how does a grounding electrode affect it?

How? Where would the imbalance be?

For external contact with the skin, it takes a lot more than 5 mA to kill. 5 mA is in the "mild pain" range, but not fatal.

Ben Miller

answer to the masses, everyone is entitled to an opinion. If you disagree with me, OK attack me, but do not confuse the issue for the people who are asking for help. I try to reduce technical questions to something that the guy next door can handle. A lot of the time the answer is hire a qualified contractor. That is another subject. This is an open forum, all can voice there opinion. Let us leave it at that and not attack each other, we all know what an ass hole is..

Lets try to be a little more applicable to the question MARK,,,, your comments should be left to people that UNDERSTAND the issue. I have worked on and installed non-grounded electrical systems. I WOULD NEVER recommend them to an residential customer. Your opinion is considered and rejected for this application. Lets try and answer the questions at hand, not take the average guy into territory that is way un - charted for the average guy.

We are here to help each other, I for one have learned alot in the newsgroups.

my thoughts alone all negitative email should come to me not the group.

The manufacturers always take th high road for liability claims. Although you are suppose to drive the ground rod and ground the generator frame to it, most personal users do not. However, if you have employees, neighbors, or children about I would take the high road and ground it.

My answer wasn't meant as an attack. I'll give an opposing view to anyone, so don't take it personally....

S250.34 Portable and Vehicle-Mounted Generators. (A) Portable Generators. The frame of a portable generator shall not be required to be grounded and shall be permitted to serve as the grounding electrode for a system supplied by the generator under the following conditions: (1) The generator supplies only equipment mounted on the generator, cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and (2) The non-current-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame.

Now if you want to power your house with this generator through a transfer switch, then you would be required to ground it and I would agree that it is a good thing. But I think you over reacted to christmas lights on trees being powered by an ungrounded portable generator.

So for the "average guy", why is an ungrounded portable generator unsafe? I can think of a few answers of why it may not be good for the generator, but none dealing with safety.

-- Mark Kent, WA

Thank you all for your replies.I look at this newsgroup from time to time and must say that it is refreshing to be able to read the different opinions that you guys have. I don't understand most of it but it is still enjoyable.

Thanks