Separately Derived emer. generator

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  #1  
Old 03-13-06, 07:18 AM
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Separately Derived emer. generator

I'd like to get some clarification on the Code referencing portable generator grounding. In particular, 702.10 talks to portable generators getting their own grounding electrode (or not). Do I read correctly in that if the generator is NOT wired into the home wiring system that it NEEDS to have a ground rod or some similar grounding system. AND, if you are wired into a transfer switch to an existing service it is NOT to have its own grounding system?
Also, lets say we have a 240/120 genset, portable. Do I require a 4 wire cord or a 3 wire cord for connection to a transfer switch? This would be the case where it is defined as "nonseparetely derived system". In such a case, ground fault currents would not appear at the generator?
 
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  #2  
Old 03-13-06, 09:20 AM
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Well... you probably wont find consensus.

As I understand 702.10 (A) and (B) both address being wired in an approved method through somekind of transfer switch. Question is about the bonding of the neutral at the generator. If the generator has bonded neutral (frame bonding with equipment ground) the transfer device must switch the neutral from the service, thus making the generator a seperately derived source and a ground rod is required at the generator.
If the generator has a "floating neutral" (not bonded to frame) then the transfer device must not switch the neutral and is therefore solidly connected to the the house service neutral and grounding electrode system and is a non-seperately derived system. You would not use a ground rod at the generator.

I'm not sure about the 3 or 4 wire cords, generally all portables now days use a 4 wire cord for the 220 volt receptacle, this was not always they case though.

You will find much disagreement on this subject so rather than expand I'll give you a few hours reading......

Maybe the answer will become clear somewhere in all this......I offer only for informative use I dont necessarily agree with everything.


http://www.imsasafety.org/journal/marapr/ma5.htm

http://www.schneider-electric.ca/www...pl_Note_EN.pdf

The following two links take a little selective browsing to get the pertanent info...

http://members.rennlist.org/warren/generator.html

http://members.rennlist.org/warren/gt5000c.html

If you really like to read try this....

http://www.selfhelpforums.com/showthread.php?t=7878
 
  #3  
Old 03-13-06, 06:15 PM
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ok, i got it.

my genset has the neutral and chassis bonded. So, to make code, i can't ALSO have my load center connect the neutral and ground when running the generator. If I do it anyway, i'll have a small voltage on the frame of the generator due to the neutral voltage drop, proportional to the L1/L2 unbalance in the load (like a couple of volts). Sounds just like the 3 wire vs 4 wire range wiring scenario. Now, if I loose the neutral connection altogether, what happens? Now, it appears the genset frame is "floating", but now is likely to be a shock hazard due to the L1/L2 leg unbalance. Something to think about with those sometimes corroded twistlocks.
 
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Old 03-13-06, 08:02 PM
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> my genset has the neutral and chassis bonded.
> So, to make code, i can't ALSO have my load center connect the neutral and ground
> when running the generator.

True.


> If I do it anyway, i'll have a small voltage on the frame
> of the generator due to the neutral voltage drop

Maybe.
Where exactly is this bond?


> Sounds just like the 3 wire vs 4 wire range wiring scenario.

Not really at all.


> Now, if I loose the neutral connection altogether, what happens?

Your 120V circuits can have 0 to 240V.


> Now, it appears the genset frame is "floating",

If you lose the bond on the genset itself? How could that happen?


> but now is likely to be a shock hazard due to the L1/L2 leg unbalance.

It's not at all likely to occur.


> Something to think about with those sometimes corroded twistlocks.

Not if the genset is bonded a you said.


The problem having a second bond somewhere else is that you get neutral current over EGC if you use four-wires.
 
  #5  
Old 03-13-06, 11:02 PM
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One of the things I find interesting about this is....these are portable generators.....how many people do you suppose are going to install a ground rod? Or even know to install one? Or if one is even required. Most of these generators people buy are job site type designed for cord and plug equipment. I'm not sure if the manufacturers ever intented for people to power their house wiring with them.
 
  #6  
Old 03-14-06, 07:13 AM
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let me explain better my example. I was describing a case where a portable generator is feeding a home. Getting to be a very typical situation these days post storm. People arn't using a ground rod at the generator, so the grounding is dependant on the twist lock connector. Most of these generators have internal neutral bonding; that is, the neutral is connected to the frame and to the ground contacts on all the recepts.
So, in typical operation, neutral current is shared between the EGC and the neutral conductor. "Typical" here means common, but not per code. Now, what happens when the twist lock pins get corroded and you have intermittant ground and neutral connections. Now, the chassis becomes hot, the voltage dependant on the unbalance of the two legs.
All I'm saying is that I expect a much larger incidence of blown out 120v appliances and somewhat of a shock hazard with these home generator plants.
 
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Old 03-14-06, 10:06 AM
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> Most of these generators have internal neutral bonding;

[1] Okay, we're assuming that it is so bonded.


> in typical operation, neutral current is shared between the EGC and the neutral conductor.

[2] It shouldn't be, but let's suppose that it is.


> Now, what happens when the twist lock pins get corroded
> and you have intermittant ground and neutral connections.

[3] This is very unlikely, but let's suppose the worst case you can think of.


> Now, the chassis becomes energized, the voltage dependent on the unbalance of the two legs.

[4] Absolutely not. See [1] above.


> All I'm saying is that I expect a much larger incidence of blown out 120V appliances

[5] Very unlikely. See [3] above.


> and somewhat of a shock hazard with these home generator plants.

[6] Absolutely not. See [1] and [4] above.

[7] A ground rod would not substantially change any of this, even in the absence of a neutral-ground bond on the genset (contrary to [1] above).
 
  #8  
Old 03-14-06, 04:26 PM
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Please let me better explain my response.

The generator is internally bonded. I assume that this bond is inviolable.

Now consider voltage.

Voltage is not something that exists at a single point in space.
(An electrical charge can exist at a point, basically excess or deficit of electrons; but we're talking about an alternator, not a Van de Graaf generator.)

A voltage is a potential difference. It is the difference between any two points.
A bird can rest on an energized high voltage electric wire because the voltage (the difference) between its two feet is 0.0000V.

So for there to be a shock hazard or touch potential, there must be a difference between two points.

What are those two points?


In order for there to be a voltage, electricity must be able go from the shell through your body, and to the internal neutral.
Not only does such a pathway not exist, even if it could be created (say by grabbing the neutral wire), such a path would have far higher impedance than the metallic pathway of the internal bond that for all practical purposes, the voltage is 0.00V.

Touching the shell of the alternator is no different really than touching any other grounded metal object.


Consider this:
If an alternator (or any SDS) is not grounded and not otherwise electrically coupled, you cannot get shocked by it unless you get between two of its conductors. Any single point cannot have a potential difference; it takes two.
 
  #9  
Old 03-15-06, 07:57 AM
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Look at the generator situation this way. No matter the bonding/grounding issue, there will be 240v across L1 to L2 on the twistlock. The generator also forces the voltage of L1 to neutral = L2 to neutral= 120V, fairly independant of load. Most all portable generators of this type have the neutral permanently connected to the generator frame. So, the frame = generator neutral voltage. This also holds true for any load, balanced or unbalanced. OK? In our case, which I will claim to be very typical, the generator is sitting on vibration dampning rubber feet and wheels, has a steel case, and is not tied to a grounding system, like a water pipe or ground rod.

Now, I wire the generator to the house. In my example, i'm going to sustain a unbalanced load, therefore there will be a neutral current. This would be a very typical situation, do to the random nature of which 120v loads are on. More exactly, lets say we have a 20 amp L1 load and a 5 amp L2 load. OK, now there is a problem with the neutral/ground to the generator due to loose connection or corroded twistlock. Let's go ahead and say that connection to neutral/ground opens. What happens??

Well, just like a utility pole that lost the neutral connection, 240v is still supplied, but each leg is not 120v off of ground anymore. In our case, the L1 load is 120/20 = 6 ohms. The L2 load is 120/5 = 24 ohms. L1 to L2 current is at 240/(5+24) = 8.3 amps. You now find that the more heavily loaded L1 leg is left with V1 = 8.3*6= 49.6v and the lightly loaded L2 leg gets the balance of the 240v, which is V2= 8.3*24= 192v. Ok, so I've got some dim lights and some brite lights. What is happening to my generator? Well, it is still running, providing 240v at 8.3 amps. It appears happy. But wait, what about its chassis? Since the house has its neutral still connected (bonded) to local ground via water pipes, and ground rods, etc. its forcing the generator neutral to off balance, that is, the L1 and L2 are not 120v from ground anymore. In fact, its neutral is at 192-120= 72v! Since the neutral is connected to the chassis, the chassis is also at 72V ref the dirt it is sitting on, which clearly is a shock hazard. (the complete path of the current is stator/L1 to house load to house neutral to house grounding electrode thru dirt to the area under the generator thru your body to the generator frame to the gen. neutral connection thru the stator/N).
 
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