Shore power

[Jim & Wilma]

18 minutes ago, NeverEasy said:

Normally, what goes in must come out.  If there is a difference, it trips, sensing that some of the current is being sent down a path (you or a faulty device) other than the one it is supposed to.

Without the ground connected to neutral, I don't understand how that alternate current path is completed, creating an inbalance between line and neutral.  What am I missing?

[SuiteSuccess]

Randy,

My dad did the "penny in the fuse box" also.  I thought it was a bad idea even at age ten.  I remember that year he had me standing on an old furnace floor register and reaching up into a wall to pull a wire down for some of his re-wiring.  I grabbed the wire or should I say it grabbed me and I also had to change my britches afterward.  Electrons definitely like to move.

[Alie&Jim's Carrilite]

Question-

I have a Generac GP5500 mounted on the back of the truck, generator frame bolted to the deck.  Either the truck or the camper can be plug into it at the 30amp socket on the generator.  Is there anything specific I need to do with it?  I have not installed / wired anything to the ground lug on the generator frame. When plugged into the truck, the shore power runs thru the inverter that has an automatic transfer switch, that feeds the outlets and I have not had any issue with it.

When the RV is plugged in, the GFCI breaker in the RV that controls the kitchen and bath outlets usually trips when the generator is first plugged in.  I have not noticed a hot skin condition on either the truck or RV, but why is that breaker tripping?

[I'mdonewiththis forum]

8 hours ago, pratten said:

Without the ground connected to neutral, I don't understand how that alternate current path is completed, creating an inbalance between line and neutral.  What am I missing?

The path is created using the hot and neutral wires. Think of it this way: The black wire carries power to the load and the white carries it back to where it started. The ground wire is a safety wire not designated to carry operating current. That is what the neutral is for. Under no circumstances except for a short circuit is the ground to carry any power. The ground and neutral are to be bonded at the source not where the load is.

[RandyA]

14 hours ago, SuiteSuccess said:

Mike Sokol at noshockzone.org has some great info on all this although Randy's is just as good.

Well… since you cracked open the lid to Pandora’s Box I might as well open it all the way.  I know this thread started with a simple question and now I have made it into a complex issue.  If I did not think it is important I would not have taken the time to respond.

Mike and I agree on how to connect shore power to a RV, or in this case a truck being modified for inclusion of 120 VAC shore power.

But, we do not agree on how to wire in an inverter or generator.  Mike’s answer complies with the language of the National Electric Code, section 551.30, for generator installations in recreational vehicles.  Sticking to the NEC is a safe and proper way to answer the question related to neutral/grounding wire bonding.  In our often litigious society this answer can keep the respondent safe – after all it is from the NEC.  Equipment manufacturers play it safe by sticking to the NEC standards – even if they know they are outdated.  The language of this section has not changed in at least 30 years even with the introduction of newer safety devices and the evolution of “portable” and vehicle mounted power systems.  If you read Mike’s blog he answers a question from someone as to why his Fleetwood (motor home) cannot be powered by his Honda 3000i generator.  He references the on-board vehicle power management system as the problem because it is looking for a connection between the grounding safety wire and neutral which is not present in the Honda 3000i.  Same is true for the add-on power management boxes.  These power management systems will fault and shut off equipment power if the neutral to grounding connection bond is not detected.  He is right again when he says that bonding the grounding wire and neutral together to the vehicle frame when using a generator with this type of power management system will allow the generator to work.  But, these power management systems were designed for shore power connections where the neutral and grounding wire are suppose to be bonded at the main service entrance panel – they were not designed for generators or inverters.  The majority of towable RV’s are not factory equipped with these advanced power management systems while some high end (read $$$) vehicles are.  Those with installed power management systems will require neutral to grounding wire bonding to work with a portable generator connected to the shore power supply cord.  This bond is created when a generator is installed in the vehicle through an approved transfer switch. The power management system is an added level of protection from electrical shock when properly wired.

So far it appears that I should shut up and exit.  Believe the NEC and take it at face value without questions. But, that is rarely what I do.  I constantly question rules and conditions and want to know why they exist.  My background in electricity and electronics is extensive.  I am proud to hold an EE degree along with Master Electrician certification.  I have spent quite a few years teaching Electrical Engineering and Electronics classes at both the high school and college level.  I have kept up with the introduction of the newer aspects of the technology through continuing education, seminars and trade publications. I have served as a technical advisor to one of the more successful manufacturers/importers of Chinese built generators and parented one of the longest surviving threads on RV.net for “3,000 Watt Chinese Generators”.  I have an in-depth understanding of RV electrical systems.  So, one might think that what I advocate for wiring a generator or inverter into a truck (RV) just might just have merit even if it contradicts the NEC.  I have explored and tested both my approach and the NEC’s for installing a generator and found that the NEC’s requirement falls short of providing personal safety.

First, let’s take a look at the NEC language.  Specifically Article 100 where it defines Bonding.  “The purpose of bonding is to establish an effective path for fault current that, in turn, facilitates the operation of the overcurrent protection device.”  Nothing wrong with that except most of the synchronous generators being used today have extremely slow responding thermal circuit breakers (inverter generators have a much faster response time).  These are necessary so that the generators can supply the “surge” power needed to start things like the air conditioners used on RVs.  The response time of these overcurrent devices are so slow that if a fault should occur with a path through your body you could easily be dead before the device opened the circuit.  In regard to personal safety it doesn’t make sense.  “Something” faster acting than the overcurrent protection device is needed.  Ground Fault Circuit Interrupters can do this – but they are not specifically prescribed by the NEC for generators or inverters.  I believe they should be.

Second, an engine driven AC generator, or alternator, does NOT have a grounding connection to good old Mother Earth like a properly wired shore power hookup.  Both supply wires are at a voltage potential above  earth ground – neither wire has the benefit of a true earth ground.  Pick one of the wires and name it neutral.  The other is called the hot wire.  That is how it works.  Take the wire you named neutral and bond it to the vehicle frame along with the equipment grounding wire and you meet the requirements of NEC article 551.  But the code fails to acknowledge that the metal parts of the vehicle are now connected to a power generating device that does not have the benefit of an earth ground.  Again, the stated purpose is so an overcurrent device will open if a fault should occur between the hot and designated neutral which is bonded to the vehicle frame.  This might be OK if you carried a bunch of 8’ grounding rods that you would drive into moist earth and connected to the vehicle frame every time you stopped and used the generator or device – but that is not going to happen.

OK – by bonding the designated neutral from a generator to the vehicle frame we violate every NEC requirement for creating only a neutral bond to earth ground.  Read section 250 if you don’t believe me.  The danger of the designated neutral bonding to the grounding wire and to the frame is apparent when one studies the laws of electron flow – particularly those that define capacitance.  We live in an AC world. Every atom around us has at least one electron. When electron potential is not equal between two surfaces the electrons in the surface with a higher potential will try to move.  This condition is easily created when one of the AC power producing wires of a generator or inverter is connected to the metal vehicle frame.  Your body is a better conductor of electron flow than the air between the vehicle and earth ground.  Stand on earth ground and touch the frame of a vehicle with a generator’s power lead connected to the frame and you create a path for the movement of the electrons through YOU.  Not good, not good at all.  Yes, this will require conditions that one might call a “Perfect Storm”.  But the odds of this happening are better than winning the lottery.  Anyway, I have personally witnessed this happening.

So how do you fix this?  Well, you do NOT bond the neutral to the grounding conductor and the frame.  You connect the generator the same way you wire a RV for shore power – with a floating neutral in the vehicle.  If the feared fault between one of the two (yes two) hot wires from the generator comes in contact with the vehicle frame without an earth grounding path nothing immediately obvious will happen.  You are like a bird sitting on a high voltage wire between power poles.  The next step is imperative to provide personal safety from a fault.  A GFCI must be used with a floating neutral.  The GFCI goes after the generator, before the transfer switch and the frame of the generator.  The generator or inverter is not bonded to the designated neutral and any contact between the generator frame and the vehicle frame does not impose an electrical shock risk.  All other NEC requirements for installing a generator or inverter including the proper transfer switch and a #6 copper grounding wire from the frame of the device to the vehicle frame should be followed.

That’s it.  Wire the generator or inverter through a transfer switch just like shore power is wired but add a GFCI which will disconnect power in no more than 6/1000^th of a second if a fault occurs.  Much, much faster than a circuit breaker - even a magnetic breaker.

While I am on the subject of GFCI’s….. they are NOT overcurrent protection devices.  You still need a circuit breaker or fuse in your supply line for 15 or 20 amp rated devices installed in an outlet box.  Some GFCI’s combine overcurrent protection and fault detection.  This type of GFCI is typically installed in the circuit breaker box in place of a conventional breaker.

 

[Jim & Wilma]

Randy, without a ground neutral connection somewhere on the line side of the GFCI, I don't understand how a generator GFCI functions on a single ground fault.  Seems to me we'd have to have two ground faults, one on the line side of the GFCI and one on the load side.  What am I missing?

[Darryl&Rita]

34 minutes ago, pratten said:

Randy, without a ground neutral connection somewhere on the line side of the GFCI, I don't understand how a generator GFCI functions on a single ground fault.  Seems to me we'd have to have two ground faults, one on the line side of the GFCI and one on the load side.  What am I missing?

It's simple math. The current in the black wire minus the current in the white wire must be less than the threshold of the GFCI. If it increases, the GFCI will trip, as current is leaking somewhere it shouldn't. The GFCI devices are rated for different trip points, based on where they'll be used. The most common is probably 5 mA, but they range to over 30 mA trip settings.

[Jim & Wilma]

17 minutes ago, Darryl&Rita said:

It's simple math. The current in the black wire minus the current in the white wire must be less than the threshold of the GFCI. If it increases, the GFCI will trip, as current is leaking somewhere it shouldn't. The GFCI devices are rated for different trip points, based on where they'll be used. The most common is probably 5 mA, but they range to over 30 mA trip settings.

I have a pretty good grasp on differential current monitoring.  It's used throughout the power distribution system from transmission substations down to the bathroom outlet.

What I don't understand is how a GFCI functions on a generator with no neutral ground connection.  If it's a floating system, with no ground current path "upstream" of the GFCI, the leaking current doesnt appear to me to have a return path to ground.  If there's no return path for the leak, then how do we get a differential current between L and N at the GFCI?

[Darryl&Rita]

The return path will be through your hand, across your body, through your feet, into the puddle you're standing in...Click.

[Jim & Wilma]

16 minutes ago, Darryl&Rita said:

The return path will be through your hand, across your body, through your feet, into the puddle you're standing in...Click.

Funny guy.  

For current to flow, there has to be a complete path or loop .  So when I'm standing in the puddle with my hand on a live 120 volt wire of a floating system, that's one side of the circuit to ground.  Where's the other side of the circuit ground path connected to get back to the source, in this case the generator?  Unless the circuit is completed back to the generator, how does the GFCI trip?  Call me slow,  but I still don't get it.

[Darryl&Rita]

Ok, you're slow.

 

Think back to differential current monitoring. The GFCI doesn't need to see current to ground, it only needs to see a difference between the 2 floating power leads. Any difference greater than the device rating will cause it to trip. It can stay energised all day, assuming that nothing touches the bare copper on that old, chafed extension cord you're using to power the truck cab. If the copper should happen to touch the metal edge of the hole you didn't put a protector on, the cab metal can become energised, but the GFCI still won't trip. The rubber tires insulate the truck metal from ground. Walk over, grab the door handle or grab handle, and now you've completed the fault circuit. With thick rubber soled boots on dry caliche ground, your body impedance is too high to allow enough current to hurt you, or trip the GFCI. Grab on with a little moisture in the ground, or a little beer bottle sweat on your hands, or high enough atmospheric humidity, or a big sweat stain across your shirt, or..., and you've altered the impedance of your body enough that current will now pass through you. The GFCI is always monitoring the 2 power leads for any imbalance between them, so it will trip to protect you, before the current can rise to a level to cause you harm

 

Whew, is this where I say "Drive on...(Watch the Skin current)"?

[I'mdonewiththis forum]

The way I understand electron flow is that once an electron leaves the source it is wants to go back to the source by any means it can. What I see Jim is asking is that if the generator is mounted to the truck then it has no connection to the earth therefore the electron is not capable of returning to the source through the earth. If that were to make the GFCI trip, then wouldn't that make the earth a giant capacitor?

[Lou Schneider]

A capacitor is nothing more than two surfaces with some sort of separation between them.

In this case the capacitor consists of the mass of the generator being in proximity to the earth.

There's no D.C. connection, but the capacitive connection is enough to pass the few milliamperes of current needed to shock someone getting between a voltage carrying conductor and the earth.

This divergence of current from the conductor to the earth unbalances the currents in the source and return leads coming out of the generator, tripping a GFCI even though there isn't any direct connection to ground.

[RandyA]

On 4/10/2017 at 3:32 PM, Alie&Jim's Carrilite said:

Question-

I have a Generac GP5500 mounted on the back of the truck, generator frame bolted to the deck.  Either the truck or the camper can be plug into it at the 30amp socket on the generator.  Is there anything specific I need to do with it?  I have not installed / wired anything to the ground lug on the generator frame. When plugged into the truck, the shore power runs thru the inverter that has an automatic transfer switch, that feeds the outlets and I have not had any issue with it.

When the RV is plugged in, the GFCI breaker in the RV that controls the kitchen and bath outlets usually trips when the generator is first plugged in.  I have not noticed a hot skin condition on either the truck or RV, but why is that breaker tripping?

Jim, I don't know from your description if the GP5500 has a floating neutral or a bond from the designated neutral to the frame which serves as the grounding point.  If it is bonded and the frame is bolted to the truck you will be putting the connected metal parts of the truck at the voltage potential of the bonded lead - same when you plug in the trailer.  Personally, if it were mine I would have the neutral floating and not connected to the frame.  As far as the GFCI tripping when you first plug in the generator I can only tell you the GFCI is detecting a current imbalance or it would not trip.  Incidentally, an imbalance can occur with a backfeed on one of the downstream wires.   No way I could tell you where without having the truck and doing some testing.  But, I would bet a dollar to a doughnut that the GFCI would not trip when the generator first starts if there was no bond between the generator/vehicle frame and the designated neutral.

[RandyA]

13 hours ago, pratten said:

Randy, without a ground neutral connection somewhere on the line side of the GFCI, I don't understand how a generator GFCI functions on a single ground fault.  Seems to me we'd have to have two ground faults, one on the line side of the GFCI and one on the load side.  What am I missing?

No additional explanation of how a GFCI works is needed - other contributors have done an excellent job of explaining operation.

When you say "the line side" I am interpreting that to be the two power producing wires from the generator into the GFCI?  OK - let's look at possible fault paths before the GFCI and after the generator output wires, which are typically a 3-wire plug-in or twist lock power cord. To reduce the possibility of a fault in this path I would recommend type SOOW or SJOOW cord to the watertight enclosure for the GFCI.  For twin Honda EU2000i units capable of producing 27 amps of current #10 AWG is OK for less than 25' runs and happens to be what you will typically find in manufactured cord sets.

1st fault - the two generator power leads come in contact with each other.  The generator overcurrent device will open.

2nd fault - just one of the generator power leads comes in contact the vehicle frame.  Remember, there is no real neutral connected to earth ground so it can be either one - they are both the same.  If either lead contacts the frame it will put us back to square one - the generator will become bonded.  Most likely, nothing immediately different will be noticed.  That is unless the GFCI senses a fault on the output side, a fault that I have determined is more likely to occur when the system grounding wire is bonded to one of the generator power output wires.

 

[RandyA]

9 hours ago, Mntom said:

The way I understand electron flow is that once an electron leaves the source it is wants to go back to the source by any means it can. What I see Jim is asking is that if the generator is mounted to the truck then it has no connection to the earth therefore the electron is not capable of returning to the source through the earth. If that were to make the GFCI trip, then wouldn't that make the earth a giant capacitor?

Yes - earth is one surface of a giant capacitor.  Seems strange, but look at a lightning bolt for an example.  It's path is from earth to the charged cloud.  The charge builds just like it does when we charge a conventional capacitor.  Eventually the charge builds to the point where the dialectic breaks down, electrons quickly move between the two surfaces using the path of least resistance.

[Heavymetal]

11 hours ago, RandyA said:

Yes - earth is one surface of a giant capacitor.  Seems strange, but look at a lightning bolt for an example.  It's path is from earth to the charged cloud.  The charge builds just like it does when we charge a conventional capacitor.  Eventually the charge builds to the point where the dialectic breaks down, electrons quickly move between the two surfaces using the path of least resistance.

Randy,

 I have read and reread all this post and just have one more question. Is plugging in a Edison plug the best way to insure safety? If not I would like to continue this conversation face to face before I hook up my generator. 

 

Richard

[RandyA]

17 minutes ago, Heavymetal said:

Randy,

 I have read and reread all this post and just have one more question. Is plugging in a Edison plug the best way to insure safety? If not I would like to continue this conversation face to face before I hook up my generator. 

 

Richard

Richard, I will be at the ECR Wednesday afternoon until departure on the 29th.  Are you going to be there?  Can't say that a Edison plug is the cure (haven't heard that term in a while - you must be really old )  Now, if you are forgoing a transfer switch and intend to move your shore power cord from a residential/ park power source to the generator as needed, my approach, which I have noted does not agree with the NEC, is to make sure your genny does not have a connection between the frame and one of the generator power wires.  You can determine if this bond is present by using an Ohm Meter.  With the generator off, but the genny CB ON,  check to see if there is an internal bond by putting one Ohm meter lead in the longest Edison slot and the other in the round grounding hole.  For unbonded the reading should indicate an open connection, for bonded you will get a continuity reading.  Check the short slot in the same manner just to be sure someone has not reversed connections on the duplex.... er, Edison outlet.    If it is bonded, I would remove the bond - which may be visible or behind the outlet panel with a jumper wire between the long slot and grounding hole screws. 

In any event that little, inexpensive GFCI box should be used between the genny and the vehicle or trailer.  It is really cheap insurance. Here are photos of a GFCI box for Edison type plugs that I made a few years back.  I have since found several commercially built units for under $100 like this one that needs plugs added or this one for 30 amp campers that includes both surge guard and GFCI protection that render collecting parts and building about equal in cost.   The GFCI  I made is in a weather tight box so it can be outside.  The GFCI device I used in my build is often not found in home stores.  It is a 30 amp rated feed thru outlet box device. But a 20 amp home store GFCI would work fine for a 20 amp circuit which is usually all you need to power a truck AC system.  You really need this no-matter-what type of bonding or floating neutral your genny or truck stop inverter has.  Also use one for shore power input- even if you have residential GFCI's downstream mounted in your RV or truck.

 

[Heavymetal]

Richard, I will be at the ECR Wednesday afternoon until departure on the 29th.  Are you going to be there?  Can't say that a Edison plug is the cure (haven't heard that term in a while - you must be really old ) 

 

 No not old just done a little wiring in my short life (54) and yes we will be in about Wendsday also. I was going to forgo a transfer switch because I was thinking of using the same shore plug to plug the genset into, but I can include one. I just figured that it wasn't needed for the way I was setting it up. I did not use GFI plugs instead I use the GFI breakers. I really don't have a problem wiring the system to keep from short circuiting the electronics I have implanted in me I just really want to protect the trucks electronics. 

 I will see you at the rally so we can discuss it face to face. 

 

Thanks for the help I really do appreciate it. 

 

 

Richard (aka) Heavymetal

[NoDirectionHome]

First off, this is a tremendously valuable and educational thread for myself as  mechanical engineer.

My question is, then do I assume I wire in a generator to the transfer switch bypassing the Progressive Industries Surge protector?

[Alie&Jim's Carrilite]

I have seen many campers using this modified plug in their Honda gensets so their GFI circuits work.  Is this safe or practical?

[Heavymetal]

1 minute ago, Alie&Jim's Carrilite said:

I have seen many campers using this modified plug in their Honda gensets so their GFI circuits work.  Is this safe or practical?

"The Edison plug" 

[Alie&Jim's Carrilite]

Alright, so the "Edison " plug connects the ground and neutral, and the devices Randy showed just have a built in circuit breaker.  Got it.

[Darryl&Rita]

35 minutes ago, Alie&Jim's Carrilite said:

I have seen many campers using this modified plug in their Honda gensets so their GFI circuits work.  Is this safe or practical?

That jumper is installed to "fool" the electrical management system into allowing floating neutral power into the coach. Oit doesn't matter whether the EMS is a Progressive Industries, or another manufacturer. It has NOTHING to do with GFCI operation.