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Author Topic: How do new S&S Class A motorhomes handle neutral bonding?  (Read 2253 times)
belfert
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« on: May 23, 2006, 10:53:44 AM »

Any time a transfer switch, generator, and inverter are mentioned in the same posting, the neutral binding issue always comes up.

How are the makers of S&S Class A motorhomes handling this?  Alost every new Class A has both an inverter and a generator installed plus shore power.

Brian Elfert
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FloridaCliff
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« Reply #1 on: May 23, 2006, 11:13:57 AM »

The ones I have seen use a premade unit that has contactors on the inside.

They are wired so as to give a particular input a priority.

They use the NC or NO controls on say the three input contactors.

I would guess that the generator is priority one, if its on all others are disconnected.

Then shore power and last inverter.

I have the contactors to make my own. 

Currently I have three seperate input outlets and move the plug which feeds my panel.

Cliff
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1975 GMC  P8M4905A-1160    North Central Florida

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DrDave
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« Reply #2 on: May 23, 2006, 12:06:59 PM »

The transfer switch units also switch the neutral bond along with the hot leads.

Multi-Contact relays are used in transfer switches. If you use Physical plugs rather than automatics
you neutral source is determined by your plug wiring.

Note: ( Neutrals from 3 sources should NEVER be hard wired together! )

The only common item is the Ground lead, It is common between the generator, line and chassis. (GREEN wire)

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belfert
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« Reply #3 on: May 23, 2006, 12:47:57 PM »

The transfer switch units also switch the neutral bond along with the hot leads.

If a transfer switch has three inputs for shore power, inverter, and generator, wouldn't it be real easy to overload the inverter?  Also, wouldn't it then be difficult for the inverter to charge the batteries when there is other power?

Brian Elfert
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FloridaCliff
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« Reply #4 on: May 23, 2006, 01:03:16 PM »

Yes, it could overload the inverter.

Some power management is required unless you have a large inverter/battery bank.

I have a 2500 watt inverter and only run my electric frig and lights when on it.

Cliff
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1975 GMC  P8M4905A-1160    North Central Florida

"There are basically two types of people. People who accomplish things, and people who claim to have accomplished things. The first group is less crowded."
Mark Twain
Dallas
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« Reply #5 on: May 23, 2006, 01:59:34 PM »

Yes, it could overload the inverter.

Some power management is required unless you have a large inverter/battery bank.

I have a 2500 watt inverter and only run my electric frig and lights when on it.

Cliff

Cliff,
I also have a 2.5K inverter, made by the world famous Mexican company, El Cheapo SA, a subsidiary of Dewey Cheatham & Howe.
I run my 12Cu' fridge, 2 computers 20" TV and house lights from it.
I have 4 group 31 house batteries and can go about a day and a half without recharge.
The only problem I found is that when the fridge comes on I get a low voltage alarm for about 1/10th of a second. This happens whether or not anything else is running.

Dallas
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Jerry Liebler
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« Reply #6 on: May 23, 2006, 02:28:08 PM »

Brian,
   It all depends on the inverter/charger,  some inverter/chargers handle the neutral bonding internally while others require an external neutral bonding relay.  I once had a Tripplite APS 2012 that had one AC input but internally handleed the neutral bonding when no AC was feeding it.  I used it behind a conventional transfer switch to switch both hot and neutral between shore and generator.   I now have a Trace SW2512 which requires an external neutral bonding relay and has 2 AC inputs (#1 for shore and #2 for generator).  Internally the Trace  only switches the 'hot' leads so it requires external contactors to complete the transfer switch.  The features of the trace make the 'pain' of a proper installation well worth the effort.
Regards
Jerry 4107 1120
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NCbob
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« Reply #7 on: May 23, 2006, 03:27:34 PM »

Let me see if I can explain, "my way" and believe me when I state that I've spent many years in dealing with power systems, generators, paralleling and marine systems.  Each source of power has it's own neutral (not to be confused with earth , safety, ground)

Neutral is the center in the Generator, Shore, or Inverter winding

If you have a 120/240 generator (source) it uses 3 legs for 120/240 (the third being neutral to the winding of the generator) and the 4th for ground (earth,  safety, source) but in this equation there is no earth..only source and safety.

In this case the 240 (L1-L3) is converted to 120/120 by adding in L2 (generator winding neutral) and L0 which is ground (but must go back to the case of the generator since it is Source.  Hence L1, L2, L3, L0

Let's go to the shore power.....

Again, if you have 120/240 shore (source) it uses 3 legs for 120/240 (again the 3rd leg being neutral to the winding at the transformer on the pole).  Being that the ground is both common the earth and the powehouse (...because the powerhouse is also earth grounded...source)

You have 240 VAC (T1-T3) and bring in T2 (neutral in the winding of the transformer on the pole) and T0 which is ground (but must go back to the earth ground at either the transformer or the pole.  Hence T1, T2, T3, and T0.

In the case of the inverter.  It being the 'source' of the 120VAC (or 240VAC)  it too will need to have either 3 wire 120VAC or 4 wire 240VAC and the 4th wire will need to be the "ground (not to be confused with 'neutral') back to the 'source'.

Were I using an inverter I would do as I'am doing...using a 4 pole rotary transfer switch, with 4 positions (Off, Gen, Shore, Inv)
and break all 4 wires to insure that whatever "source" I'm using is isolated from every other "source" as far as Neutral and Ground are concerned.

You might have to read this slowly a couple of times in order to grasp what I'm trying to say...I've re-read it a number of times so as not to mislead anyone.  Each "Source" and it's Neutrals and grounds...need to be seperated from every other source.

Some Electronics guru's will tell you otherwise, but theirs is a completely different application.  Electricians will disagree with me..but then most of them don't live aboard Boats or travel in Motor Homes

Backfeeds along neutrals and grounds are what cause fires...which kill people while they're sleeping!

Some of this might not apply to such things as aluminum hulled boats where they go so far as to use isolation transformers to keep the electricy used aboard from eating up the hull...but that's a debate I will glady engage in with the more learned among us.

NCbob
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Jerry Liebler
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« Reply #8 on: May 23, 2006, 03:43:25 PM »

NCbob,
    Please check and correct your understanding!  Here are the facts:  The ground wires should  never be disconnected and always remain connected to each and every source.  The neutrals however must be switched and only the neutral associated with the 'source' actually supplying power should be connected to the load(s).  The only place neutral is connected to ground is at each power source.  Disconnecting ground is acceptable if the connection is through a plug and socket but NOT if the source disconnect is a switch.  For a 120 volt transfer switch exactly 2 poles are needed.  For a 120/240 transfer switch exactly 3 poles are required.  Switching grounds in a transfer switch violates the NEC and is unsafe.
Regards
Jerry 4107 1120
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NCbob
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« Reply #9 on: May 23, 2006, 05:49:03 PM »

Well Jerry, this is where the NEC and I have disagreed for years.  I have seen instances where lightning has come back through the (earth) ground and caused catastrophic damage to a standby generator and associated switchgear because the (earth) grounds were interconnected.  Had the standby generator been allowed to act as the (earth or source) ground, and isolated from the Utility (earth) ground the system would not have been so badly damaged.  We do understand that extreme high voltage knows no limitations.

That's one of the reasons I studied so hard to be certified in High Voltage applications.

But that's the world we live in.  Opinions and first hand experience don't necessarily count.  Only the thoughts of the ones in the Ivory Towers who've rarely been in the field to experience the damage Mother Nature can impose on our lives.

All of the above is offered in the nature of friendly debate, not at all meant to be confrontational.

NCbob
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Jerry Liebler
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« Reply #10 on: May 23, 2006, 06:17:48 PM »

Ncbob,
     I find it hard to believe that you KNOW the damage due to lightning would have been less with a transfer switch that opened the ground wires.  My house has been hit by lightning and several unplugged elictrical devices were destroyed including transistor radios and tube type televisions.  The electrical field strengths in and around a lighting strike are very hard to comprehend.  This lightning strike also destroyed a deep well pump with a ground and two 240 volt wires even though the pump was under 100+ feet of water.  The code says keep the ground wires connected because that is safest in ALL conditions including during a lightning strike.
Regards
Jerry 4107 1120
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David Anderson
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« Reply #11 on: May 23, 2006, 06:36:20 PM »

Not to poo poo anyone's take on this issue, but I've been reading threads about how to solve the neutral bonding switching issue on this board and BNO since 1999 and nothing seems to generate more opinions than this subject.  I can confidently say that no one has explained a system that is code compliant that works.  It's funny because I was just getting ready to write a thread,

"Haven't seen anyone post about neutral/ground bonding in a while",

just to get the issue fired up again hoping to see any new enlightenment about it.  

I too have a Trace SW2512 and called Trace several times and read and reread their book on the issue and never came up with satisfactory results that resolve the issue of switching the ground.  

I guess you can do it manually with 3 plugs, generator, inverter, and shore.  Whichever you are using at the time, you plug it in, but that sure defeats all the neat features on your Trace.   Transfer switches that switch 2 neutrals and 4 hots (genny and shore (2 120 volt lines)) that would integrate with 3 separate grounds (genny, shore, inverter) were not available when I put my system together.  Dick Wright couldn't even offer a solution, and he puts these things together all day long.   I could have home brewed my own, but given the complicated switching logic and the probablity of mechanical failure I devised a much simpler manual/electric system less prone to failure that didn't dilute the features of the Trace inverter.

I resolved the issue in my coach and it does not meet NEC criteria,
BUT it does bond the neutral and ground at the generator WHEN the generator is running.  
The bond at the genny is open WHEN the bus is on shore power.  
The neutral/ground bond at the inverter is open WHEN the bus is on shore power and WHEN the genny is running.  
The neutral/ground bond at the inverter is bonded WHEN there is no genny and no shore power.    
There is no neutral/ground bond anywhere  in the coach WHEN the bus is plugged into a pole.  That is the way it is supposed to be, right?

NEC 551-54(c)1----"The grounded cicuit conductor (neutral) shall be INSULATED from the equipment grounding conductors and from equipment enclosures and other grounded parts.  Bonding screws, straps, or buses in the distribution panelboard or in appliances SHALL BE REMOVED AND DISCARDED." (emphasis added) 1999 edition.   Remember, the theory of grounding is at the source of power---the genny, or the inverter, or the park power pole.  One place only, not all three or two.  That is why your main panel board should have the neutral INSULATED from the panel case and ground wire. 

I dare not show anyone this for liability issues because of the NEC prohibition of ground switching.  If I ever sell the coach I will state as such and if the buyer takes issue I'll disconnect it all and let him hire an electrician to do it his way.   It's all within reach of the bay doors.  

I am a true advocate to do everything according to the various codes for safety and liability reasons and I don't recommend otherwise.  However, as stated above nothing to this date has ever been shown to me that is compliant with simple logic and low cost.

Let the arguments begin!!!

David
« Last Edit: May 23, 2006, 10:21:08 PM by David Anderson » Logged
NCbob
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« Reply #12 on: May 23, 2006, 06:37:05 PM »

Jerry, now I know that I missed a golden opportunity to meet some very interesting people by not being able to attend Dallas' and Cats' shindig in Timmonsville last week end...you being among them.

Our dabate, such as it is, will probably not be decided until such time as we have the opportunity to sit down and have (probably a quite animated debate) over a couple of cool glasses of lemonade (or whatever)  but I'll look forward to it.

Back to the topic.....

When the signs of high voltage (above 600 VAC) gone crazy leaves tracks on pre-stressed concrete back to interconnected (earth) grounds of an electrical system, on standby power, and blows the complete generator end of a 180 KW Generator, and blows the Transferswitch completely off the wall, gives me clear evidence that the grounds should NOT have been interconnected.

Bear in mind....that the "grounds" have a difference in potentiial between the generating plant, the Utility and lightning....and that difference is ...to say the least...tremendous.

In the interest of not boring our fellow Busnuts...shall we put this debate on the back burner....to be explored..at another time when we can sit down and discuss it to the 'enth degree'?

You definitely have a mind I'd wish to explore more in depth!

cordially,

NCbob
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Jerry Liebler
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« Reply #13 on: May 23, 2006, 07:23:13 PM »

David,
    An (or even 2 strapped for 120/240 volt operation) SW2512 or SW4024 can be installed and fed from shore or generator in a fully code compliant manner.  The preference is generator priority but that is only to take advantage of generator autostart while suplementing a small shore cord.  The installation requires a normally closed bonding relay (120 volt coil) with a 60 amp rating and two 3 pole normally open contactors (120 volt coils) rated for 60 amps and a 4pdt relay (120 volt coil) with contact ratings greater than the coil curents of the other relays and contactors.  The relays and contactors need to be mounted in a suitable enclosure. In my installation I also have a switch that allows  inverter bypass and disconnects everything.
Regards
Jerry 4107 1120 
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FloridaCliff
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« Reply #14 on: May 23, 2006, 08:07:02 PM »

Not bonding your grounds is a serious mistake.

Not only does it violate the NEC, but it puts your life at risk.

90% of the incidences I have investigated involving someone being shocked have involved a loss of the neutral and our ground being used as a return.

When our ground was disconnected most of the electrical equipment in the home burned up and someone was shocked.

The Power, Telephone, Cable all bond together to avoid a difference of potential on the ground.

I wonder why?

Even though my current set up involves moving a plug, My generator is bonded to the chassis, my inverter is bonded to the chassis, and my shore power is bonded to the chassis.

I have a question to those who don't bond ground?

Is your generator 100% isolated from the chassis? No bolts to the housing, up on isolaters.

Is your inverter mounted in any way to the chassis without isolators and disconnected from the battery ground when not in use?

Does your shore power go to a disconnect that is 100% isolated from the chassis?

You probably have a bond to ground, just not the best one.

Ok, I feel better now! Grin

Cliff

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1975 GMC  P8M4905A-1160    North Central Florida

"There are basically two types of people. People who accomplish things, and people who claim to have accomplished things. The first group is less crowded."
Mark Twain
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