Grounding a Trace SW-12 inverter

[madlabs]

Hi All,

I was recently given a Trace SW2512 by a friend who upgraded to a 48V system. I'm in the process of hooking it up and just want to make sure that I have the grounding right.

In the manual (yes, I did RTFM) it clearly states that the AC ground and the DC ground should BOTH be connected to earth ground. However, I seem to recall reading that this can be a bad idea. Is this the best practice?

The inverter has a ground lug on the case. Should I connect the AC ground of my trailer to the lug, or home run it down to the ground rod?

Thanks for the handholding. My friend who gave me the inverter is out of town, so I can't ask him how he had it.

Jonathan

[Airstream]

Lets not mix up automotive-style temporary use inverters & the SW-Series inverters, the portable ones are made to keep isolated - the "neutral bond" issue of mixing household distribution items that have ground and the neutral AC wire bonded together that blows up cheap inverters.

You mentioned the word "trailer".  BZZZT. Are you permanently off grid?

Your SW2512 Manual states:

NEUTRAL-TO-GROUND BOND SWITCHING (RV AND MARINE APPLICATIONS)

...inverter/charger installations in the U.S. that are used in RV or Marine applications (must) employ ground-to-neutral switching....

....The SW Series Inverter/Charger does not include Neutral-to-Ground switching and must be provided in the AC installation....

....When the unit is operating as an inverter, the AC output neutral should be connected or "bonded" to the frame/hull (chassis ground). When an external AC source (AC shore cord) is provided, the inverter's AC output neutral should be disconnected from the frame/hull (chassis ground) and allow the "bond" to be provided by the external AC source....

Anyhow - one way around that is a 'lock-out' style manual breaker arrangement that will switch incoming AC and ground to open circuit and only then allow the inverter circuit and ground to be enabled (and vice-versus)...

Good question - I am about to be wiring up my trailer and want to hear what others say about this, THANKS!

[madlabs]

Airstream,

My manual does not seem to have the section you mention. Is yours a PDF version? Mine is a paper version that shipped with the inverter. I'll poke around on the web and see if I can find a newer version.

Yes, I am permenatly off grid. The trailer has seperate nuetral and ground busses.

If I understand the snip you gave from the manual, when I am using the inverter I should have the chassis ground connected to earth ground. When I am using a generator to charge batteries and run loads, the chassis ground should be disconnected and the generator connected to earth ground. Is this correct? It seems odd, I must be misunderstanding.

What about the battery ground? Should it be connected to the earth ground?

[madlabs]

OK, I found the updated manual that Airstream was refering to. It has a much better explanation and diagrams of grounding for the system. I think I get it.

My question is: The docs say to use a relay to perform the neutral to ground switching. It does not say anything about powering the coil of the relay. I'm guessing that I should use the generator AC out to energize the coil. Will this be quick enough?

Let's say that I am using the inverter and want to fire up the generator for charging. I start the generator and its output energizes the relay and switches the ground/nuetral bonding. Will the relay energize and protect the system in a timely fashion? Or do I need to manually switch over before starting the generator? In that case I might as well just use a switch.

Thanks for the help folks! The SW2512 may be old but I don't want to wreck it! It is a serious upgrade for me.

Jonathan

[Ungrounded Lightning Rod]

Let me chime in.

If I understand the snip you gave from the manual, when I am using the inverter I should have the chassis ground connected to earth ground.

Yes - in exactly ONE place.

When I am using a generator to charge batteries and run loads, the chassis ground should be disconnected and the generator connected to earth ground. Is this correct? It seems odd, I must be misunderstanding.

Yes you are misunderstanding.  The issue relates to how the outside source is connected internally, not to whether it's outside.

That statement refers to plugging into a GRID system - where the neutral-ground bond is already made somewhere out there - probably at the meter drop / main breaker box.  Connecting the two at the inverter also means you bond in TWO places.  This is a no-no for several reasons, among them:

 - It splits the load current between the ground and neutral wiring.  This causes the voltage drop in the ground wire (and possible connector corrosion) to raise all the grounds in the affected system (such as the cases of electrical equipment, doorknobs, handrails, plumbing, stairway, hitch handle, ...) to dangerous voltages above other local grounds (such as the next trailer over, the ground water, the water pipe, ...).  Electrocution hazard.  Corrosion hazard. ...

 - If lightning hits nearby the double-bond encourages the bolt to travel through the wiring to get from one bond to the other, greatly increasing the amount of damage and fire risk.

If you're plugging into an external generator you need to check THAT GENERATOR to see whether IT has a ground-neutral bond.  If it doesn't, you leave the one in the inverter connected.  If it does you disconnect the bond in the inverter - or the one in the generator, your option.  And you also make sure there is ONE connection from the system ground to a ground rod/water pipe/whatever is appropriate.

[madlabs]

UGL,

I found the manual Airstream is using, and it has pretty pictures that mkaes it very easy to see how the system should be grounded. My generator DOES have the ground/neutral bonded, so I will need the relay. Would it be work to disconect the ground/neutral bond in the generator and skip the relay, at least until I get one?

Any input on my question below about using the AC out of the generator to energize the relay?

Thanks!

Jonathan

[Airstream]

Devices monitoring generator power have a watchdog circuit to ensure the power source is stable, twenty or thirty seconds delay once generator start is demanded... Active frequency & voltage monitoring is a good thing since a simple relay in circuit would probably chatter on pick up & drop out etc. or worse.

The breaker I mentioned above with lock-out to keep both sources from ever meeting is what I am planning on using, I nabbed a marine ship-shore power selector panel a while back.

Depending on the man'f intended use of your generator that bonded neutral-ground link might be only intended for (example) construction site use where a snakes nest of extension cords would be safely grounded at the sources power panel. The link might be safely lifted in some circumstances, but definitely refer to the OEM manual or support services to stay within code.

Xantrex website has a forum - the questions you still have might be better answered there by searching or asking for specifics there...

[Ungrounded Lightning Rod]

Would it be work to disconect the ground/neutral bond in the generator and skip the relay, at least until I get one?

Sure (if you remember to connect it again later, before you use the generator for other purposes).  IMHO that's the better one to lift, if the generator design makes it easy.  That way, if you unplug the genny, your trailer still has a bond.

It would also work to manually disconnect the jumper in the trailer while you have the generator connected.  (Preferably with the power off while you're working inside it or plugging things in and out.  B-) )

What really matters is whether the two are jumpered (and whether there's current on the neutral between the jumpers).  You should disconnect one or the other whenever the trailer is plugged into the generator, whether the genny is running or not.  (Though if the genny is off and the inverter is on there is not likely to be a lot of current in the line to the genny - unless the inverter has screwed up and is trying to backfeed it - which would probably pop the breakers or shut down the inverter.)

You could use a switch instead of a relay, or play with the jumper by hand.  The relay is for automating the process - on the assumption that anything you plug your trailer into, containing an external ground-neutral bond, will be live.

[cardamon]

Grounding is so confusing because it refers to several different things, some of which have nothing at all to do with the ground/dirt.  Dirt grounding is divided into two parts: Equipment grounding with is the connection of metallic raceways, frames, enclosures and other parts to dirt, and the other is system grounding which is the connection of one conductor of an electrical system to dirt.  Equipment grounding is required by the NEC, and system grounding is required by the NEC for certain electrical systems.  Both do not really accomplish much other than to waste copper and steel.  Remember, electricity has nothing to do with ground, it doesnt care about it, it doesnt seek it, what it does do is wants to return to its source.  This is why equipment grounding and system grounding do not provide any protection from shock.  Now the other aspect of an electrical system that we call grounding is the one that has nothing to do with ground/dirt at all but it is by far the most important.  This should be called equipment bonding and what it does is connect  metal parts together and provides a low impedance path back to the source so that an over current protection device will de-energize the circuit.  For example, lets take a normal household wiring system and see what happens when a frayed wire contacts the metal frame of an appliance.  The frame is connected to the bonding wire ('ground wire') which provides a low impedance path back to the service panel.  From there, it travels through the main bonding jumper which connects the bonding wire to the system neutral and from there it travels back to the center tap of the utility transformer, the source.  If all was done correctly, this was a low impedance path back to the source which caused a large amount of current to flow thus tripping a breaker or blowing a fuse, alerting us that there is a problem.  Note that the earth/dirt has nothing at all to do with this process.  Now take an inverter supplying a premise wiring system, just like with the conventional wiring system, the bonding conductors ('grounds') need to provide a path back to the source so they are bonded to one of the system conductors (again typically the neutral) at or near the inverter.  Without this connection, the current has no place to go (it doesn't care about the ground rod, it doesn't want to go there)that frayed wire that energized the frame of your fridge, will remain energized until someone finds out by getting a shock.

A few tips when evaluating an electrical system for correct system grounding, equipment grounding, or equipment bonding:

1. take them one at a time and treat the separately, because they are three distinct systems serving different purposes.  

   
   

2.  For equipment bonding, imagine a fault at any point and follow the current back - it should have a path (not involving dirt because it is a horrible conductor) back to the source. This means that the bonding conductors must be attached to one of the system conductors (usually a neutral) somewhere and in one location only.

   
   

3.  Equipment bonding which allows fault clearing is the big one and has nothing to do with earth.  System and equipment grounding serve other minor purposes and IMO rarely result in a benefit.

   
   
   

Hope this helps.

[madlabs]

Yay! The boards are back up! I was lonely.

I got it all worked out. I had to remove a jumper inside the Trace as well as set up the relay. It's all working now, and boy am I happy. I don't have to turn off the fridge when making coffee anymore, I feel so grown up now. (My old inveter couldn't run both at once).

I may still but a switch on the genny to un-bond/bond the ground, as the relay I have is only rated for 15 amps, and when using the generator it has to pass all the current through the neutral.

As always, thanks to everyone for all the help.

Jonathan

[Airstream]

I'd love to know the sequence of events that led to discovery of the internal jumper...

[madlabs]

Air,

It is visible when you are looking at the in/out terminal block. It is a bit of metal with three tangs that connect all three neutrals together. Since I knew I was looking to sperate the neutrals, I knew it had to go. It has one black wire soldered to it that goes to the PCB of the inverter. After talking with Trace, they said to flip the jumper so that the wire was above neutral out, then bend the other two tangs up so they are not connected to the neutral in 1+2.

Thanks for the help, BTW. The manual that I found online because of your first post made all the the issues much clearer. No wonder they updated the grounding section, the one in my manual doesn't cover neutral/ground bonding at all.

Jonathan

[Airstream]

"Talking to Trace" (Xantrex) traced it out - that is excellent!

Thanks back at you for posting, another use-it-or-lose-it tidbit of knowledge for these archives... : )

[Airstream]

rats, forgot to ask the real question...

What, where and how?  Is this an off-the-shelve solution or did you shoestring in a relay?

THANKS

[madlabs]

I put in a relay. The coil is energized from the generator. I used a DPDT rated for 15 amps, which is a little low. So, I tied the contacts of both sides together. Since the inverter waits for 1 minute after the gen starts to start using power, the relay won't be switching with any current to speak of flowing. So, it should be OK to share the load on the two sets of contacts.

Jonathan