Dual 30A/120V Shorepower Grounding

[Beausoleil]

I'm still trying to resolve the gremlins in my AC system. As I've noted in earlier postings, Beausoleil has two 30A/120V shorepower inlets, a 120/240V 60A genset, and a 2800W inverter/charger. I know that with a 120/240V genset, I should have 120/240V 50A shorepower, but that's not changeable right now, (and I'm still P.O.'d at the installer).

The gremlins are as follows: When running off the genset, the inverter (which is on L2 - second 30A leg) will trip its internal breaker whenever I run high loads on L1 - say the hot water heater (pulling 11A) and the auxiliary battery charger #1 (pulling 10A), and sometimes even when turning on auxiliary battery charger #2 (on L2 - same leg as inverter/charger).

I've eliminated tripping due to excessive amperage - as measured with a clamp-on ammeter (Sperry DSA-600), I never see more than about 20-25A on circuit breaker #41 (load L2) or #42 (neutral N2). And this happens with all breakers on the subpanel off - there're no loads at all on L2 to compete with the inverter/charger.

As best as I can tell, the system is wired according to the attached PDF drawing. I made some changes from the boatyard installer's initial wiring with regards to the neutral N2 bus and fully separating the AC grounding bus so it's only interconnected at the genset and the main DC ground bus.

The inverter (a Magnum MS2812) has a "Shore Max" feature which will allow to to set the maxium AC current it will use when charging. This allows you to let the charger scale back when AC loads downstream are in use - in order to prevent tripping of the upstream circuit breaker (#41/42 in this case). Changing it from its default (30A) down to even 5A makes no difference. There's also a VAC dropout setting, where you can set where the inverter will cease charging if the input AC voltage drops to the VAC dropout setting - 80V default. The lowest I measured was 121V when the inverter tripped.

Any ideas?

[CharlieJ]

Capt Jon-You have a mismatch of systems and the neutrals are doing you in. Two 30 amp shore power services must have two separate neutral busses and a single safety ground bus supporting the downstream loads. This is almost how your shore power is set up. There should only be one safety ground bus installed. By the way, a single 60 amp galvanic isolator would have sufficiently protected both shore power safety grounds.

Conversely; a 120/240 VAC ("split phase") service, has two hot legs and a single, common neutral leg. This is not how your genset was wired and I believe this is the source of your problem. I do not see how you can combine two single phase systems (your two shore power inlets) and a split phase system (your genset). I believe that you are developing a neutral imbalance and the MMS2812 is protecting itself by tripping its internal breaker.

[Rick]

I believe Charlie J is correct. I've had problems with boats having two ac input lines that have a common neutral bus. Whatever loads that are fed by the inverter (either using the internal pass-through tranfer switch or the inverter output) must have their neutrals fed only to the inverter/charger/transfer switch neutral and no other neutrals on the boat.

This necessitates, sometimes, a long process of chasing down all of the separate neutral wires and labeling both ends.

Please note Charlie's advice to use a galvanic isolator for each ac input cable. This will prevent a transfer of neutral current to the green wire grounds due to a differential in neutral voltages with two ac input cables which naturally occur with heavy onboard loads as "seen" all the way back to the shore power grounding system.

[s/v Jedi]

I don't understand the replies. Jon writes that he has these problems when running on the genset, in which case the shore power inlets are disconnected from the system and there is only one neutral involved.

But I agree that the balancing might be the source of the problem. I would think about an auto-transformer to solve that, eliminating the neutral from the genset completely (it makes a new neutral). See http://www.victronenergy.com/upload/...rmer%2032A.pdf for more info.

cheers,
Nick.

[CharlieJ]

S/V Jedi-The system mismatch is that while on the genset, he is feeding two legs of 120 VAC from the split phase system but he has split the neutrals. I have to think about the auto-transformer as a solution.

On reading the original post, it appears that the genset is a relatively new installation. If so, than it should be relatively easy to re-configure the output from the genset from 120/240 VAC to 120 VAC only. But this results in only one leg of 120 VAC.

A straight forward solution would be to convert your two 30 A shore power services to a single 50 A service. This would match your sources and you could leave the I/C as shown.

[GordMay]

Originally Posted by CharlieJ

The system mismatch is that while on the genset, he is feeding two legs of 120 VAC from the split phase system but he has split the neutrals. I have to think about the auto-transformer as a solution...
... A straight forward solution would be to convert your two 30 A shore power services to a single 50 A service. This would match your sources and you could leave the I/C as shown.

Are you thinking of a Scott T connection?

When more than 1 shore power inlet is used, the shore power neutrals shall NOT be connected together on the boat.

Cap'n Jon has a miss-matched system, for which there is no simple solution (excepting to correct the shore power wiring). Any "patch-up" will require a horrendous switching scheme.

[s/v Jedi]

Originally Posted by CharlieJ

S/V Jedi-The system mismatch is that while on the genset, he is feeding two legs of 120 VAC from the split phase system but he has split the neutrals. I have to think about the auto-transformer as a solution.

With the auto transformer, you only connect the two 180 degree shifted hot phases from the genset to it, the neutral is left unconnected. This means the genset is always loaded balanced with 240V output. The transformer creates the neutral and absorbs any inbalance with 120V devices at the consumer end.

Still, you write that the neutral is split when running on the genset. All I see is one neutral bus with the genset neutral connected halfway that bus. Pls. explain what you mean with "he split the neutrals" because I am affraid I'm missing something... the genset only has 1 neutral and that is used for all neutral wiring when the transfer switch is in the "gen" position, right?

A straight forward solution would be to convert your two 30 A shore power services to a single 50 A service. This would match your sources and you could leave the I/C as shown.

But that doesn't change anything when the transfer switch is in the "gen" position which is where he has the problem???!!

cheers,
Nick.

[Zach]

Gord,

With two shore power sources coming off the dock, would it require the boat have two main breaker panels?

Thanks,

Zach

[ChuckSK]

Cap,n Jon

If turning on high loads on the generator temporarily causes the ac voltage to drop to the inverter turn-on threshold, then the inverter neutral output would be grounded at the inverter and the inverter neutral input would be grounded at the generator, thus tying the inverter neutral input and output together. This condition might cause the inverter to trip. Try disabling the inverter nuetral ground feature to see if the problem disappears.

A completely separate ground system for inverter subpanel circuits might be a permanent solution. I don't know if that would be legit though.

Regards
Chuck

[GordMay]

Originally Posted by Zach

With two shore power sources coming off the dock, would it require the boat have two main breaker panels?
Zach

Yes; each 120VAC service requires it’s own 2-pole main breaker, positive bus (feeding branch circuit breakers/fuses), and neutral bus. The 2 pairs of bus bars must remain separate.

You have no assurance that the (2) 120VAC shore sources are on separate legs (which would give 240VAC between them).

Here’s a basic guide to Marine AC Systems wiring:
http://www.marinco.com/files/media/g...r%27sGuide.pdf

[chala]

If it is CB3 (30A) Figure 3-2, Power Flow - Standby Mode that is tripping, I see no reason for it to do so. I would try to reduce the charge rate.

05 Charge Rate: This setting can be used to turn off the charger, limit the amount of current that the charger can use (leaving more current available to power loads); or to ensure small battery banks are not overheated because of a charge rate that is too high.

http://www.magnumenergy.com/Literatu...%20Series).pdf

[Beausoleil]

Guys,

Thanks for all the replies!

Let me fill in a little more details. The whole system is basically brand spanking new (last year, anyway). I originally designed the system as a 120/240V 50A shorepower and genset system. But the electrician doing the install for me was not familiar at all, it turns out, with the Magnum inverter/charger and its ability to handle a 120/240V split phase system. He told me he thought the system as designed would be "unbalanced" and he would talk to a "marine electrician with years of experience" about it. I'm a telecom guy with a BSEE and an MSEE - I don't know much about power systems, right?

Well, I deferred to the guys with "experience", and what you see in the diagram is what they came up with (and my minor corrections using what I had on hand).

ChuckSK - I've verified (within the limits of my DMM) that turning on "high loads" does not drop the genset's AC voltage down to the VAC dropout voltage setting on the inverter. On the contrary, it appears (however illogical) that when I turn on a high load, the voltage actually will increase - and that's before I can hear the inverter/charger's circuit breakers kick in. Yes - that doesn't sound right...

CharlieJ - the genset isn't "new" to the system - the shorepower was re-worked along with the rest of the AC system. As I said, against my better judgement and original design, the shorepower system went in as two 30A 120V connectors rather than a single 120/240V 50A inlet with a single neutral. I may just have to re-work that as an last-ditch effort. I was hoping I could work my way around it.

And when running off shorepower, the two neutral systems are totally separated - unless there's something I'm missing in the drawing...the transfer switch breaks any connection between the two that would be taking place back at the genset.

chala - I've tried changing the Charge Rate setting, and it makes no difference. It shouldn't anyway, since the Magnum's DC output is only 125A, and my battery bank (AGMs) is 800Ah - 40% of C would be 320A, 25% would be 200 - well above the 125A capability of the inverter/charger.

Gord/all: The problem seems to only manifest itself when running off the genset. And yes, the boat essentially does have two main breaker panels, even though one is labled "subpanel". If you look at the diagram, the main panel is basically a single leg panel, except that a portion was sectioned off to provide a second leg, with breakers 41 & 42 serving L2 and N2 to the inverter and subpanel (along with two other main-panel breakers serving L2/N2. The two ship's neutrals are connected back at the genset. When running off the inverter, we're only using the L2/N2 side.

I'm with s/v Jedi - the problem seems to only happen when running off the genset. I'll confirm that tonight, as we're actually in a slip on shorepower for the next day or two.

Rick - the shorepower system is currently setup with two separate 30A galvanic isolators, not a single 60A as CharlieJ suggested.

[ChuckSK]

A transient condition may not show up on your DMM.

I would still temporarily disable the grounding relay in the inverter as a test.

Regards
Chuck

[CharlieJ]

S/V Jedi-You are quite right...when running the genset, there is only a single neutral involved. My quick review of the drawing initially missed that rather unorthox splitting of neutral off the genset into N1 and N2 only while on shore power.

Cap'n Jon- The schematic indicates that the MMS2812 was wired in a dual in/dual out configuration. After reviewing the wiring Table 2-3 in the Magnum techman http://tinyurl.com/qay83w I have to question this decision as you have no 240 VAC loads, and you are not supplying the I/C with 120/240 VAC split phase. The internals of the I/C may be "expecting" two legs of split phase power but only "seeing" a single leg.

Some final points:
1. You really need to hand over hand the as built wiring to ensure that the wiring diagram truly depicts what is reality.
2. Have you contacted Magnum Energy yet? I have had to talk to their technical support extensively on a problem that I had with an installation and I found them to be incredibly responsive and knowledgeable. If you need a point of contact, please contact me.
3. Finally, some problems are just too complex be solved in a forum atmosphere. One small omitted factoid can skew the entire logic tree. For example, if a high impedance DMM is used to check continutiy across a switch, the switch will test satisfactory. The fact that the switch had high resistance across the contacting surfaces would be masked by the characteristics of the DMM. This small ommission could direct the forum participants way down the wrong path. It is my opinion that your problem is complex enough so that a qualified electrician needs to be on site with the proper instrumentation in order to resolve it.

73-
Charlie

[chala]

Cap'n Jon
In my humble opinion if the inverter was unhappy with it supply, (neutral unbalance, etc) it should disconnect from it supply and run as an inverter and not trip its internal breaker. Is the inverter faulty or just not up to the task?

Charlie
The inverter is wired as per Magnum Figure 2-12, AC Wiring for Single In - Single Out (60 A) Configurations
See previous link