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Old 27-06-2021, 07:08   #1
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AC DC common ground

Hello, I have been mapping the wiring in my new to me sailboat. I have heard that the AC ground bus should be connected with the DC negative bus. I am not seeing evidence of this in my wiring. Is this necessary? I do have an inverter charger (isolation transformer) and inboard engine (DC negative grounds to block / prop shaft) in the system. Thank you for input from your experience.
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Old 27-06-2021, 07:28   #2
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Re: AC DC common ground

It is required.

The primary reason is the scenario where your shore powered battery charger develops a short between the shore power AC side and the DC output side which would put AC voltages on the DC battery circuit. This in turn connects to the engine block and through most transmissions to the underwater metal that could kill a swimmer in the water.

By joining the two negatives together a voltage leak like that will be grounded back to the source.

The risk of such a failure is miniscule however the consequences can be catastrophic.
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Old 27-06-2021, 07:50   #3
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Re: AC DC common ground

So we got new docks last year, and with the new docks came new fancy safe circuit breakers. (I'm sure government mandated)
Many boats that never had an issue were immediately tripping these new breakers.
The electrician that wired the docks said first thing to check for on our boats, was to make sure AC and DC grounds were NOT connected or sharing the same buss.
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Old 27-06-2021, 07:52   #4
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Re: AC DC common ground

Just noted CSY 44.
MANY fond memories chartering the CSY 44 in the BVI in the mid 80's!
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Old 27-06-2021, 08:33   #5
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Re: AC DC common ground

The shore power grounding bus (green wire) should be connected to the DC ground (Black or Yellow Wire). DO NOT connect the AC neutral (White Wire) to the DC ground (Black or Yellow Wire)/AC ground (Green Wire) together.
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Old 27-06-2021, 08:38   #6
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Re: AC DC common ground

Quote:
Originally Posted by James M B View Post
So we got new docks last year, and with the new docks came new fancy safe circuit breakers. (I'm sure government mandated)
Many boats that never had an issue were immediately tripping these new breakers.
The electrician that wired the docks said first thing to check for on our boats, was to make sure AC and DC grounds were NOT connected or sharing the same buss.
That is incorrect. The most common reason for tripping a shore power ELCI breaker is a connection between AC neutral and AC safety ground. This should not be done on a boat, but as it is required in houses, it is often done mistakenly on boats.

Connecting neutral and ground provides and alternative neutral path to the dock. The ELCI breaker measures the current on both the hot and neutral legs. If the current is more than 10 milliamperes different, it trips.

Connecting AC safety ground to DC ground ensures that there are both at ground potential, but they should be connected at only one point to prevent a grind loop.
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Old 27-06-2021, 08:46   #7
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Re: AC DC common ground

IIRC AYBC requires the green SAFETY GROUND of the AC system be connected to the traditionally black (now yellow) GROUND of the DC system. We have ELCI main breakers and GFCI outlets in our boat and do not have any problems with breakers tripping. If you have an inverter built into your system, I -think- that it makes this connection internally.

I do know that the green SAFETY GROUND is NOT to be connected to the same bus as the white NEUTRAL in the AC system. (Unlike a house) This will cause some breakers to trip.

Some older boats were built before all the requirements were written, others just ignored them. (Some new boats too. . . )
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Old 27-06-2021, 09:54   #8
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Re: AC DC common ground

This subject is deadly important so I want to clear up some points in the comments made.

@Yandina @#2:
+1. Absolutely correct with the additional comment that this path back to the source will ensure enough current flows to trip a protection device on overcurrent.

@Jones_M_B #3:
Quote:
So we got new docks last year, and with the new docks came new fancy safe circuit breakers. (I'm sure government mandated)
Not government mandated, it is and has been a requirement of the National Electrical Code and came about because every summer people were dying from electric shock drowning in freshwater lakes and reservoirs.
Quote:
The electrician that wired the docks said first thing to check for on our boats, was to make sure AC and DC grounds were NOT connected or sharing the same buss.
That electrician is very poorly informed and flat wrong. As noted down thread, the tripping of the ELCI occurs because of an incorrect or inadvertent N>G bond or, on boats with multiple shore power cords, the N from the branch load from one cord has been landed on the N bus for the other cord causing an imbalance on N current.

@Sainted #6:
Quote:
The ELCI breaker measures the current on both the hot and neutral legs. If the current is more than 10 milliamperes different, it trips.
Concept is correct, however; the ELCI trip limit is required to be 30mAAC maximum, in a maximum of 100mSec. Most will trip at about 26mAAC.
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Old 27-06-2021, 10:12   #9
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Re: AC DC common ground

Also depends where the boat was built. May have changed but in Europe it was forbidden to connect the A/C ground to the DC neutral. This seems sensible, D/C neutral is a 'floating ground' because ti is a closed loop on the boat it does not need a reference ground and does not care what voltage it is relative to the world outside the boat. The A/C side however must always have H2 or neutral joining the ground wire at the power source. If this is an onboard generator or inverter the ground and H2 or neutral should connect there. If A/C comes aboard from shore power then the H2 or neutral goes back ashore with the ground. This creates a ground that is reference to the outside world. The marina should have a big copper stake in the ground to maintain this reference.
Now in theory all that happens if you connect A/C and D/c grounds and all is working correctly is that you now reference your D/C system to external ground. Does no harm and the D/C side does not care. When you get a fault a few things can happen. As mentioned above if the shore power charge shorts with a fault that allows voltages to flow into the D/C neg and raise it above ground it can put a dangerous charge in the water. This can only happen if the shore power breakers also malfunction and the simplest way to avoid it is to add you own RCD where shore power feeds into the boat. The other, and in my view much more likely scenario, is that your boat is fine but there is a fault on the shore power earth line. Now if there are faults in any A/C equipment none of the breakers or fuses will trip and all the cases on the A/C equipment will be live and if the A/C ground an C/C neg are connected all your D/C and engine block will also be at A/C voltages. You may get enough leakage to the surrounding water to pop a trip but if you have one of those plastic flexible coupling without a bypass you may not even know - Until of course you do touch something grounded...
My preference is separate ground systems for A/C, Radio and corrosion with a floating D/C system. Also I ensure full onboard protection from RCD's and don't rely on shore power grounding for safety. The only thing connected to shore power is the battery charger, all equipment runs off the inverter (or if you have one can be done with an isolation transformer.
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Old 27-06-2021, 10:33   #10
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Re: AC DC common ground

Following. There was a similar thread some weeks ago, but some of the comments/answers confused more than they cleared up. Our boat was built in Asia, for the Asian market; now that we're around US power (USVI, and PR), we installed an Victron auto-sensing isolation transformer, to bump the US 110v marina powerpole power to the 230v(3-wire, not 4 wire US 220v!) the boat requires. We also kept the galvanic isolator in the system, although we've been told it is not required. Between the isolation xfmr and US power,and the (factory installed, but now 31 yr-old)bonding system, there is much to confirm things are correct.
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Old 27-06-2021, 13:43   #11
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Re: AC DC common ground

ALL grounds, ac and dc, regardless of voltage, must be tied to a common bus.
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Old 27-06-2021, 14:16   #12
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Re: AC DC common ground

Quote:
Originally Posted by James M B View Post
So we got new docks last year, and with the new docks came new fancy safe circuit breakers. (I'm sure government mandated)
Many boats that never had an issue were immediately tripping these new breakers.
The electrician that wired the docks said first thing to check for on our boats, was to make sure AC and DC grounds were NOT connected or sharing the same buss.
Clearly not an ABYC Certified Marine Electrician.
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Old 27-06-2021, 14:17   #13
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Re: AC DC common ground

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Originally Posted by roland stockham View Post
Also depends where the boat was built. May have changed but in Europe it was forbidden to connect the A/C ground to the DC neutral. .
Please provide a citation for this
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Old 27-06-2021, 15:06   #14
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Re: AC DC common ground

@roland_stockham #9
Quote:
Also depends where the boat was built. May have changed but in Europe it was forbidden to connect the A/C ground to the DC neutral.
DC neutral?? If you truly meant DC ground or B-, than the latest version of ISO 13297 that I hold (2012) contradicts your statement.

ISO 13297-2012
Quote:
4.4 The a.c. protective conductor(s) shall be provided with a final (single) connection to the hull of a metallic hull craft, or if the craft has a non-metallic hull, to the main grounding/earthing point of the craft.
roland_stockham #9
Quote:
If A/C comes aboard from shore power then the H2 or neutral goes back ashore with the ground. This creates a ground that is reference to the outside world. The marina should have a big copper stake in the ground to maintain this reference.
The "big copper stake" has nothing to do with fault clearance! Current is returned via the neutral in a USA 120V system. The safety ground (green wire) is a low impedance redundant conductive path back to the source, usually the secondary of the marina's transformer, to allow enough fault current to pass to trip a protective device somewhere in the circuit.

This is a seriously dangerous subject. Please, do not feel obligated to type something if you do not consider yourself well versed in the subject matter, as misinformation can do some actual harm if it is applied by a novice.
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Old 27-06-2021, 16:18   #15
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Re: AC DC common ground

Quote:
Originally Posted by CharlieJ View Post

This is a seriously dangerous subject. Please, do not feel obligated to type something if you do not consider yourself well versed in the subject matter, as misinformation can do some actual harm if it is applied by a novice.
Water and electricity. Sorta flows and keeps flowing.
Will tense muscle hence if with a grip, grip won't let go and flow will keep flowing until tripped.
I plan my projects but use an electrician to verify building of.
Seen the bruising on a friend. Burn bruising from electrical flow.
Up thumb along arm, around heart upto head, eye lids, down other arm and out of other thumb.
Even though simple. Double check, triple check.
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