DC diagram review

[rgleason]

Picking fuses and circuit breakers is not a simple project. The ANL fuse blow point is much higher than other fuses. See Blue Seas Choosing Circuit Protection

[flee27]

Spot-The pictures represent MRBF fuses (marine rated battery fuse)

Sparx-I think in my situation the cut off switches will service me better. The bus bar location would be harder/more time consuming to get to in an emergency (windless sticks on). I would have more flexibility in the mounting locations for the cut off switches so they are easily accessible.

smak999-I agree on the MRBF fuse capability for the main feed. I would change this to a class T most likely.

Thanks for all the suggestions and opinions. I will verify all the wire gauges, devices, and connections are up to the job.

Keep in mind this is only the DC side of things. I will lay out my AC side separately but welcome all comments and suggestions on that also.

Foster

[goboatingnow]

Originally Posted by MerMike

The theory behind requiring a separate AC case ground is that it provides a more direct path to AC ground for a fault than via the DC case ground.

The AC ground, the DC ground and the DC case ground should be tied together at the DC ground bus, but the committee decided that that is not a sufficiently direct path and that a separate wire from the case to AC ground is needed.

We don’t tie ground together this side of the pond anyways.

Logic would suggest tying the case to earth is superfluous given the inverter is not part of the shore ( earth referenced ) mains supply

The same logic would suggest your vhf radio case should be earth connected

[CharlieJ]

Case grounds.

The case ground is a “grounding” conductor in that it is not normally a current carrying conductor. It’s function is to provide a heavy duty redundant path back to B- should a DC fault occur internally. If there was an internal DC ground faultand the normal B- conductor was compromised, the AC safety ground would be.the return path and it would not be able to carry the DC. Melting insulation and other damage would ensue.

[goboatingnow]

Originally Posted by CharlieJ

Case grounds.

The case ground is a “grounding” conductor in that it is not normally a current carrying conductor. It’s function is to provide a heavy duty redundant path back to B- should a DC fault occur internally. If there was an internal DC ground faultand the normal B- conductor was compromised, the AC safety ground would be.the return path and it would not be able to carry the DC. Melting insulation and other damage would ensue.

This doesn’t make sense (a) it would assume that mains earth is connected to dc 0V , which is a recipe for impressed corrosion

And secondly a DC fault in a big inverter may not conduct to the case anyway ( mind isn’t connected to the dc input ) and could potentially cause 100A to flow via a connected earth system. No AC earth system on a boat can handle that , and that fault current would have to make its way back to the DC - terminal via a circuitous route

Again electrically grounding the case makes no sense and it never did.

[CharlieJ]

@goboatingnow #20

Well, it may not make sense to you but it makes sense to the ABYC and ISO and to all reputable manufacturers of this equipment.

From ISO 13297 AC Systems:
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.

From ABYC A-31 Battery Charges and Inverters:
31.7.5. DC Grounding Connections
31.7.5.1. The DC grounding conductor (see FIGURE1,FIGURE 2, FIGURE 3,and FIGURE 4) shall:
31.7.5.1.1 be connected from the metallic case or. chassis to the engine negative terminal or its bus, and
31.7.5.1.2.shall be of an ampacity equal to that of the DC positive conductor.
EXCEPTION: The DC grounding conductor may be one size smaller than the minimum size conductor required for the DC current-carrying conductors (see ABYC E-11, AC and DC Electrical Systems on Boats, “Allowable Amperage of Conductors For Systems Under 60Volts”) providing the overcurrent protection device in the DC positive conductor is rated no greater than 135% of the ampacity of the DC grounding conductor and the conductor is no smaller than 16 AWG.
NOTE: The DC grounding conductor is not necessary if the case is made of plastic or nonconductive material and the case completely isolates the DC conductors from any grounded metallic internal parts, and there are no grounded metal parts the ungrounded DC terminal could contact.

This doesn’t make sense (a) it would assume that mains earth is connected to dc 0V , which is a recipe for impressed corrosion

Not so. AC will very, very rarely cause stray current ("impressed") corrosion.

And secondly a DC fault in a big inverter may not conduct to the case anyway ( mind isn’t connected to the dc input ) and could potentially cause 100A to flow via a connected earth system. No AC earth system on a boat can handle that , and that fault current would have to make its way back to the DC - terminal via a circuitous route

Who is to say how an internal fault is going to manifest itself, hence the requirement. The bolded phrase above is precisely why the requirement exists for a robust case ground.

And if those requirements are not enough to sway you from your position; every single metal case marine battery charger and inverter/charger that I have installed including, ProMariner, Victron, MasterVolt, Xantrex, and Magnum require a case ground as described above.

[flee27]

Even if ones does not agree with the practice of grounding the cases of this equipment, why not do it as a precaution (not to mention the ABYC standards)? Is there a proposed negative other than a little time, wire and connectors?

Foster