Bus Switching - Best Practice?

[Wotname]

Disclaimer - I could well be overthinking this!

What is considered best practice for a bus interconnect switch wiring (AKA Emergency Connect etc)?

The blue option or the red option on the attached schematic?

While it is normal to have the interconnect switch between the load sides of the house & start circuits, with the inclusion of a charge bus, there are two places downstream of the house bank where the interconnect could be considered suitable. I see more situations using the red option but I feel the blue is better.

Thoughts?

EDIT - Assume house and start battery chemistries are the same.

[Reefmagnet]

I could be reading your diagram wrong, but the issue with the blue connection is you need to switch both the interconnect and house bank off to isolate your house electrical system from supply. I know this because I kind of have the blue system with a VSR connection (a temporary situation waiting for the LFP's to go in once the 8yo AGM's give up the ghost, the usual system being a manual m/o switched contactor override) and my house power stays on after I've turned the house bank off until the VSR shuts down. I do have a switch that isolates the start battery from the engine, but the VSR lies between the switch and start battery.

[Wotname]

Originally Posted by Reefmagnet

I could be reading your diagram wrong, but the issue with the blue connection is you need to switch both the interconnect and house bank off to isolate your house electrical system from supply. I know this because I kind of have the blue system with a VSR connection (a temporary situation waiting for the LFP's to go in once the 8yo AGM's give up the ghost, the usual system being a manual m/o switched contactor override) and my house power stays on after I've turned the house bank off until the VSR shuts down. I do have a switch that isolates the start battery from the engine, but the VSR lies between the switch and start battery.

Perhaps I haven't drawn it clearly enough or explained it well enough but there is no connection between the start and house batteries unless the Interconnection is ON. It's default position is OFF.

[Reefmagnet]

I understand that. The issue is you can't isolate your house supply in an emergency situation if you need to use the house bank for your engine circuit.

[Wotname]

Originally Posted by Reefmagnet

I understand that. The issue is you can't isolate your house supply in an emergency situation if you need to use the house bank for your engine circuit.

The house bank isolation switch will isolate the house bank in any circumstance. The start battery normally provides for the engine circuit.
The interconnect switch is primarily there should either the house or the start battery fails. If either one fails, then really there is only one battery suppling everything and if that fails, then it is all over - electrically speaking.

Or am I missing something basic???

[Reefmagnet]

Apologies. Distracted watching the footy is my excuse. I mistook your house load block for battery bank. This actually makes blue more suitable than red, but I'd still say that if the intention is for emergency use, you want to be able to isolate as much extraneous circuit as possible, and connecting directly to the battery (after it's fuse, of course) is the best choice. The logic being you don't lose any thing in the process for what is essentially a permanently wired jump starter.

[zstine]

see attached for another example...

[Wotname]

Originally Posted by zstine

see attached for another example...

In essence, this is the same as the RED option in my OP. However this example has some minor differences, namely it has only a single alternator and a VSR.

The setup in the OP has a normally independent house and engine circuits each with its own alternator.

[Lake-Effect]

What is the "engine switch"?

If you have a start "emergency", it would most likely be because the start battery is discharged or defective... in which case I would not like to now dump the house battery into it. So instead of the engine switch and interconnect switch, I would replace them with a single 1-2-both switch that chooses whether the start or the house batteries are connected to the engine.

And you have the 'both' position for when you need to power the house panel from the start battery.

[Wotname]

Originally Posted by Lake-Effect

What is the "engine switch"?

If you have a start "emergency", it would most likely be because the start battery is discharged or defective... in which case I would not like to now dump the house battery into it. So instead of the engine switch and interconnect switch, I would replace them with a single 1-2-both switch that chooses whether the start or the house batteries are connected to the engine.

And you have the 'both' position for when you need to power the house panel from the start battery.

Engine switch connects the start battery to the engine (and its associated circuits e.g. engine instruments, starter motor, engine alternator).

The mere action of posting the question on CF has helped me to clarify my thinking about the possible arrangements. At this stage I will most likely to opt for the BLUE option as that allows for the connection of the charge bus to the start battery while maintaining the ability to disconnect the house loads. Using the RED option requires the house loads to be ON should one wish to connect the house charge bus to the start battery.

Note, the default arrangement has completely seperate charging between the house and start.

[ayates]

Here is an early version of my plan...

[smac999]

Neither. Before the engine switch.

If I turn the engine switch off I want %100 no power to engine if someone is working on the engine.

In your diagram it’s possible to have power to the engine with the engine switch off which is dangerous to mechanics

Using a 1-2-all switch to feed engine is safer. Fed from both batteries. And it’s also Becomes the Parellel switch

[Wotname]

Originally Posted by smac999

Neither. Before the engine switch.

If I turn the engine switch off I want %100 no power to engine if someone is working on the engine.

In your diagram it’s possible to have power to the engine with the engine switch off which is dangerous to mechanics

Using a 1-2-all switch to feed engine is safer. Fed from both batteries. And it’s also Becomes the Parellel switch

Interesting and .... surprising.

Surprising because I usually find myself in lockstep with your posts about electrical matters yet this time I am not.

Interesting because your suggestion highlights the different possible perspectives / requirements of bus switching. My proposed bus switching allows for house bank isolation, house load isolation, start battery isolation and 'emergency' tie from either battery to both loads. Obviously the default position of the interconnect switch is OFF. Should either battery fail, it can be quickly removed from circuit and it's load provided by the other battery.

I understand the rationale behind your method and I can see why some might like it and use it however the following reasons against it hold more weight for me than the advantages you proposed.

• If I have understood your description correctly, there is no quick way to switch the house loads to the start battery should the house bank fail.
• I have never had a mechanic work on any of my boats - YMMV.

[Reefmagnet]

Id be inclined to agree with smac. If you have a major issue with your house bank, there's a chance it may not necessarily be related to the batteries themselves. The last thing you want is to introduce your last remaining good battery that your engine depends on to a potential problem.


The better solution, I reckon, is to have an independent second house bank.

[Dockhead]

Yet another possible approach: Keep them separated; charge the start batt with an echo charger. Emergency interlink by jumper cables.

My boat is a little different because house is 24v and engine is 12v with separate alternators and mains chargers, and there is no connection of any kind between them.

We all need a backup method of starting, however, so I keep jumper cables (which I've never used for starting the main engine in 12 years with this boat). The advantage of this vs. a hardwired system is that you cannot accidentally interconnect it -- it takes a deliberate act to connect the jumper cables.


Another advantage is that with jumper cables you are not necessarily interconnecting a good battery with a bad one, which is usually a bad thing. You can disconnect the bad battery before jumping to the good one.


Previous boat had one of those infernal 1-2-both-off switches and it happened regularly that the switch somehow or another got put into the wrong position with various bad results. My advice is keep them as nearly totally separated as possible. Have you thought about a second alternator?