Alternator Service Switch

[Scott T-Bird]

On one of MaineSails wiring diagrams, there is an On/Off switch that is labeled "Alternator Service Switch". I'm assuming it could be a Blue Seas 9004E. It is in line between the alternator and an "unswitched/charge buss" for + terminations that also include leads from a solar charger and a.c. charger. This is a buss that connects the power lead from the House Battery Bank.

What is the purpose of this switch? My understanding is that the alternator field disconnect (AFD) is installed to protect the alternator diodes if the battery switch is inadvertently turned off. But in the diagram, the alternator output doesn't go thru the battery switch, so why would there be a desire to add a switch that isn't needed? I also understand that the AFD is only used with a external regulator (it appears there are F1 and F2 posts that take separate leads from the alternator and external regulator) and it works with a secondary single-pole switch to assure that alternator output current isn't interrupted when the primary switch is thrown off.

I am confused by the diagram because it appears that the alternator output current goes directly to the House Bank and to the Aux Bank (via ACR) without passing through battery switch(es). So why add the Alternator Service Switch? The diagram also doesn't show an external regulator, but I still assume that AFD will not function unless there is one. What am I missing? Explanations should be geared to a "3rd grade" understanding, please.

 

[LeslieTroyer]

It protect the guy working on the engine, or if you need to remove the alternator. The Alternator output in a 3 switch model is always hot (going directly to the house bank) so working in and around the alternator you could accidentally short the alternator output to ground with bad bad results. By installing and turning off the “alternator service switch” you can work safely around the alternator.

Les

 

[dlochner]

What @LeslieTroyer said. In order to remove the alternator safely the out put needs to be disconnected from the battery if not and the alternator output wire is removed from the alternator and it touches the engine block the full force of the battery bank will pass through the wire and ground to the block. A Big Bang, some smoke and skipped heart beats will result if lucky. Current will flow in either direction on the alternator cable. When the alternator is running current flows to the battery, but when the cable is disconnected, current is able to flow from the battery through the cable and back to ground if the cable touches the engine block.

With the service switch located near the alternator it is easy to safely disconnect the the alternator from the battery without having to disconnect the battery cable. Once the switch is turned off, there is no current flow from the battery to the alternator, thus if the wire touches the engine block nothing will happen.

I'm assuming it could be a Blue Seas 9004E.

Yes that would work or the 6000e (M series switches). The M series is a little smaller but up to the job. I installed an m series on my boat.

My understanding is that the alternator field disconnect (AFD) is installed to protect the alternator diodes if the battery switch is inadvertently turned off.

Correct, but if the alternator cable goes directly back to the battery, there is no need for the field disconnect as there is always a load on the alternator which prevents the voltage surge that occurs when the load is suddenly stopped.

The field voltage energizes the electro magnet that allows the alternator to generate electricity. The higher the field voltage, the more current than can be produced. This is important, but unrelated to the service disconnect. However, if while the engine is running and the alternator is producing electricity and you turn the switch off, then there will be a voltage spike and the diodes may well blow. Locating this switch in the engine compartment prevents the uninitiated from turning the switch off. Presumably any mechanic working on your engine would know enough to not turn the switch off while the engine is running. If they don't, they'll get to pay for rebuilding the alternator.

This article may help with understanding what an AFD does. https://shop.pkys.com/What-is-the-a...ct-terminal-on-a-battery-switch-for_b_26.html

If the alternator is wired to the battery switch it should use a "make before break" switch. This switch is set up so that when switching from one battery to another on a 1-2- both switch a battery is always connected. When changing batteries say from 1 to 2, there is a momment when both batteries are connected before the first one is disconnected, making the new connection before breaking the old connection.

Is this helpful?

 

[Scott T-Bird]

Very helpful! I wasn't thinking about the return current danger! I feel relief that I didn't find out the hard way! I think I'm going to make a label and stick it on the alternator for future reference!

 

[dlochner]

I feel relief that I didn't find out the hard way!

Yeah, but just imagine the story you could have told.....
maybe.

 

[Scott T-Bird]

Boy am I glad I asked this question. That is a scary thought. Warning is going on the alternator and over the switch cover!

 

[Scott T-Bird]

I don't have an external regulator, so I assume that the AFD is essentially non-functioning because I won't be able to detour the field wire. Therefore, if I have the switch off while working around the alternator, I must remember to turn it on before starting engine again, correct? Otherwise, diodes blow?

 

[Stu Jackson]

Yes.

 

[dlochner]

Therefore, if I have the switch off while working around the alternator, I must remember to turn it on before starting engine again, correct? Otherwise, diodes blow

I don't think so. The diodes blow because there is a spike in the output voltage. But that is an interesting question.

 

[Stu Jackson]

I don't think so. The diodes blow because there is a spike in the output voltage. But that is an interesting question.

Dave, it's "kinda like that." If there is nothing connected to the AO (alternator output) then the diodes could blow but there is no spike involved at all. In addition, in the olden days when folks had the battery switch in the AO line and the brother-in-law turned off the switch when the engine was running, sometimes the diodes didn't blow if the battery bank was full. If the bank was low and the regulator was calling for output then they would blow.

Scott, there should be a fuse at the battery end of the AO, whether you have a switch or not. I haven't gotten around to installing my switch yet, so I simply remove the fuse when working on or near the alternator.

 

[dlochner]

Dave, it's "kinda like that." If there is nothing connected to the AO (alternator output) then the diodes could blow but there is no spike involved at all. In addition, in the olden days when folks had the battery switch in the AO line and the brother-in-law turned off the switch when the engine was running, sometimes the diodes didn't blow if the battery bank was full. If the bank was low and the regulator was calling for output then they would blow.

Scott, there should be a fuse at the battery end of the AO, whether you have a switch or not. I haven't gotten around to installing my switch yet, so I simply remove the fuse when working on or near the alternator.

The issue may be whether the alternator is internally or externally regulated. If internal, the regulator would "see" the battery as not accepting a charge and would not excite the alternator and the alternator would not be producing any current. An external regulator would look to the battery and perhaps see that it needed a charge and would excite the alternator and cause it to produce current which had nowhere to go.

Where's @Maine Sail when we need him?

 

[jssailem]

Scott the switch is a convenient tool. You could just disconnect the batteries from your system. Thus the alternator would be disconnected from the batteries. But it would be more labor intensive. And thus sometimes skipped by skilled electrical workers until they have a Big Bang and short out the electrical system.

Some folk risk it with heavy tape wraps of the wire end.

 

[Stu Jackson]

If internal, the regulator would "see" the battery as not accepting a charge and would not excite the alternator and the alternator would not be producing any current.

A regulator is a regulator, doesn't matter where it is. Although external regulators can have separate battery sense wires, the AO is always above the resting battery voltage.

 

[Maine Sail]

Unless you want to tap into the brush pack on the stock alt then an AFD switch won’t help. This is why the service disconnect always gets mounted in the engine room near the alt. It should be clearly labeled.

With an external reg such as the Balmar MC-614 you simply place regulator B+/pos feed on the alt side of the service disconnect switch so the reg can’t physically boot with that switch open.

 

[Scott T-Bird]

Scott, there should be a fuse at the battery end of the AO, whether you have a switch or not. I haven't gotten around to installing my switch yet, so I simply remove the fuse when working on or near the alternator.

Thanks Stu, I have the standard 80A Hitachi alternator that came with my new Yanmar. The only work that I did was when I disconnected from the starter wiring and installed the alternator cables to the buss in the winter when the batteries were removed, which was just dumb luck because I probably wouldn't have even thought about the danger. I swear that I live under a lucky star (a good one!) because I think that I have escaped unscathed thru many dangers that I never knew existed! In any case, I'm sure that I have read about this issue in the past but it just didn't sink thru the skull until I finally became curious about the diagram. I've been interested in doing the electrical work both on the boat and in my home remodeling (in the past) because it seems like relatively logical, connect-the-dots type work. But a lot of it is still mysterious to me and it shakes me up a bit when I realize that I might overlook a critical aspect of it!

I do have a 100 amp fuse but I installed it at the + buss, which is about a 5' run from the alternator. I also intended to install the switch at that location because it is more convenient than finding a spot near the alternator. Based on MaineSails post about the switch location, I think I will have to re-think the locations of both the switch and the fuse. I used 4 AWG for pos and neg cables based on 10' round trip and 105 degree insulation in engine space. The calculator that I used (Blue Seas?) seemed to indicate this is suitable. I'm not sure if the fuse is a bit higher than recommended.

 

[dlochner]

I think I will have to re-think the locations of both the switch and the fuse.

Scott, keep the fuse near the battery. The fuse is protecting the wiring and the greatest source of current is the battery, not the alternator, so the fuse should be close to the battery. Placing the switch in the engine compartment is both a matter of convenience and safety. With a short run of cable from the alternator to the switch, it's purpose will be quite clear.

If I recall correctly the fuse should be 1.5 times the alternator's max output, which would be 120 amps for your Hitachi. You'll probably be OK with 100 amps because the Hitachi is a super dumb alternator (see Maine Sail's post on this) and you may never reach peak output or reach it for long.

 

[Ward H]

Size the wire to reduce voltage drop. Size the fuse to protect the wire.
I followed Maine Sail's advice and installed the Alt Service Disconnect switch right next to the alt so I wouldn't forget it or forget to tell the mechanic that the alt is wired direct to the house bank. I could have pulled the fuse when I worked near the alt but with that red switch right in my face I won't forget or take the lazy way out.
Here's a pic.