Chassis Grounding in DC only system

[dsaint1884]

I am building a camper van setup and plan to run 4x100w 12v solar panels in 2s2p. This will bring the panels up to 24v with an Voc of 48.6v. (I mention this just because I've heard that 50v DC is where you need to be more concerned about grounding).

Anyway, right now I'm running every negative wire back to the batter/bus bar. I ran duplex wire so everything is run with two wires, the positive to the load, and the negative back to the battery. I have no chassis grounding, I've not grounded the solar panels, or anything else so far. It's all just a big loop trying back to the battery. Right now there is no shore power, DC-DC charge from the alternator or an inverter in the system. If I add AC to the mix I'd ground that system but that's a different animal than DC.

Do I need a chassis ground anywhere in this DC only system for safety? Alternatively, should I just stick to parallel solar panels and the lower voltage so I have less to worry about, if it's a worry at all?

I've not included any fuses or switches in this diagram but this is what it would look like to start as far as the path of all the circuits.

 

[Bobert]

Do I need a chassis ground anywhere in this DC only system for safety? Alternatively, should I just stick to parallel solar panels and the lower voltage so I have less to worry about, if it's a worry at all?

Grounding tithe chassis insures that if you have a positive wire shorted to chassis or a shorted to chassis device you will blow a fuse on that circuit instead of making you chassis potentially intermittently live . It also makes test wires easier as any metal part of the body can be used as a ground when testing wires. I would recommend grounding it. If you are using a pwm charge controller you want to have your panels parallel. If you are useIng an MPPT controller serial is ok.

 

[dsaint1884]

Grounding tithe chassis insures that if you have a positive wire shorted to chassis or a shorted to chassis device you will blow a fuse on that circuit instead of making you chassis potentially intermittently live . It also makes test wires easier as any metal part of the body can be used as a ground when testing wires. I would recommend grounding it. If you are using a pwm charge controller you want to have your panels parallel. If you are useIng an MPPT controller serial is ok.

I think I get what you're saying. So if I have a 40a fuse in place and I somehow become inside the circuit due to a shorted wire touching the chassis/body of the vehicle then it would cause the fuse to blow and prevent shock.

Maybe I'm missing something but in an isolated DC system it seems like it would be really difficult to find myself connected inside the circuit to both positive and negative. On the other hand, once I connect ground to chassis then any shorted wire would have a negative terminal to run to. It almost seems like the negative ground on the chassis gives the positive short a circuit to run through but without running that ground wire nothing would happen at all and it would be just like touching the positive terminal of the battery in isolation - nothing happens. That's clearly wrong based on your reccomendation, I'm just trying to better understand the mechanics behind it.

 

[Vigo]

So im an auto mechanic (ASE Master, certified on hybrids blah blah) and in my roughly 2 decades of sweating heavily while working on 12v systems i’ve ‘felt’ 12v many times. Some of those times my arm actually touched both positive and negative terminals on an actual battery, but other times i was ‘connecting’ some voltage to chassis ground. In those cases i would NOT have been forming a connection if there WAS NO chassis ground. And that’s exactly what’s done with actual high voltage systems, total isolation with no chassis ground, and monitoring to make sure it never happens or disable the system if it does. So there is a way of thinking that says more ground to touch means more chances to form a human circuit with the positive side. But i wouldn’t consider it a serious enough issue to rule out doing it for other, possibly more important reasons, at least on a 48v system. I personally think the increased danger of ‘accidental welding’ with nice/expensive parts bothers me more than the risk of ‘48v of brief discomfort’, but then as a mechanic ive taken it upon myself to constantly lift 5000lb objects and then stand under them pushing on things, so your personal risk assessment may vary.

 

[Bobert]

I think I get what you're saying. So if I have a 40a fuse in place and I somehow become inside the circuit due to a shorted wire touching the chassis/body of the vehicle then it would cause the fuse to blow and prevent shock.

I wasn’t particularly worried about shock it is possible with 12v but rather unusual. In a camper van you are likely to have multiple 12v devices in use at any given point. It’s pretty hard to keep a system completely isolated in a metal shell. All it takes is one screw touching between any 12v device that has a grounded case and a metal part of the vehicle and you have a partial ground. If positive from your house system accidentally makes the frame of the van “live” you will now measure 24v between the negative of the house battery and the positive of the van battery. Trouble shooting wiring problems can get pretty squirrely if this has happened. It’s just better all the way around to take the guesswork out and ground the house system.

 

[Vigo]

As far as electrocution goes, sadly there is not really any kind of ‘protection device’ like a fuse or breaker that protects the human body, they just protect the electrical sustem itself from melting or catching fire etc. The amount of current that needs to pass through your body to kill you is far smaller than basically any circuit hooked directly to a battery. It takes milliamps to kill you (in a perfect storm scenario), and the smallest fuses youd find on a typical 12v system are rated for thousands of milliamps.

All the ‘protection’ gets done on the human side. Insulated gloves (all gloves offer SOME insulation) and other workwear, insulated tools in some cases (all plastic handles offer SOME insulation), and most importantly, knowing what will happen when you touch something before you touch it, aka training and knowledge. But with 12v systems the risk is very low. Ever held a 9v battery to your tongue? You’re 75% there!. With 48v the risk is.. mild. Generally with dry skin you can touch 48v and whether it even qualifies as pain is open to interpretation, but what’s REALLY important is what parts of your body current passes through! If it goes through skin and muscle, ouch whoops dont do that again. If it passes through your heart or more central parts of your nervous system it can be life threatening. Basically all training will say ‘treat any source of potential electrouction as possibly life threatening’ but the fact that DIY hardware has sort of ‘settled’ around 48v max while the systems car manufacturers bury and protect deep in the vehicle are 300-800v, is a pretty strong insinuation that 48v systems are not very likely to kill you. Still, best to have some idea of how not to become part of the circuit, and reasonable precautions to take.

 

[dsaint1884]

In those cases i would NOT have been forming a connection if there WAS NO chassis ground.

This was sorta my line of thinking...if I connect a chassis ground then any time I'm touching the vehicle and there's an accidental positive connection, now I'm in the circuit, right? Where if there is no chassis ground I'm just touching positive only and there is no issue there. It seemed like not grounding was better and it would make sense just to isolate the system. I suppose this might already exists if the van has chassis ground from the van system itself.

I personally think the increased danger of ‘accidental welding’ with nice/expensive parts bothers me more than the risk of ‘48v of brief discomfort’

Could you elaborate on this more? I'm not worried about the electronics or parts as much as I am human safety for both myself and others who may touch the van.

 

[dsaint1884]

All it takes is one screw touching between any 12v device that has a grounded case and a metal part of the vehicle and you have a partial ground.

I appreciate your help so please forgive me as I'm trying to learn....if something is touching a metal part of the vehicle and creates a partial ground, does that mean I'm already creating a chassis ground, albeit unintentionally? or are you referring to the positive side likely touching some metal part of the van?

If positive from your house system accidentally makes the frame of the van “live” you will now measure 24v between the negative of the house battery and the positive of the van battery. Trouble shooting wiring problems can get pretty squirrely if this has happened. It’s just better all the way around to take the guesswork out and ground the house system.

Thank you, I understand your push for grounding the house system. I don't doubt what you said, I'm just a bit slower than others at this so I'll have to spend more time thinking about why that creates 24v between the negative of one battery and the positive of another. In my mind they are isolated systems.

 

[dsaint1884]

Grounding tithe chassis insures that if you have a positive wire shorted to chassis or a shorted to chassis device you will blow a fuse on that circuit instead of making you chassis potentially intermittently live.

I spent some more time thinking about this part. What I think you are saying here is that if the positive wire connects to the steel frame of the van and makes a connection with the chassis ground, the idea is that it's likely to blow the fuse on that line and shut down the power. So in theory as long as I'm not in the middle of it at the time then the circuit would go dead and I would know I have a wiring problem. I should also know which wire is to blame, because it would be the one with the fuse that blew. Am I seeing that as the advantage you were trying to highlight in chassis grounding the system?

Thank you again for your feedback and for helping me learn.

 

[Bobert]

I appreciate your help so please forgive me as I'm trying to learn....if something is touching a metal part of the vehicle and creates a partial ground, does that mean I'm already creating a chassis ground, albeit unintentionally? or are you referring to the positive side likely touching some metal part of the van?


Thank you, I understand your push for grounding the house system. I don't doubt what you said, I'm just a bit slower than others at this so I'll have to spend more time thinking about why that creates 24v between the negative of one battery and the positive of another. In my mind they are isolated systems.

When you take the positive post of a 12v battery and connect it to the negative post of another 12v battery you place them in series and the voltage across the 2 remaining posts will measure 24v. Accidentally grounding a positive wire from your house battery will create a series voltage between the negative of the house battery and the positive of the chassis battery. If the house battery is grounded to the chassis, accidental connection between a positive wire will result in a blown fuse or breaker ( if they are properly sized) You will know you are miswired and the affected circuit will be relatively easy to trouble shoot.
Not all 12v devices are connected internally to the negative wire of the incoming power but some are. A car radio for instance usually has a case that is connected directly to the incoming ground wire and will cause the negative wire to make a connection with the chassis if it is screwed to it in addition the antenna will also connect the radio to the chassis in most cases. If the positive wire of your “isolated” house battery was to come in contact with the chassis of your van in this situation it could cause the ground wire on the radio to melt instead of blowing the fuse of the faulty circuit. This can be a fire hazard although not statistically likely. The more likely scenario is that you will eventually have a confusing electrical problem that would be straightforward to repair if the negative terminal of house battery was grounded to the chassis.

 

[dsaint1884]

When you take the positive post of a 12v battery and connect it to the negative post of another 12v battery you place them in series and the voltage across the 2 remaining posts will measure 24v. Accidentally grounding a positive wire from your house battery will create a series voltage between the negative of the house battery and the positive of the chassis battery. If the house battery is grounded to the chassis, accidental connection between a positive wire will result in a blown fuse or breaker ( if they are properly sized) You will know you are miswired and the affected circuit will be relatively easy to trouble shoot.
Not all 12v devices are connected internally to the negative wire of the incoming power but some are. A car radio for instance usually has a case that is connected directly to the incoming ground wire and will cause the negative wire to make a connection with the chassis if it is screwed to it in addition the antenna will also connect the radio to the chassis in most cases. If the positive wire of your “isolated” house battery was to come in contact with the chassis of your van in this situation it could cause the ground wire on the radio to melt instead of blowing the fuse of the faulty circuit. This can be a fire hazard although not statistically likely. The more likely scenario is that you will eventually have a confusing electrical problem that would be straightforward to repair if the negative terminal of house battery was grounded to the chassis.

This all makes a lot of sense to me. Thank you again for the time helping me understand a bit better.