Chassis is shocking us when connected to ungrounded shore

[slipperysam]

We have now felt this a couple times when opening the metal door of our recently converted bus. For context, our electrical system is largely based on Explorist.Life diagrams (e.g. this one) with a few diversions (24v battery bank, no alternator charging, both 12v and 24v distribution, and a 50a inlet (but just one leg)). In particular there is a single ground wire from chassis to negative bus bar. The Multiplus and SCC are also both grounded to the negative bus bar.

The house I'm renting is old, and the 15A outdoor outlet we had plugged into looked very old as well. I also confirmed the shocks: when plugged into this outdoor outlet, I stuck one end of my voltmeter into the ground and one onto the metal door: ~ 115V.

I then plugged into the outlet in our house's bathroom, which is presumably grounded. The differential went down to ~5V.

My thoughts are that (1) the first outlet likely wasn't grounded and (2) the multiplus just senses that I'm connected to shore power and its internal transfer switch primitively assumes that the shore has bonded neutral to ground, so it keeps them separate, when really in this case they _should_ get bonded together to avoid this "rv hot skin".

Are my thoughts correct? Can anyone help me understand what is happening here and what a potential fix looks like? For my edification, hypothetically what would happen if I just had a rod from my chassis that I spiked into the earth to avoid a differential?

 

[Pierrot]

Hello, have a look at this chapter: 7 : Ground, earth and electrical safety

 

[tigerwillow1]

The fix is that the chassis of the bus need to be earth grounded. Required, not an option. The "normal" way to do this is via the ground pin of the outlet the bus is plugged into, and the ground wire in the power cord. It is unsafe to be plugged into an ungrounded outlet. A single wiring failure in the bus could make the chassis hot with potentially lethal current. With a proper ground in place, this same failure would instead trip a circuit breaker. Even with no wiring faults, the ungrounded chassis will be hot with tingle-level current from capacitive coupling in the wiring and leakage in some electrical devices in the bus.

 

[Zil]

Do not plug into that outlet ever again. You or your family could die. With a shore connection the chassis must be connected to a true earth ground. To the utility company's approved earth ground. You could have that outlet replaced and rewired from the service panel with the proper 2+ ground wiring.

 

[slipperysam]

Hello, have a look at this chapter: 7 : Ground, earth and electrical safety

I have read that, and re-read it for good measure. I still find it odd that my bus was almost exactly 120V from the earth. The neutral from the ungrounded outlet was presumably bonded to earth ground at the main panel of the house. So, the neutral should have the same potential as the earth and the hot is 120V from that. In my bus, the hot and neutral should remain the same, and the chassis ground of my bus is essentially disconnected from that entire system (earth, neutral, hot). Is it just a coincidence then, that the earth and chassis ground differ by 120V?

 

[slipperysam]

The fix is that the chassis of the bus need to be earth grounded. Required, not an option. The "normal" way to do this is via the ground pin of the outlet the bus is plugged into, and the ground wire in the power cord. It is unsafe to be plugged into an ungrounded outlet. A single wiring failure in the bus could make the chassis hot with potentially lethal current. With a proper ground in place, this same failure would instead trip a circuit breaker. Even with no wiring faults, the ungrounded chassis will be hot with tingle-level current from capacitive coupling in the wiring and leakage in some electrical devices in the bus.

Interesting. Are you basically saying that my idea of attaching a rod to the chassis to drive into the earth is a reasonable idea? I thought it outlandish because I've never heard of anyone else installing this in their mobile dwellings. Would this only be relevant for an ungrounded shore outlet? EDIT: sorry I just reread and realized that this is still outlandish. You're saying the ground from shore power is the fix. I agree with that! It would still be good to have protection against a faulty shore power ground, since in many places this is unknowable, if you are just parking at a random campground. Is there any common solution for that?

So presumably the "capacitive coupling in the wiring and leakage in some electrical devices" is the reason the chassis was ~115V difference from the earth? Do you have any theory as to why the difference was 5V from the earth when connected to the grounded outlet? I would have hoped it would be zero.

 

[slipperysam]

Could I protect myself from improperly grounded shore outlets in the future with a GFCI breaker? Either as my main breaker in my AC panel or just an inline breaker on the shore inlet before the Multiplus?

 

[time2roll]

Start by checking the polarity of the outlet. 3 prong outlet tester for $10 at any hardware store needs to be in your toolbox.

GFCI is not intended to replace the ground connection. Fix the ground at whatever cost or replace the outlet with an old 2 prong and find a grounded outlet for the RV.

For real protection get a Progressive Industries full EMS and surge protector. This will not allow power to connect unless all voltage, polarity and ground is correct.

https://www.progressiveindustries.net/our-products

 

[tigerwillow1]

At a very minimum, a responsible RVer checks a campground outlet with a 3-light tester before using it. As noted above, an EMS automates this process and also verifies the quality of the power. In theory a GFCI would protect you, but I would not advise taking the risk on theory. I do however advocate plugging an RV into a GFCI outlet whenever possible because it can give an early warning of failures. It's fairly common for failing water heater and refrig heating elements to have leakage to ground, and once it starts it's not going to get any better. But any electrical consumer can have faulty leakage to hot or ground.

As far as the 5 volt difference to earth ground when plugged into a grounded outlet goes, you're right that it in theory it should be zero difference. In reality different earth points can be at different potentials. I'm a bit surprised by the difference being 5 volts and would at least make an attempt to see why it's that large a difference. If everything in the bus was perfectly balanced and insulated, the ungrounded chassis would float at the midpoint between the hot and neutral voltages. In reality they tend to float below the midpoint, if for no other reason that there is likely a little resistive leakage through the tires to earth ground, and a capacitive coupling between the chassis and earth ground. I measured my former 5th wheel and current small motorhome for this and if ungrounded they both floated at about 40 volts. If you're parked near a power line, the ungrounded chassis could be really hot, voltage wise. I once visited somebody whose house backed up to a very high voltage transmission line. He could hold up a fluorescent tube from his back deck, and when parallel to the line, the darn thing lit up dimly.

 

[Tecnodave]

Serious advice.....

Run a 3 wire 10 guage service to a dedicated outlet for the R.V. Use a NEMA TT-30 socket and connector to plug in your bus to shore power. This TT-30 connector is very specifically designed for travel trailers and RV’s

This is exactly how all marketable R.V.’s are wired to code.....and yes, you do need to be “code” even if it is not a building.......your are living in it.....

 

[Pierre]

We are fortunate in ZA that we use a 230v 50Hz single phase grid supply and therefor do not have to create artificial midpoints / split phases etc. All appliances are rated at a nominal 230v. Much thinner wiring and lower currents to supply same rated loads at 120v.
As far as vehicle / RV installations are concerned , the supply at the campground is in the form of a 3 wire supply i.e.
Live , neutral and earth. The earth wire is connected to the chassis of the RV and live / neutral run through a 30mA earth leakage unit in the DB.
When building DC equipment for RV’s I never use the chassis as a negative conductor and always run two wires everywhere. The negatives are never tied to the chassis and when using a DC/DC converter from the vehicle supply it is always one with isolated in/outputs. When installing an inverter in the RV only the casing ( if metal ) is connected to chassis ground and neutral is not internally connected to the casing.

 

[12VoltInstalls]

Start by checking the polarity of the outlet. 3 prong outlet tester for $10 at any hardware store needs to be in your toolbox.

Ya, there’s I think three things going on and polarity was the first thing that crossed my mind.

my bus, the hot and neutral should remain the same, and the chassis ground of my bus is essentially disconnected from that entire system (earth, neutral, hot). Is it just a coincidence then, that the earth and chassis ground differ by 120V?

There’s more to it than that.
So the neutral and bare/green are not connected in the bus in any way, right?
And you have a way of disconnecting/transferring Hot, Neutral, and Earth/green from the RV cord to onboard inverter, right?

unrelated issue but a lot of good resources for understanding in the later pages concerning ‘grounding.’

 

[TacomaJoe]

Here's my related experience. Short of reversed polarity on the old 15A outlet, I'd agree with capacitive coupling. In our traffic signal cabinets we have an RFI (radio frequency interference) filter that consisted of an inductor and capacitor to knock down the pops on nearby AM radios. Mind you this is 50 year old technology in the old mechanical controllers. Even today there are filters with a capacitor between the hot and ground. Not a problem with a 3 wire cord but if you only have a 2 wire, you are missing a bond and will measure 120VAC between the cabinet (chassis) and the earth ground. I suspect that your 5V is just inductive leakage in the open circuit (other end of the wire isn't connected). Just lay a length of wire near energized conductors and you can measure leakage.

 

[Rednecktek]

Interesting. Are you basically saying that my idea of attaching a rod to the chassis to drive into the earth is a reasonable idea? I thought it outlandish because I've never heard of anyone else installing this in their mobile dwellings. Would this only be relevant for an ungrounded shore outlet? EDIT: sorry I just reread and realized that this is still outlandish.

Actually, I had wondered about something similar for things like the home brew solar generators on hand trucks and the like. Could you just have a metal spike with the ground wire? Stick it in the dirt once you place your cart and step on it to sink it in? It's a ground, right?

 

[Zil]

Actually, I had wondered about something similar for things like the home brew solar generators on hand trucks and the like. Could you just have a metal spike with the ground wire? Stick it in the dirt once you place your cart and step on it to sink it in? It's a ground, right?

Not nearly close to good enough. Occasionally you may get lucky and it becomes a decent earth ground, but mostly it ain't!

 

[Jred]

Not nearly close to good enough. Occasionally you may get lucky and it becomes a decent earth ground, but mostly it ain't!

So how would you do it?

 

[Hedges]

Could I protect myself from improperly grounded shore outlets in the future with a GFCI breaker? Either as my main breaker in my AC panel or just an inline breaker on the shore inlet before the Multiplus?

Only if it is "all pole", like a portable GFCI.
Typical permanent outlet GFCI only interrupts the Hot wire, not the Neutral.
Which works great if Hot is the one carrying 120V, and Neutral is grounded.
If wired backwards, so Neutral is 120V, that GFCI won't do you any good.

GFCI doesn't fix the problem, only shuts off when it measures you getting a shock.

Robot or human?

 

[Alloy]

Could I protect myself from improperly grounded shore outlets in the future with a GFCI breaker? Either as my main breaker in my AC panel or just an inline breaker on the shore inlet before the Multiplus?

After having hot skin on/off for over a year friends had their 13yo RV catch fire at 2:00am and were woken up to someone pounding on the side. Fire investigation found a 110V wire shorting to the frame.

The shore power on my trailer runs through 50A AFCI/GFC breaker and a Progressive EMS. EMS has a remote display + switch so it can be turn off when running (unbonded) on a generator

Progressive Industries EMS-HW50C

Details page of Progressive Industries EMS-HW50C RV Hardwired Surge & Electrical Protector.

www.progressiveindustries.net

 

[Hedges]

An isolation transformer would make the RV immune to polarity issues of the shore power supply.
So long as "ground" terminal isn't hot, and connected to chassis.

 

[Zil]

So how would you do it?

Read post number 10 and post number 4 in this thread. If that is not enough read some Mike Sokol; https://www.rvtravel.com/?s=hot+skin
We are not talking about a stand alone generator, we are talking about connecting to the utility mains. The Grid.