I am working on a vehicle which has electronics equipment inside it. A neutral electrical charge of the ground connection is desired.

When I measured the negative terminal, which is connected to chassis and batteries negative, compared to the ground/earth wire in the wall socket, the voltmeter read ~ +6V DC. I also tried to measure the current from chassis to wall socket earth. The (cheap) voltmeter showed ~40 mA for a very brief moment, then zero milliamps, and the voltage difference was neutralized. Immediately after removing the current meter, the voltage difference would return to ~ +6V.

The system is: vehicle, batteries, solar charger, inverter. (and appliances but those were disconnected)

Sometimes I feel a spark when touching the vehicle while grounded.

The system is 12V. I'm not quite sure where the 6V comes from.

a) Why isn't chassis ground at zero potential? Isn't it being earthed via the rubber tires?

b) Is there a way to achieve a better earthing, e.g. a loose wire that hangs and touches the ground?

c) Is there something wrong with my configuration, or is this completely normal?


3 Answers 3


Aswer to a:

The car is not earthed trough the tires. Remember that sometimes someone get a static shock when leaving a car.

Answer to b:

Some people purchase and install a special flexible wire (carbon strap) that lays on the ground removing static charge. You can most probably find it in a car shop.

Answer to c:

Since the current drops to zero everything seems to be ok. So dont worry.

  • \$\begingroup\$ Hmm, I mentioned that the tires were conductive because I read it somewhere. And various reasons for static shock, e.g. that the seat/pants act as insulators. But this wouldn't explain why, when you stand on the ground, and then touch the vehicle, you can experience a static shock. In that case, I think it can only be that the vehicle is not grounded, i.e. the tires are not conductive. \$\endgroup\$ Sep 29, 2016 at 18:01

In addition to the earlier answers, don't assume that the ground terminal of a wall socket is at 0V relative to true ground. If you want a true ground reference, hammer an earth spike into the dirt (such as a flower bed or lawn), well away from any electrical installations.

(If you don't have a proper earth spike handy, try a large screwdriver, garden fork or anything else that's pointy and metal)


Rubber tires are excellent high voltage insulators and can even store a large static charge.

You will need to make differential measurements.

Dragging a chain or carbonized strap is often used to prevent static buildup on cars.

Keep in mind chargers use line filter caps to ground (Y2) with up to 0.5mA of ripple current which also have DC leakage to ground and transformers even SMPS have coupling capacitance and DC leakage (but very low). These Y2 caps and your current meter are producing the observed effect.

  • \$\begingroup\$ Depending on where you live and how recently the tires have been purchased, tire manufacturers have started adding conductive material to tires. \$\endgroup\$
    – vini_i
    Sep 29, 2016 at 16:40
  • \$\begingroup\$ It's about time. I asked my nephew, a top mgr in Michelin 5 yrs ago and he said not yet. They use e-beam gigavolt polymerization to cross-link strengthen the material in tires and this zaps out microscopic flaws like ESD zapped particles. Adding carbon or similar static dissipation material would have to be compatible with e-beam processing plants and not cause melting of passivator due to corona. ( I think) When Firestone first added braided wire steel belted radial tires they all eventually failed due to frictional tearing of strands and side bulges \$\endgroup\$ Sep 29, 2016 at 17:39
  • \$\begingroup\$ global-sei.com/technology/tr/bn75/pdf/75-09.pdf e-beams \$\endgroup\$ Sep 29, 2016 at 17:45
  • \$\begingroup\$ I tried googling for an image of a carbon strap, but found a bunch of irrelevant images. Is it something like this? \$\endgroup\$ Sep 29, 2016 at 18:05
  • \$\begingroup\$ most of them dont have carbon and are pure plastic 1e+12 R and useless but still sell ... search "motion sickness anti static strap car" in images i00.i.aliimg.com/wsphoto/v0/561935948_5/… \$\endgroup\$ Sep 29, 2016 at 18:11

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