# If two objects are grounded, why do they still need to be bonded? And if they are bonded, why does each still need to be grounded? (fire safety)

When transferring a flammable liquid, both the container and the receptacle are supposed to be grounded and bonded.

• If they are both grounded, shouldn't the change on each be equal to ground? And if so, why do they still need to be bonded?

• And similarly, if they are already bonded, then there shouldn't be any sparking when the come in contact. So why do they still need to be grounded?

• Bonding insures they are the same potential if grounding is an issue. Just common sense safety. – StainlessSteelRat Mar 14 at 16:02
• Why double insulate wires? – Andy aka Mar 14 at 16:21
• Bonding them prevents them from being at separate potentials from each other. The benefit of that should be obvious. Connecting at least one of them to earth potential is a good idea to make sure there is no static charge buildup. It does seem that connecting them both to earth is unnecessary but it may provide some slight extra margin of safety (or probably that is what they were thinking when they wrote the requirement). A case could be made that there should only be one connection to earth. – mkeith Mar 14 at 16:49

They are bonded together to equalize charge between them so no sparking will occur.

They are grounded to equalize charge between them and anything else that might come in contact with them like workers or other equipment (that should be grounded as well).

If they were just bonded together and not grounded you could get a large spark from external equipment providing a charge path to ground.

If they were just each grounded and not bonded it would be a better case, but sometimes the impedance between grounded objects isn't controlled well and bonding provides an extra measure of safety should a ground fail.

They need to be grounded to equalize the voltage between the container and the ground to prevent static discharge. The grounding wire also provided a current pathway to ground to provide a pathway in the event of static discharge

They need to be bonded to equalize the voltage between the source container and and the container to be filled and to provide a current pathway to ground

The other nice thing about grounding and bonding is the electrostatic discharge would go on the outside of the container, and then through the wire.

If there wasn't a grounding or bonding wire, the pathway could potentially be through the flammable liquid (if conductive) or through the hose, thus creating sparks near the nozzle and air inside the container that is most likely saturated with gas from the liquid that is being poured. You need both for a current pathway. I wouldn't see much difference if you grounded both containers instead of bonding one to the other.

In order to reduce any stray vapour charge voltage in the air, the tanks, earth AND fluid need to be at the same potential. ( or as low as possible below flashpoint or LEL).

All fluids are dielectrics and all dielectrics have capacitance. Charge can be created by tribolectric friction of some materials and depleted by fluid transfer (charge transfer). Neglecting the former, I assume;

$$\Ic=C\cdot dV/dt+V\cdot dC/dt\$$ where dC/dt is the charge depletion rate of fluid flow from some unknown charge Q=CV.

I expect detonation to occur with some threshold above breakdown voltage in air Vth but this threshold gets reduced by Ic from a high dV/dt which is related to current and limited by V/R of the interface.

One usually controls ESD in workplaces by a selected R of 1M to reduce the Ic=V/R for some kV/Megohm where Ic=CdV/dt for a HBM of 100pF and 300pf for a cart with R bonded to earth ground.

Since vapours of combustible gas have a lower explosive limit (LEL) such as H2=4%, this level may be exceeded by some electric field of kV/mm or V/um if the dielectric voltage rises from the above fluid flow effects.

simulate this circuit – Schematic created using CircuitLab

I don't know what OSHA recommends for confined air spaces but one smells explosive fluid in the air in an enclosed space with > 4% H2 , I would make connections with high resitance 1st to neutralize charge then clamp to keep at 0V difference, like ESD wrist and heel straps at low < 1mA current ( 1Meg) then clamp with low R when walking around.

• I wonder what is wrong with this answer (-3) disgruntled EE's – Sunnyskyguy EE75 Mar 15 at 1:27