Static electricity and grounding

At gas stations, we often see gas attendants touching a metal disc - an electrostatic discharge device - before lifting their guns to refuel. This subtle action is backed by a sophisticated science that concerns the safety of life and property. For refueling machines with complex structures and long-term exposure to flammable and explosive oil products, the internal static electricity and grounding system are the "lifeline" for their safe operation. This article will delve into why the static discharge line on the fuel gun is crucial, and how the grounding system of the entire refueling machine can build a solid anti-static barrier.

1、 The danger of static electricity: invisible sparks

Before explaining the specific design, we first need to understand why fuel dispensers are so afraid of static electricity.

(1) Generation of static electricity: When two objects of different materials rub, come into contact, and then separate, electrons will transfer, causing one object to carry a positive charge and the other to carry a negative charge, which generates static electricity. During the refueling process, the oil flows at high speed through the fuel hose and gun, causing intense friction with the pipe wall and generating a large amount of static charge. At the same time, vehicle movement and friction between personnel's clothing can also cause static electricity to be carried on the vehicle body and human body.

(2) Accumulation and discharge of static electricity: If these charges cannot be directed to the ground in a timely manner, they will accumulate. When the charge accumulates to a certain extent and the electric field strength exceeds the breakdown strength of the surrounding medium, electrostatic discharge occurs, producing electric sparks.

(3) Deadly sparks: In the gas station environment, the oil and gas escaping from the fuel gun mix with the air, making it highly prone to form an explosive gas environment. At this moment, a tiny electrostatic spark is enough to become the "fuse" that ignites this mixed gas, causing serious fires or even explosions.

Therefore, one of the core safety goals of fuel dispensers is to eliminate any electrostatic discharge that may cause ignition sources.

2、 Oil gun static discharge line: the "guardian" of the last line of defense

The fuel gun is a component that directly contacts the vehicle's fuel tank opening and is also one of the areas with the highest concentration of static electricity risk. The static discharge line on the oil gun (usually built into the refueling hose) plays a crucial role as a "guardian".

(1) Direct elimination of static electricity in oil flow: The static charges generated by the friction of oil flowing in the hose will be guided out in real time through the metal wires or semiconductor layers embedded in the hose, preventing the accumulation of charges on the oil gun and hose.

(2) Establish an equipotential bridge: During refueling, the metal part of the fuel gun may come into contact with the vehicle's fuel tank opening. Due to the insulated rubber of vehicle tires, the body of the vehicle may carry static electricity while driving. The electrostatic discharge line of the fuel gun passes through the fuel dispenser body and is ultimately connected to the ground. When the oil gun contacts the fuel tank opening, this wire builds a "charge bridge" between the vehicle body and the ground, placing the vehicle body, oil gun, and ground at the same potential (equipotential). In this way, even if there is an electric charge, there will be no violent discharge sparks between the two, but they will flow smoothly towards the ground.

(3) Ensuring personnel safety: Similarly, if a refueling attendant carries static electricity on their body, when they come into contact with a grounded fuel gun, the charge will be safely released through the gun's grounding system, avoiding the danger of sparks generated by human contact with the fuel tank opening.

The resistance of the oil gun to ground should not exceed 10 ohms. This strict resistance value standard is to ensure that the static discharge path from the oil gun to the ground is sufficiently "unobstructed". The smaller the resistance, the faster and safer the discharge of charges. If this wire breaks or has poor contact, it is equivalent to cutting off this lifeline, and the risk is self-evident.

3、 The overall grounding system of the fuel dispenser: a tightly sealed "safety net"

The static discharge line of the fuel gun is only the endpoint, and behind it is a tightly grounded "safety net" that covers the entire fuel dispenser.

(1) Grounding of fuel dispenser body:

Independent ground wire: The fuel dispenser should be equipped with a separate ground wire, and the resistance to ground of the fuel dispenser should not exceed 4 ohms. This means that each fuel dispenser must have a dedicated line directly connected to the grounding electrode (a metal body buried in the ground) to ensure that the potential of the machine itself is consistent with the ground. This is the cornerstone of the entire grounding system.

(2) Pipeline and flange cross connection:

The fuel delivery pipeline of the refueling machine is composed of multiple metal pipes connected by flanges (connecting plates). If the gasket between the flanges is insulated, it will block the flow of charges, causing charges to accumulate on a certain section of the pipeline.

Both ends of the flange of the pipeline should be connected to a jumper wire, which ensures that even after passing through multiple flanges, the entire oil pipeline is electrically continuous and conductive, and static charges can flow unobstructed to the grounding electrode.

(3) Grounding of power and signal systems:

After the cable is connected to the explosion-proof junction box, the plug should be tightened to ensure explosion-proof performance. The ground wire (PE wire) in the power cord provides working and safety grounding for the electronic devices of the fuel dispenser (such as the motherboard and power board), preventing the shell from being charged in case of equipment leakage or failure.

The wiring connection of the control cabinet also requires that the grounding wire (GND) must be correctly connected, which provides a common reference potential and fault current discharge path for all electrical control units.

(4) Equipotential connection:

The core of the entire design concept is equipotential bonding. By reliably connecting all conductive parts such as the fuel dispenser casing, oil pipeline, electrical equipment casing, and oil gun together and ultimately connecting them to the same grounding system, it is ensured that there will be no significant potential difference between these components under any circumstances, thereby fundamentally eliminating the possibility of internal sparks.

4、 Daily maintenance and safety awareness: an essential aspect that cannot be ignored

Even the most perfect design requires proper maintenance to ensure.

Installation and maintenance: Explosion proof tools must be used during installation to prevent the generation of mechanical sparks. After installation, the grounding resistance must be strictly measured to ensure that it is less than the standards of 4 ohms (fuel dispenser) and 10 ohms (oil gun).

Personnel protection: Staff are required to wear anti-static work clothes and gloves (mentioned in tool preparation) to prevent human body from becoming a static power source.

Daily inspection: Regularly check the conductivity of the static electricity outlet line of the oil gun, and inspect each grounding connection point for rust or looseness.

Conclusion

The static electricity and grounding system of a fuel dispenser is a comprehensive and three-dimensional protection project from macro (the entire machine and the ground) to micro (oil flow friction). The seemingly ordinary static discharge line on the fuel gun is the nerve endings of this "lifeline". It, together with the independent grounding wire of the fuel dispenser, the flange jumper of the pipeline, and the protective grounding wire of the power supply, forms an invisible safety net. This net silently and continuously introduces dangerous static charges into the earth, guarding the calmness and safety of every refueling. Understanding and respecting this system, installing, using, and maintaining it strictly in accordance with regulations, is not only the responsibility of technical personnel, but also a reflection of the safety awareness of every gas station practitioner. It is the most reliable barrier between us and potential dangers.