Introduction
Ground distribution for a wiring harness, also called grounding allocation, covers whether individual ground connections are needed or whether grounds can be shared. Power distribution and grounding design are core parts of harness design. Proper grounding ensures reliable power delivery and signal transmission; poor selection of grounding points can cause interference and impair component function. This article focuses on grounding allocation for wiring harnesses.
1 Single-wire system and negative grounding
The vehicle grounding system has two important concepts: single-wire system and negative grounding. A single-wire system means that each load is connected to the power source with a single conductor while the vehicle body, chassis, or engine metal structure serves as the common return conductor. Because a single-wire system reduces wiring, simplifies routing, and eases installation and maintenance, modern vehicle electrical systems commonly use it.
When a vehicle uses a single-wire system, one battery terminal is connected to the vehicle body, referred to as grounding. If the battery negative terminal is connected to the body, this is negative grounding; if the positive terminal is connected, it is positive grounding. According to national standards, vehicles in the Chinese market use negative grounding.
2 Types of grounding points
1) Power ground: the zero-potential at the battery negative post.
2) Chassis ground: conductive vehicle body panels, chassis, or engine parts that are electrically continuous across the vehicle.
3) Power-signal ground: the power feed returns for various electrical components. Based on the magnitude or waveform of the loop current, these can be classified as "dirty ground" or "clean ground".
Clean ground: grounding loops with peak currents below 1 A, such as sensor feedback or low-level control signals (for example, network communication).
Dirty ground: grounding loops with peak currents above 1 A, including PWM loads or switched loads above 1 A, such as motors and switching loads.
4) RF ground: used to control radio-frequency interference. These grounds are typically attached directly to body panels and must not be used to bypass any grounding currents.
5) Antenna ground: for antenna installations such as a radio antenna ground.
3 Ground allocation
Based on component installation locations, load types, and ground categories, assign grounding points to each component.
3.1 General principles
1) Ground locally where possible. Minimize ground loop length to reduce loop voltage drop, cost, and mass.
2) Minimize unnecessary interference between subsystems.
3.2 Mechanism of interference
Ideally each component would have its own local ground, but this would create too many ground points, complicating assembly and increasing mass and cost. Therefore shared or consolidated grounds are typically used. When combining grounds, potential interference must be assessed.
When only component 1 operates, I1 = U/(L1+R1); when only component 2 operates, I2 = U/(L1+L2+R2); when both operate, I1+I2 = U/(L1+R1) + U/(L1+L2+R2). If I2 is significantly larger than I1, the potential difference between component 1 and ground increases and may affect its operation.
R1 = resistance of component 1, L1 = conductor resistance from the stud to the ground point, R2 = resistance of component 2, L2 = conductor resistance from component 2 to the stud.
Design standards typically prohibit combining dirty and clean grounds on the same stud. If the difference between high and low ground currents exceeds 1/5, the two ground loops should not share a stud. This is a conservative rule; with detailed analysis and validation, sharing may be acceptable. To minimize interference from shared studs, reduce L1 by locating the stud close to the ground strap and increasing conductor cross-section where necessary.
4 Requirements for grounding points
1) Grounding points should be accessible for installation and maintenance, and the contact surface must meet torque requirements.
2) No more than two ground straps should be connected to a single grounding point.
3) Ground points should not be located in high-splash or water-collecting areas.
4) Ground points should not be placed on removable metal parts joined by bolts, such as doors. Dashboard structure ground points are allowed only if the structure is welded to the body.
5) For body panels thinner than 3 mm, welded nuts are recommended. For thicker plates greater than 3 mm, self-tapping screws are suggested for ground attachment.
5General ground allocation principles
1) Critical components that affect vehicle performance or safety and are susceptible to interference, such as engine ECU and ABS, should have dedicated grounds. Components prone to interference from other electrical devices, for example audio or fuel-level sensors, should also have dedicated grounds.
2) Airbag system grounds must be dedicated and use redundant grounding so the system remains grounded if one point fails.
3) RF systems should have separate grounding to avoid interfering with other systems.
4) Low-level sensor grounds are best kept independent and located close to the sensor to preserve signal integrity.
5) Other components may share grounding points based on layout; the guiding principle is local grounding to avoid unnecessary voltage drops from long ground runs.
6) Battery negative and engine ground straps have large conductor cross-sections; keep their lengths and routing controlled to minimize voltage drop. For safety, the engine and body are generally connected directly to the battery negative.
7) Separate electronic grounds from power grounds. Separate analog signal grounds from digital signal grounds to reduce mutual interference.
6 Conclusion
The above covers key points of low-voltage wiring harness grounding design. In practice, designers should apply these principles flexibly and validate the design through necessary tests to ensure reliability.
