That distinctive concave metal washer (often called a cup washer, belleville washer, or spring washer) placed under the head of a grounding bolt serves several critical functions:
Enhanced Contact Pressure & Biting Action: Its conical shape acts like a spring when compressed during tightening. This maintains constant, high pressure on the connection, forcing the bolt head and the underlying surface (e.g., busbar, lug, structural steel) together. Serrations (teeth) on its inner or outer edge bite through paint, rust, or oxidation, creating a clean metal-to-metal contact path essential for low electrical resistance.
Vibration Resistance: The spring action compensates for material relaxation (like soft copper lugs compressing) and resists loosening caused by vibration or thermal cycling, which are common in electrical environments. This prevents the connection from becoming loose and developing a high resistance over time.
Protection & Indication: It protects the surface of softer grounding conductors (like copper) from being gouged by the sharp edges of the bolt head during installation. The visible deformation of the washer after proper torque application also serves as a visual indicator that sufficient pressure was achieved.
In essence, this "iron bowl" is a vital component ensuring the grounding connection remains tight, clean, low-resistance, and reliable throughout its service life, directly contributing to electrical safety.
