Bus Surge Arrestor
A bus surge arrestor is a surge protective device installed directly on an electrical distribution busbar or bus duct to limit transient overvoltages and divert surge energy to ground. It uses a nonlinear resistor to maintain voltage below the Basic Insulation Level of connected equipment, protecting insulation systems and apparatus from lightning and switching surges without causing power interruptions.
On the shop floor, bus surge arrestors are installed at main service switchboards, motor control centers, and process line switchboards to protect multiple loads like VFDs, PLCs, and CNC machines from surges. They clamp overvoltages from lightning or switching events, reducing nuisance trips and component failures. Direct bus-bar connection achieves low let-through voltage, ensuring uniform protection for all downstream equipment. Compliance with NEC requirements for safety interlock circuits and emergency systems is achieved through these devices, enhancing worker safety by maintaining reliable operation of safety circuits and reducing arc-flash risks.
- Using non-listed or non-certified devices with incorrect voltage ratings, leading to premature failure or inadequate protection.
- Improper installation with long lead lengths or poor grounding, increasing let-through voltage and reducing surge diversion effectiveness.
- Lack of inspection and maintenance, causing degraded arresters to fail catastrophically under major surges, posing arc-flash or fire hazards.
How should a bus surge arrestor be coordinated with equipment BIL on an MV bus?
Select a metal-oxide arrester class and rating such that its maximum residual voltage under specified surge current (e.g., 10 kA impulse) is below the BIL of bus insulators and transformer windings, with margin for aging. Apply arresters phase-to-ground at strategic bus locations per ANSI/IEEE C62.11 or CSA CAN/CSA-C233.1 standards.
What is the distinction between a bus surge arrestor and panel-level SPDs?
A bus surge arrestor is a metal-oxide device connected directly to a busbar for system-level protection on distribution buses. Panel-level SPDs (Type 1/2/3 per UL 1449) are installed at service equipment or downstream panels via terminals or breakers, with Type 3 covering receptacle devices per CSA C22.2 No. 269.3.
How do bus-bar connected SPDs achieve lower let-through voltage than cable-connected SPDs?
Direct bus-bar connection eliminates or minimizes lead inductance, reducing voltage drop during surge current. This lowers effective let-through voltage, providing more effective suppression of transient events for all connected loads, as per Eaton's SPD guidance.