Understanding Tombstone Sockets in Commercial LED Retrofits
When retrofitting fluorescent fixtures in commercial offices, shops, and warehouses, understanding the internal wiring of the lamp holders—commonly referred to as tombstone sockets—is critical. The distinction between shunted and non-shunted sockets determines how electrical current is delivered to the lamp and directly affects safety, compatibility, and code compliance.
This distinction is especially important when performing ballast-bypass (Type B) LED retrofits, where incorrect socket selection can result in short circuits, driver damage, or unsafe conditions.
What Is a Shunted Socket
A shunted socket has its internal contacts electrically connected, or “shunted,” together. This creates a single conductive path between the two pins on one end of the lamp holder.
Shunted sockets are most commonly found in fluorescent fixtures designed for instant-start ballasts. Because the contacts are tied together, shunted sockets cannot safely accept both line and neutral conductors on the same end of a lamp.
What Is a Non-Shunted Socket
Non-shunted sockets contain two isolated internal contacts, providing separate electrical paths for each pin. This configuration allows both line and neutral conductors to be terminated at the same lamp holder.
Non-shunted sockets are standard in rapid-start and programmed-start fluorescent fixtures and are required for most single-ended (Type B) LED tube installations.
Socket Requirements for LED Tubes
The required socket type depends entirely on the LED tube wiring configuration specified by the manufacturer.
- Single-Ended (Type B) LED Tubes: Require non-shunted sockets. Both the line and neutral conductors are connected to the same end of the lamp. Using a shunted socket in this configuration will result in a direct short.
- Double-Ended (Type B) LED Tubes: Typically compatible with shunted sockets. Line is supplied to one end of the lamp, and neutral to the opposite end.
- Ballast-Compatible (Type A) LED Tubes: Operate through the existing fluorescent ballast and generally do not require socket modification, though long-term reliability and efficiency depend on ballast condition.
How to Identify Tombstone Sockets
Visual inspection alone is often unreliable, as shunted and non-shunted sockets frequently appear identical externally. The most accurate identification method is continuity testing with a digital multimeter.
- Set the multimeter to the continuity or resistance setting.
- Insert one probe into each pin opening on the same side of the socket.
- If continuity is present, the socket is shunted.
- If no continuity is detected, the socket is non-shunted.
Safety and Compliance Considerations
Installing LED tubes with incompatible tombstone sockets can result in immediate fixture failure, damaged drivers, or fire hazards. During commercial ballast-bypass retrofits, replacing existing sockets with new non-shunted tombstones is often the most reliable approach.
This practice ensures compatibility with the most common single-ended LED tube designs and reduces long-term maintenance and liability risks.
Related Commercial Lighting Categories
Correctly identifying and specifying tombstone sockets is a foundational step in any commercial LED retrofit. Matching socket type to tube wiring ensures safe operation, predictable performance, and long-term system reliability.
