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.
Frequently Asked Questions
What is the difference between shunted and non-shunted tombstone sockets?
A shunted socket has its two pin contacts internally tied together, creating one electrical path. A non-shunted socket keeps the two pin contacts isolated, creating two separate electrical paths.
Which socket type is required for single-ended Type B LED tubes?
Single-ended Type B LED tubes typically require non-shunted sockets because line and neutral are landed on the same lamp end, using separate contacts for each pin.
Can a shunted socket be used with a single-ended Type B LED tube?
No. A shunted socket ties the two pin contacts together. If line and neutral are terminated on that same end, the socket can create a direct short and damage the lamp or wiring.
Which socket type is commonly compatible with double-ended Type B LED tubes?
Double-ended Type B tubes are commonly compatible with shunted sockets because line is supplied to one end of the lamp and neutral to the opposite end. Compatibility is determined by the tube wiring diagram and the fixture wiring method.
Do Type A ballast-compatible LED tubes require shunted or non-shunted sockets?
Type A tubes operate through the existing ballast and usually do not require socket changes. Socket type is typically not the controlling factor unless the retrofit also changes the fixture wiring.
How can you reliably identify whether a tombstone socket is shunted?
Use a continuity or resistance test with a digital multimeter across the two pin openings on the same socket. Continuity indicates shunted. No continuity indicates non-shunted.
Why is visual inspection not a reliable identification method?
Shunted and non-shunted sockets often share the same external form factor and markings can vary by manufacturer, so the electrical configuration is not consistently visible from the outside.
What ballast types are commonly associated with shunted sockets?
Instant-start fluorescent ballasts are commonly associated with shunted sockets because the ballast design does not require isolated contacts at the lamp holder.
What ballast types are commonly associated with non-shunted sockets?
Rapid-start and programmed-start fluorescent ballasts are commonly associated with non-shunted sockets because the fixture wiring typically uses separate contacts at the lamp holder for lamp operation.
When should tombstone sockets be replaced during a retrofit?
Replace tombstones when the LED tube wiring method requires a different socket type, when existing sockets are heat-damaged or loose, or when the contact tension is poor and causes intermittent lamp operation.
What is the main safety risk of using the wrong socket type during ballast-bypass retrofits?
The primary risk is creating a short circuit or miswiring condition that can damage the lamp, overheat conductors, or create a shock hazard during servicing.
What information should be confirmed before choosing shunted vs non-shunted sockets?
Confirm the LED tube type and wiring method from the manufacturer documentation, including whether the lamp is single-ended or double-ended, and follow the provided wiring diagram for line, neutral, and any ballast removal requirements.
Brandon Waldrop
As the lead technical specialist for our commercial lighting technical operations, Brandon Waldrop brings over 20 years of industry experience in product specification, outside sales, and industrial lighting applications.
His career began in physical lighting showrooms, where he focused on hands-on product performance and technical support. He later transitioned into commercial outside sales, working directly with architects, electrical contractors, and facility managers to translate complex lighting requirements into energy-efficient, code-compliant solutions.
Today, Brandon applies that industry experience to architect high-performance digital catalogs and technical content systems, helping commercial partners streamline the specification process and deploy lighting solutions with total technical confidence.