The Difference Between Shunted and Non-Shunted Sockets: Which is Best for Your Lighting Needs?

Understanding Tombstone Sockets in Commercial Retrofits

When converting older fluorescent shop or office fixtures to LED technology, understanding the internal wiring of the lamp holders—commonly referred to as "tombstones"—is essential. The distinction between shunted and non-shunted sockets determines how electrical current is delivered to the lamp and is a critical factor in both safety and functionality during a ballast-bypass (Type B) LED retrofit.

What is a Shunted Socket?

In a shunted socket, the internal contacts are physically joined or "shunted" together. This means the socket provides a single path for the electrical current to travel. Shunted sockets are most commonly found in older fluorescent fixtures utilizing "Instant Start" ballasts. Because the contacts are connected, you cannot use them to send a neutral and a hot wire to the same end of the bulb, as this would cause a direct short circuit.

What is a Non-Shunted Socket?

Non-shunted sockets feature separate internal contacts that are not connected. This creates two distinct paths for the electrical current. These are standard in "Rapid Start" or "Programmed Start" fluorescent fixtures. In the context of modern LED upgrades, non-shunted sockets are required for "Single-Ended" LED tubes, where both the live and neutral wires must be connected to the same lamp holder.

Which Socket Do You Need for LED Upgrades?

The requirement for shunted or non-shunted sockets depends entirely on the type of LED tube being installed:

• Single-Ended LED Tubes: These require Non-Shunted sockets. One pin on the lamp holder receives the "Hot" wire, and the other receives the "Neutral" wire. Using a shunted socket here would cause an electrical short.
• Double-Ended LED Tubes: These are generally compatible with Shunted sockets. In this configuration, power is sent to one end of the tube and neutral to the other.
• Plug-and-Play (Type A) Tubes: These work with the existing ballast and typically do not require a socket change, though they are less efficient for long-term commercial use.

Identifying Your Sockets

Visual inspection is not always reliable, as many shunted and non-shunted sockets look identical from the outside. The most accurate way to identify a socket is to use a digital multimeter set to the "continuity" setting:

1. Place a probe into each of the two holes on a single side of the lamp holder.
2. If the multimeter beeps or shows zero resistance, the socket is Shunted.
3. If the multimeter does not beep, the paths are separate, and the socket is Non-Shunted.

Safety and Compliance

Using incompatible sockets during an LED retrofit can lead to fixture failure, burnt drivers, or fire hazards. When performing a ballast-bypass in a commercial shop or warehouse, it is often more cost-effective to replace the old tombstones with new non-shunted versions to ensure compatibility with the most common "Single-Ended" LED tubes on the market.

Conclusion

Identifying the difference between shunted and non-shunted sockets is a vital step in any commercial lighting modernization project. By ensuring your sockets match your LED tube specifications, you can guarantee a safe, flickering-free, and energy-efficient lighting system for your facility. If you are retrofitting a large shop or garage, always verify your tube wiring requirements before beginning the installation.