Shunted and non-shunted tombstones can look almost identical, but using the wrong type may cause wiring issues or even safety concerns, including loss of UL compliance.
The difference comes down to internal wiring: shunted tombstones have internally connected contacts, creating a single current path. Non-shunted tombstones have isolated contacts, providing two separate current paths.
This guide explains how each type works, typical use cases, and simple ways to identify them in the field.
A light tombstone is the socket that holds a linear lamp and connects it to the electrical supply. It secures the lamp in place while providing a safe and reliable electrical connection.
It is easy to recognize shunted and non-shunted tombstones if you understand how they work.
Shunted tombstones contain internally connected contacts that create a single electrical path. A conductive bridge inside the socket connects both terminals, allowing current to flow without an external wire between the contacts.
Because the contacts are already connected, shunted tombstones simplify fixture wiring and are commonly used in fluorescent fixtures equipped with instant-start ballasts.
In instant-start ballast systems, power is supplied to only one end of the lamp — which is why shunted tombstones, with their single current path, are the compatible choice.
Non-shunted tombstones have isolated contacts, allowing each terminal to be wired independently. Unlike shunted tombstones, there is no internal connection between the contacts. This allows each pin of the lamp to receive power independently, which is required by many LED direct-wire installations and fluorescent systems using rapid-start or programmed-start ballasts.
Although some tombstones include markings that identify their type, shunted and non-shunted versions often look nearly identical from the outside. The most reliable way to verify the type is with a multimeter set to continuity mode. A shunted tombstone will show continuity between the contacts, while a non-shunted tombstone will not.
Understanding this distinction is the foundation for selecting the correct tombstone. The next step is matching it to your specific ballast type or LED tube design.
● Electrical Connection: The most significant difference is the presence or absence of a built-in electrical connection between the terminals.●
● Wiring Complexity: Shunted types are easier to wire because the terminals are already connected.
● Application: Shunted tombstones are commonly used in parallel lighting circuits. Non-shunted tombstones are more suitable for series circuits and dimmable lighting systems.
● Compatibility: Shunted ones are often limited to specific bulb types and wattages.
| Feature / Aspect | Shunted | Non-Shunted |
|---|---|---|
| Internal Wiring | Terminals are internally connected | Terminals are electrically isolated |
| Current Flow | Power flows through a single path | Power flows through separate paths |
| Typical Use | Fluorescent fixtures with instant-start ballasts | LED retrofit fixtures with ballast-bypass wiring |
| LED Compatibility | Works with Type A (plug-and-play) LED tubes only | Required for Type B (direct-wire) LED tubes |
| Ballast Dependency | Requires a ballast for operation | No ballast required |
| Installation Complexity | Easier to wire | Requires more careful wiring |
| Common in Fixtures | Older commercial fluorescent setups | Modern LED retrofits and updated installations |
Understanding the basics of how these lamp holders function makes it easier to determine when each is needed for your lighting requirements. While numerous tables and charts outline specific examples, the choice often hinges on the ballast used for the fixture.
For fluorescent lights, the majority require a non-shunted design, with instant-start ballasts being the only exceptions. T8 fluorescent lamps, however, can be non-shunted instead of shunted with an external wire.
Many LED designs are compatible with both shunted and non-shunted tombstones. Plug-and-play products may require shunted tombstones, while most others, especially those using ballast bypass, are compatible with non-shunted ones.
If uncertainty lingers, seek advice from a local lighting specialist to ensure the right types are chosen, mitigating potential risks. Always confirm with your lighting expert or the lamp manufacturer to make sure you're using the correct type.
The primary difference between shunted and non-shunted T8 lamp holders lies in their internal wiring and compatibility with different lighting systems. Shunted tombstones have internally connected contacts, meaning both pins share the same electrical circuit. On the other hand, non-shunted tombstones have isolated contacts, requiring separate wiring for each pin.
To test a power lamp holder, plug in a working device or use a voltage tester. For network tombstones, use a cable tester or connect a device to check connectivity. Always prioritize safety.
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