What Type of Socket Do I Need (Shunted vs. Non-Shunted)?


Why is it important to use non-shunted vs. shunted tombstones? Traditional fluorescent tubes are typically dual-end powered (DEP), with one end hot or live and the other side neutral. Shunted tombstones (a socket or lampholder named after their shape) allow power between contacts and receive voltage with L/N on different sides. On the other hand, no power flows between contacts in non-shunted tombstones. They allow separate points of entry for the wires with L/N on the same side.

Sunco requires non-shunted tombstones for our direct wire LED T8 tubes (SEP).

Using the wrong tombstone can void the UL listing on your lamp and shorten the life of your LED tube. It can also cause an electrical short and potentially cause a fire hazard.

How the electrical current flows through the fixture is the difference between shunted and non-shunted tombstones. Each LED lamp type requires power to flow in a specific way. Some require a ballast and some, like Sunco T8 LEDs, need the ballast removed.


What is a ballast?

Ballasts are required for fluorescent and HID light sources to provide appropriate circuit conditions. A ballast manages the distribution of energy in a fixture to ensure necessary voltage, current, and waveform to start and operate the lamp.

Electricity flows from the ballast through the tombstone and into the lamp pins.

In fluorescent lights, before the lamp is turned on, tombstones provide power through the lamp’s cathodes. Unlike traditional fluorescent tubes, LED tubes include an internal driver. Sunco LED T8s are instant on and do not require power in advance of illumination.

Direct wire LEDs like we offer at Sunco do not use a ballast. Instead, our T8 LEDs bypass the ballast and use non-shunted sockets with a small amount of wiring work to complete the modification or retrofit of the linear tube fixture.


Non-Shunted Tombstones

Electrical current flows through multiple tracks or paths in a non-shunted tombstone. Their contacts are not connected or joined. This type of socket is used with Sunco LED T8 tubes, and also in dimming ballasts, programmed or rapid start ballasts.

Sunco uses non-shunted tombstones for our direct wire LED T8 tubes.

To retrofit a single-ended power (SEP) LED tube, the electronic ballast needs to be removed. An electrician can quickly cut the L/N wires from the ballast, cut the socket lead wires and connect the input wires to one end of the fixture in order to bypass the ballast. The LED tube will connect directly to the voltage line flowing into the sockets. The second tombstone acts as a stabilizer to hold the tube firmly in place. No electricity flows through that side. For safety reasons, a qualified electrician should perform any retrofitting.

Be sure your electrician attaches the label provided with our T8s to your retrofitted light fixture to ensure someone does not accidentally use a fluorescent linear tube instead of an appropriate LED tube after the ballast is removed. The label says: “Warning: Fixture has been modified. Ballast has been removed. Do NOT install fluorescent tubes. Use only 120/277V single-end LED tubes.” You can also notify your maintenance and office management personnel of this change.

The bi-pin end of your linear lamp tube (G13 in this case) is then secured on a tombstone in the slot between the metal contacts and conveniently placed plastic supports. This is the same for each type of socket.

Note that the grooves on the side of the socket are not just for decoration. The groves fit differently on separate lighting fixtures. Be sure to make note of where the grooves fit on your particular light fixture before beginning the retrofit process. These grooves are designed to slide your tombstone into the right position on the fixture.


Shunted Tombstones

There is a single path for the current flow inside a shunted tombstone. That means the two electrical contacts come into contact with your lamp pins and are internally connected.

If you look at the line art above, showing examples of the shunted and non-shunted tombstones, you will notice that there are little arrows pointing up inside the base (not the ones pointing into the wire connection points). The arrows inside the base point up to the lamp pin slot and the connectors. These arrows indicate the type of electricity flow in each socket style.

Shunted electricity flows from one point into the two pins. Non-shunted flows from two points into two pins; there is no connectivity. This variance is why you need to verify which tombstones are in your existing lighting fixture before modifying the unit to accept T8 LED tubes.


Determining if you need to replace existing tombstones

How do I know if an existing fixture has shunted or non-shunted sockets in place? A visual inspection will show you:

  • whether there is a ballast
  • if there are L/N wires on both ends of the fixture or just at one end
  • whether a maintenance sticker was added to indicate any changes to the fixture

A visual look at the sockets inside an existing light fixture will not necessarily indicate whether it is shunted or non-shunted. Some tombstones cover up one pair of wire connection points, revealing only two, when they are shunted, while others do not. Please do not rely on a visual investigation alone for tombstone analysis.

The best way to determine electricity flow is to test the existing sockets with a voltage meter set to “continuity.” An electrician would place a probe on each of the metal contacts in a socket to verify whether a connection exists or not.

  • If your existing light fixture shows positive continuity when testing both contacts on a single socket (usually the meter will beep), there is power flowing between the two contacts, and this is a shunted tombstone.
  • If there is no power between contacts, it is a non-shunted tombstone.

Remember that typically a T8 fluorescent fixture with an instant start ballast would have non-shunted sockets already installed.

In the case of continuity (indicating the tombstones are shunted), you would need to modify the light fixture to include a non-shunted tombstone, and also bypass the ballast, before inserting a Sunco LED T8 Tube.



Now that you understand shunted and non-shunted tombstones, how their power flow varies, you are ready to plan your retrofit of existing fluorescent linear tube fixtures to hold Sunco LED T8 Tubes.

An ideal fluorescent replacement, direct wire LED Tubes like Sunco LED T8s are more energy efficient than fluorescents, plus plug-and-play or hybrid LEDs. Next week’s blog will cover the advantages of direct wire LED T8 tubes.



NOTE: For your protection, be sure to retrofit your lighting fixtures with the power turned OFF. Also, for safety reasons rewiring fluorescent fixtures for use with LED should only be performed by individuals who have been properly trained to perform basic electrical maintenance and repair.

Please review instructions in our install guide manuals carefully before proceeding.

Do not make or alter any open holes in an enclosure of wiring or electrical components during kit installation.

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