Shunted vs Unshunted Tombstone

What are Tombstones?

Have you ever heard the term “tombstones” when people talk about lighting, but you didn’t know what it was referring to? It can be confusing because it describes the appearance of the object without telling you what its function is. Tombstones are also known as sockets or lamp holders.

The term “tombstone” is used because of its shape, which is like something that you’d see in your typical cemetery. They are used for tubular fluorescent and LED lamps with a bi-pin configuration.

They have two functions: hold the lamp in place in the fixture and provide electricity. When a lamp is installed into a fixture, it is rotated in such a way that its pins are locked into the tombstone so that it is held in place. It is then connected so that power can run through it.

The fixtures that tombstones are used in include:

Linear High Bay Fixtures

Linear high bay light fixtures are used for illuminating large indoor areas with high ceilings. They are placed anywhere from 20 to 45 feet from the floor. They can also be suspended from a ceiling with chains or cables, or they can be surface mounted. Areas that they are used include gymnasiums, warehouses, factories, and conference halls.

Drop Ceiling Troffer Fixtures

Drop ceiling troffer fixtures are used for illuminating indoor areas with a grid ceiling configuration. They are typically placed inside the grid so that they are flush with the ceiling. They can also be suspended from a normal ceiling with chains or cables. They can also be surface mounted. Areas they’re used include offices, retail spaces, and showrooms.

Strip Lights / Shop Lights

Strip lights are used for illuminating indoor areas that require abundant lighting. They can be suspended from the ceiling with chains or cables. They can also be surface mounted. Areas that they are used include workshops, retail spaces, and storage units.

Vapor Tight Fixtures

Vapor-tight fixtures are used for illuminating indoor or outdoor areas that require your lighting to be protected from harsh environments. These fixtures are gasketed so that dust and moisture are kept out of the inside of the fixture. This prevents the inside of the fixture from being damaged or shorted out. They can be suspended from the ceiling with chains or cables. They can also be surface mounted. Areas that they are used include food processing plants, parking garages, and refrigeration units.

What’s the Difference Between Shunted and Unshunted Tombstones?

The difference between a shunted and an unshunted tombstone is how the electrical current flows through the fixture.

For Shunted Tombstones

Current flows through a shunted socket through a single path.

• They have internally linked electrical contacts. This gives a single path for the electrical current to go from the ballast, through the tombstone, and to the lamp’s pins.
• They receive voltage through a single set of wires and spread it to two contacts.
• They allow power between contacts and receive voltage with Live/Neutral on different sides.

For Unshunted Tombstones

Current flows through an unshunted socket through multiple paths.

• Unshunted sockets have separate contacts (or points of entry for the wires) creating two tracks for the electrical current to travel. The contacts are not joined or connected.
• No power flows between contacts in unshunted tombstones. They allow separate points of entry for the wires with Live/Neutral on the same side.

You must use the correct tombstone type with your lamps. Using the wrong type can lead to issues such as:

• It can cause an electrical short.
• It can shorten the life of the lamp.
• It can void the UL listing on your lamp.

A look at the tombstones inside an existing light fixture will not automatically tell you whether they are shunted or unshunted. Usually, non-shunted sockets have four holes and shunted sockets have two holes, but that is not always the case. Because of this uncertainty, it’s best to use a voltage meter just to make sure. You should test the tombstones using a voltage meter set to “continuity”. If the light fixture shows positive continuity when testing both contacts on a single socket, it means that power is flowing between the two contacts, and it is a shunted tombstone. If there is no power between the contacts, it is an unshunted tombstone.

What Types of Lamps Use Tombstones?

Tubular lamps are held and powered by tombstones. They can be fluorescent or LED. They have two pins on each end that lock into the tombstones to hold them in place. Here is what you need to know about them:

Fluorescent Tubes

Fluorescent Tubes are used in conjunction with a ballast. Ballasts are needed for fluorescent light sources to provide proper circuit requirements. A ballast manages the supply of energy in a fixture to guarantee the necessary voltage, current, and waveform to start and operate the lamp. Electricity flows from the ballast through the tombstone and into the lamp pins. Fluorescent tube need a ballast. Without it, the lamps would draw a free electrical current. This would overheat the lamp and cause it to burn out.

Types of Fluorescent Tubes:

• T12 – T12s are the oldest style of fluorescent tubes. They have become obsolete but there are still some in use today.
• T8 – T8s have a one-inch diameter and are available in 2-, 3-, 4-, and 5-foot lengths. They were the successor to the T12s and are still widely in use.
• T5 – T5s have a 5/8-inch diameter and are slightly shorter than T8s. Their standard lengths are 22-, 34-, 46-, and 58-inch. They began to appear on the market in the 1990s and are more efficient than T8s.

LED Tubes

Each LED lamp type requires power to flow in a specific way. Some require a ballast, and some need the ballast removed. Direct wire LED lamps do not use a ballast. Bypassing or removing the ballast is necessary for the operation of the LED bulb so it can operate offline voltage directly. Plug-and-Play LED lamps can operate with specific ballasts (instant start and programmed start).

Types of LED Tubes

• T8 Plug-and-Play
• T5 Plug-and-Play
• Single-Ended T8 Direct-Wire
• Double-Ended T8 Direct-Wire (Ballast-Bypass)
• T8 Remote Driver

What Type of Tombstone Should I Use?

Shunted

For fluorescent fixtures, you can only use shunted sockets with instant start ballasts. Use shunted Tombstones for:

• T8 Fluorescent with instant-start ballast
• T5 Fluorescent with instant-start ballast
• T8 LED Plug-and-Play
• Double-Ended T8 LED Direct-Wire (Ballast-Bypass)
• T5 LED Plug-and-Play

Unshunted

For fluorescent fixtures, unshunted sockets are used with all other ballast types. These include programmed start, rapid start, pre-heat, dimming, and trigger ballasts. Use unshunted Tombstones for:

• T12 Fluorescent
• T8 Fluorescent with rapid-start, programmed-start, or dimming ballast
• T5 Fluorescent with rapid-start, programmed-start, or dimming ballast
• Single-Ended T8 LED Direct-Wire
• Double-Ended T8 LED Direct-Wire (Ballast-Bypass)
• T8 LED Remote Driver

Quick Selection Guide

Fluorescent Lighting

LED Lighting