Shunted vs Unshunted Tombstones

What are Tombstones?

Tombstones earned their name from their distinctive cemetery marker shape, but their function is far more practical than decorative. Also called sockets or lamp holders, these components serve dual purposes: securing tubular fluorescent and LED lamps with bi-pin configurations while delivering electrical power. When you install a lamp, rotating it locks the pins into the tombstone mechanism, creating both physical stability and electrical connection.

Various commercial fixtures rely on tombstone technology across different applications:

Linear High Bay Fixtures

Linear high bay fixtures illuminate expansive indoor spaces with ceilings ranging from 20 to 45 feet high. Mounting options include chain or cable suspension, plus direct ceiling attachment. Warehouses, manufacturing facilities, gymnasiums, and large conference halls benefit from their powerful, wide-coverage illumination.

Drop Ceiling Troffer Fixtures

Ceiling troffer fixtures integrate seamlessly into standard grid ceiling systems, sitting flush with ceiling tiles for clean aesthetics. Beyond grid installations, suspension mounting and surface mounting provide flexibility for solid ceilings. Office environments, retail showrooms, and commercial spaces rely on their even light distribution.

Strip Lights / Shop Lights

Strip light configurations deliver abundant illumination for task-oriented environments. Installation methods include suspended chain or cable mounting alongside surface mounting directly to ceilings or walls. Workshops, retail displays, and storage facilities depend on their straightforward, high-output lighting.

Vapor Tight Fixtures

Gasketed vapor-tight construction protects internal components from dust, moisture, and harsh environmental conditions in both indoor and outdoor installations. This protection prevents electrical shorts and component damage that would compromise fixture performance. Food processing facilities, parking structures, and refrigeration areas require this level of environmental protection.

What’s the Difference Between Shunted and Unshunted Tombstones?

Electrical current flow patterns distinguish shunted from unshunted tombstones, creating compatibility requirements that directly impact fixture performance and safety.

For Shunted Tombstones

Single-path current flow characterizes shunted socket operation through internally connected electrical contacts.

• Linked contacts create one pathway for electrical current traveling from ballast through tombstone to lamp pins
• Single wire set receives voltage and distributes power to both contacts simultaneously
• Power flows between contacts with Live/Neutral connections on opposite fixture sides

For Unshunted Tombstones

Multiple current pathways define unshunted socket architecture with completely separate contact points.

• Independent contacts create dual tracks for electrical current without any internal connections between entry points
• Zero power transfer occurs between contacts, requiring separate wire entry points with Live/Neutral on identical fixture sides

Matching correct tombstone types with compatible lamps prevents serious operational problems including:

• Electrical short circuits that damage fixtures and building electrical systems
• Premature lamp failure and reduced operational lifespan
• Voided UL safety listings that compromise insurance coverage and code compliance

Visual inspection alone cannot determine tombstone type since socket appearance varies between manufacturers. While unshunted sockets typically feature four wire holes versus two holes on shunted versions, this rule isn’t universal. We always recommend voltage meter testing set to continuity mode for accurate identification. Positive continuity between both contacts on a single socket indicates shunted operation, while no power flow confirms unshunted design.

What Types of Lamps Use Tombstones?

Tubular lamp technologies depend on tombstone connections for both mechanical support and electrical power delivery through bi-pin configurations on lamp ends.

Fluorescent Tubes

Fluorescent tubes require ballast operation to regulate electrical supply and prevent lamp burnout. Ballasts manage voltage, current, and waveform delivery essential for proper lamp ignition and sustained operation. Without ballast regulation, fluorescent lamps draw unrestricted electrical current leading to overheating and immediate failure.

Types of Fluorescent Tubes:

• T12 tubes represent obsolete technology with 1.5-inch diameter, though some legacy installations remain operational
• T8 tubes measure 1-inch diameter across 2-, 3-, 4-, and 5-foot standard lengths, maintaining widespread commercial use
• T5 tubes feature 5/8-inch diameter in 22-, 34-, 46-, and 58-inch lengths, delivering superior efficiency since 1990s introduction

LED Tubes

LED lamp categories require specific power delivery methods. Direct-wire LEDs eliminate ballast dependency by operating on building voltage after ballast removal or bypass. Plug-and-play LEDs maintain compatibility with existing instant-start and programmed-start ballast systems.

Types of LED Tubes

• T8 Plug-and-Play models work with existing compatible ballasts
• T5 Plug-and-Play versions integrate with T5 ballast systems
• Single-Ended T8 Direct-Wire tubes receive power from one end only
• Double-Ended T8 Direct-Wire tubes accept power from both ends after ballast bypass
• T8 Remote Driver systems separate LED driver from fixture housing

What Type of Tombstone Should I Use?

Shunted

Instant-start ballast systems exclusively require shunted socket compatibility in fluorescent applications. Choose shunted tombstones for:

• T8 Fluorescent with instant-start ballast configurations
• T5 Fluorescent with instant-start ballast systems
• T8 LED Plug-and-Play retrofits
• Double-Ended T8 LED Direct-Wire installations
• T5 LED Plug-and-Play conversions

Unshunted

All non-instant-start ballast types, including programmed start, rapid start, pre-heat, dimming, and trigger configuration,s operate with unshunted sockets. Select unshunted tombstones for:

• T12 Fluorescent installations (all ballast types)
• T8 Fluorescent with rapid-start, programmed-start, or dimming ballasts
• T5 Fluorescent with rapid-start, programmed-start, or dimming ballasts
• Single-Ended T8 LED Direct-Wire systems
• Double-Ended T8 LED Direct-Wire applications (alternative wiring method)
• T8 LED Remote Driver configurations

Quick Selection Guide

• Fluorescent Lighting:
  • T12: Unshunted
  • T8 with Rapid Start, Programmable Start, or Dimming Ballast: Unshunted
  • T8 with Instant Start Ballast: Shunted
  • T5 with Rapid Start, Programmable Start, or Dimming Ballast: Unshunted
  • T5 with Instant Start Ballast: Shunted
• LED Lighting:
  • T8 Plug-n-Play: Shunted
  • Single Ended T8 Direct Wire: Unshunted
  • Double Ended T8 Direct Wire: Shunted / Unshunted
  • T8 External Driver: Unshunted
  • T5 Plug-n-Play: Shunted