The difference between a shunted and non-shunted socket is how the electrical current flows through the fixture. Current flows through a shunted socket through a single path and through a non-shunted socket through multiple paths. If the wrong socket is used, the possibility of an electrical short and early burn out of the bulb is greatly increased.

In fluorescent fixtures, shunted sockets are only used with instant start ballasts and the non-shunted sockets are used with dimming, pre-heat, program start, rapid start, and trigger ballasts. The ballast provides a sufficient voltage amount to start the lamps and then regulates the current to keep them operating effectively. Essentially the ballast is a bouncer for the club (fixture) which allows the right number of people (volts) to get through the lines (sockets) to get the party started (turn on the lights). Without a ballast, the fluorescent lamps would draw an unregulated current, which would over heat the lamp and burn it out.

How to Identify a Shunted or Non-Shunted Socket

Identify the socket by testing the contacts of the socket for continuity (complete path of current flow) with a voltage meter. On a shunted socket, the contacts are connected internally, which means continuity is present for both of the contacts. With a non-shunted socket, there is no continuity present between the contacts since the contacts are not connected internally and require separate power leads. Depending on the voltage meter used, an identifier or beeping noise will ring if continuity is present. Typically, shunted sockets have two holes and non-shunted sockets have four holes, but since there are exceptions it’s safer to use a voltage meter to figure out which socket style is installed in the fixture.

LED retrofit lamps typically don’t require a ballast to produce a high start-up voltage like their fluorescent counterparts and are offered in two different versions, either a direct wire or plug-n-play option.  Most direct-wire LED lamps require non-shunted lamp sockets while plug-n-play LED lamps typically require shunted lamp sockets. For the direct wire function, bypassing (removing) the ballast is essential for the operation of the LED bulb, so it can operate off line voltage directly. The plug-n-play option is a game changer since many are capable of operating with specific ballasts, including instant start or programmed start ballasts.

How to Tell if You Need a Shunted or Non-Shunted Socket

The ever changing world of technology has allowed manufacturers to tweak and create different generations of each lamp type. In order to get the best functionality from your overhead lighting, whether they are fluorescent or LED, it is best practice to reference the specification sheets or manufacturer brochure the so that you won’t be left in the dark by an electrical short or damaged light bulb.

While purchasing and installing light bulbs into your existing fixtures, be sure your sockets translate the proper voltage to avoid any possible mishaps or headaches. If you still have questions, or would like help figuring out the socket needs for your specific application, don’t panic. Contact our customer service team.