Important Things to Know About Shunted Sockets

If you are working with fluorescent lighting you may need to replace the sockets in the fixtures. Although this is not a difficult job, not all sockets are wired the same in fact some are shunted sockets while others are non-shunted. It’s important to know what “shunted” means and the differences between non-shunted and shunted, so let’s explore the subject.

What’s the Difference?
If you look at a shunted and non shunted socket or tombstone you may notice little difference. In fact, a T8 shunted may take the same bulb as a T8 non shunted. However, the difference is in the circuits. With a shunted tombstone, only one side of the socket will accept wires (1-2). A non shunted tombstone can accept wires from both sides (2-4).

When you look at the bulb connections, they may appear quite similar. This is because the connections are inside the unit. In a shunted tombstone, both the left and right hand sides are the same polarity. In fact, this is a good way to check to see if a socket is shunted or not. Simply place an ohmmeter or continuity checker across the leads. If you have resistance or continuity, the socket is shunted.

Why are both connections the same polarity on shunted sockets? This is done to create a series circuit. Series connections are often used for rapid start ballasts. On the other hand, and instant start ballast will require a parallel circuit so the connections to the bulb must be of different polarities.

Series and Parallel Circuits Explained
The best way to explain series and parallel circuits is to imagine two 1.5-volt standard flashlight batteries. When you connect the 2 batteries in a flashlight, the negative (-) end on one battery connects to the positive end on the other. This doubles the voltage to 3 volts and keeps the current the same, creating a series circuit.

If you were to connect both positive ends together and also both negative ends, the negative and positive leads would still have 1.5 volts but you would double the current or output of the batteries. This is a parallel connection.

Parallel and Series Lighting
Suppose you wish to wire two 120 volt bulbs to a circuit. This can be done in one of two ways. You can connect the two fixtures in a parallel 120-volt circuit. You also can use 240 volts and connect the lights in a series circuit. However, with series circuits, if one bulb burns out, the circuit is opened and you have no lights.

Shunted sockets are designed to produce a series connection, so make sure to check each socket before installing or retrofitting.