A Basic Guide To Understanding Shunted Sockets

One of the most challenging aspects of working with fluorescent lighting is to learn all the different terminology and technology used in the system. In general, there are two types of lampholders used that will be known as non-shunted and shunted sockets or sometimes as tombstones because of their unique shape.

Both the shunted and the non-shunted sockets look almost the same, but it is possible with some designs to quickly tell the difference. The shunted lampholder will only have one pair of wire holes on the bottom “foot” area of the socket while the non-shunted will have one pair on each side.

The Test

The best and most effective way to confirm if it is a shunted or non-shunted lampholder is to use a volt meter. Set the volt meter to continuity, and then touch the positive and negative probes to either side of the contact in the socket without any power to the socket. On the shunted sockets, you will hear a signal sound because the contacts are actually connected.

With the non-shunted lampholder there will not be a sound produced as the two contacts are not connected. This is why there is the requirement for the two wires on either side of the socket compared to just one pair in the shunted lampholder.

Fixture Types

For most types of fluorescent fixtures, particularly those that are a bit older, it is very likely that the system uses the shunted lampholders. You can look at the current socket and see if there is a wire that loops from one of the pairs of wire holes in the base to the other, or if the wires run back to the ballast from both sides.

If there is only one set of wires going into the socket and no wire connecting the two, you are working with fixtures using shunted sockets. If there is no existing wiring in place and you still are not sure, you can take the socket apart and look at the formation inside the component.

In the non-shunted socket, there will be no point where the copper connectors in the inside of the socket make contact with each other. On the other hand, with the shunted tombstone, the interior copper connectors will be connected to each other, typically at the base of the lampholder.

As these are very different components, using a shunted lampholder when a non-shunted socket is required will result in a dead short in the socket. This will result in tube failure and damage to the socket, which will require the need for full replacement of the tubes and the sockets.

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