By Xuyang Xu from Britec Electric Wenzhou Co., Ltd.

Gas Discharge Tube (GDT) has two plates of metals which is separated by inertia gas and then sealed in a porcelain cylinder. When the voltage between these two plates is higher than the continuous operating voltage of the GDT, arc starts between the two plates and arc can conduct the current. Once it starts to conduct the current, a very low voltage is needed to keep the arc.

The inertia gas can cut following current smaller than 100A. Some GDT which is not filled with inertia gas can’t cut any follow current.

All the GDTs which are used in Britec’s SPDs has the ability to cut 100A following current.

If the GDT is installed between live and neutral, once the GDT is activated by surge current, the current from power line will continuously come from live to neutral causing a short circuit. That is why we should not use GDT between live and neutral.

Once a short circuit occurs, the MCB or fuse to protect the SPD shall trip or blow. The SPD itself has a build-in thermal protection mechanism which will act and cut the circuit and the indicator will turn from green to the red which reminds the user to replace the modules.

Although the protection measures have been taken into consideration, when design the product, short-circuit should be avoided from the very beginning, so we should use MOVs instead of GDT for live to neutral surge protection because MOVs have no follow current, i.e. they cut all the follow current.

For type 1 surge arresters, some special spark gap technology are used to cut the follow current so they can be used to protect surge current from live to neutral. Although the symbol are very similar to normal GDT, in reality they are not like the single gap GDT discussed above.