7) About follow-on current


What is follow-on current?
Follow-on current is a phenomenon where the current in a discharge tube continues to flow.
Surge absorbers nominally have high impedance. When a surge enters the surge absorber, it immediately drops to low impedance which allows the surge to bypass the electronic circuit it is protecting. After the surge has passed, the surge absorber should return to high impedance.
However, when the surge absorber has low impedance and there is sufficient voltage on the line to keep current flowing, even after the surge ends the surge absorber continues to discharge. The surge absorber fails to return to high impedance and the current continues to flow. This is a phenomenon known as follow-on current.
Surge absorbers that display this follow-on current are discharge or semiconductor switching. A characteristic of these absorbers is that during surge absorption (bypass), the operating voltage (remaining voltage) is lower than the starting voltage.
The advantage of these surge absorbers is that during suppression the voltage is held very low, so it reduces stress on the equipment. However, a problem arises when the line current of the equipment is high enough that it continues to drive the surge absorber even when the voltage is low.
Follow-on current mechanisms are explained further in the next chapter, along with the discharge tubes.

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