A GDT is a fold back device. In the “off-state” (black line on the X-axis up to VDC shown here), its leakage current is very low. Also, it has an interim “glow mode” region, which is not desirable. It should just be a short transition mode from inactive mode to arc mode. In normal operation the GDT should be selected such that typical operating and fault conditions do not cause the GDT to remain in Glow mode for a long time, since this is a high energy dissipation mode, which will eventually result in damaging the GDT. Glow mode is <70 V for most Littelfuse devices. Arc voltage examples: GTCx36: 16 V, GTCx38: 24 V.