surge arresters for control and measurement signals
Many 3 terminal gas tube igniter uses a centre pin to ground to match the impedance "almost short circuit" to ground.

The first is where ground is not supplied such as an isolated 24Vac transformer for your application.

The second is when ground is also supplied with source (3 wire) if the there is a common mode shield around the isolated pair.

This isolated shield is usually earthed at both ends to reduce Common Mode induction.

In both cases this protection is only limited to 500mA and allows 24 to 36Vp. Gas tubes often take 10uS to begin ionization so the transient voltage is better protected by TVS diodes .  Both may require current limiting at source to protect power source.  Another method to reduce transients is an LC line filter to attenuate spikes.