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I am trying to understand the mechanism and characteristics of Weidmuller surge arresters for control and measurement signals.Wiemuller has presented two types of surge arresters for such signals: VSSC4 and VSSC6. But there's something kinda basic about these surge arresters that makes me confused.

I can't get what is the third line in VSS6 for? Here is the data sheets and photos of these 2 arresters.enter image description here http://catalog.weidmueller.com/procat/Product.jsp;jsessionid=5CE47B92400AB638C2D6EBC242B522DE?productId=(%5b1063730000%5d)&page=Product

http://catalog.weidmueller.com/procat/Product.jsp;jsessionid=5CE47B92400AB638C2D6EBC242B522DE?productId=(%5b1064170000%5d)&page=Product

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IMO both devices are connected to the earth, the VSS6 has probaly one additional clamp contact, that you can use to connect the cable shield, so it can be used as complete terminal connection for a shielded cable.
The VSS4 is probably meant for unshielded twisted cable.
The mid point of gas arrester has to be connected to the earth, in this way it can discharge the surge. The main problem of the gas arrester is that is somehow slow compared to the TVS, so the next stage is an impedance (could be an inductor, resistor, or both RL) that limits the rate of change, then a TVS will react almost at rated protetion level. In the meantime the gas arrester will also begin to leak curent to the earth. If the discharge is persistent for long time, then the gas arrester got hot and the two wing contacts bend to form a short circuit to protect the gas arrester from exploding.

enter image description here

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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.

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