Why tranzorb is fixing in peralell of 24volt dc relay?

Answer 1

A Tranzorb connected across the coil of a 24VDC relay will prevent electromagnetically generated transient voltage spikes from damaging the relay coil driver element on de-energization. A Tranzorb placed across the contacts of a relay may help reduce contact sparking, but might more likely revert to the conduicting state - and stay that way - should a transient be generated across the relay contacts on their opening.

The clamping action of a Tranzorb is much sharper and more prompt than that of a Metal Oxide Varistor, or MOV. A MOV, in turn, offers significantly greater transient suppression that a capacitor. Homeowners who operate sensitive digital electronics on the site should know this and condition their mains service accordingly with Tranzorbs, for the sake of long-term harm reduction.

Hope that helps.