During the Aeon of Strife, protoss warriors created personal shields by focusing psionic energy. The protoss later reproduced the effect using psi-field generators. These generators could be installed in machines giving protoss vehicles shields as well. Psionically powerful protoss such as Khastiana can still enhance the power of their shields. Shields must be dropped to make use of some physical contact-based psionic abilities. Shields regenerate quickly after absorbing damage. Shields are charged via a device named the shield capacitor, also referred to as a shield battery. These devices can be drained by extended use.
After the Brood War, vast advancements were made to shield regeneration technology, which led to the phasing out of the shield battery as a means to recharge shields. However, improvements to the shield battery's ability to recharge plasma shields led to some factions reintegrating them.
Shields points act as another form of hit points and are usually lost first. Unlike regular HP, plasma shields always suffer full damage regardless of the type of attack used against them or the size of the shielded object. For example, a large-sized dragoon's shields will take full damage from concussive damage attacks such as from a vulture.
Shield points have a separate "armor" rating from regular hit points. The base "armor" is 0 and may be upgraded. This applies to both units and buildings. If an attack both depletes shields and damages HP, both shield armor and regular armor are subtracted from the attack's damage. Shield points regenerate slowly but a shield battery may be used to restore them faster. They regenerate at a rate of approximately ~1.5 points per second.
↑Karune. 2007-01-22. http://www.battle.net/forums/thread.aspx?fn=sc2-general&t=269104&p=1&#post269104 (archived 24 January 2008) StarCraft II Q&A - Batch 26. Battle.net StarCraft II General Discussion Forum. Q " If they are based on PSI energy, how can be possible for a Terran EMP attack to drain their energy?" A " the energy field projected by a pylon that powers Protoss structures cannot truly be called psionic in nature, just as the shields protecting a zealot are not purely technological constructs. Rather, the two processes are so closely interlinked that they are almost the same. As a result, Protoss power sources and particularly their shields are vulnerable to disruption by technological means such as EMP weapons. Similarly such power sources can be restored or recharged from artificially stored energy."
↑Underwood, Peter, Bill Roper, Chris Metzen and Jeffrey Vaughn. StarCraft (Manual). Irvine, Calif.: Blizzard Entertainment, 1998.