Research EMP Shockwave - 200 200
Cost: 100
The Science Vessel can fire an EMP missile. This missile has a fairly short range an affects an area. Upon impact, the shields of any units (Protoss) are dispersed bringing them to 0. The energy of any unit caught within the EMP blast is also set to 0.
This ability is primarily an anti-Protoss weapon. It is most effective against Archons. Archons have 350 shields but only 10 hit points. When EMP is used on Archons, they are reduced to 10 hit point warriors. EMP can be used on Protoss buildings as well to make them easier to destroy. Key targets include Photon Cannons (Making them easier to destroy) as well as Shield Batteries (Canceling both its shields and energy) and Pylons powering key buildings. EMP works wonders on the Protoss armies reducing their power tremendously. A skilled Protoss player will run from the battle once hit with EMP to recharge at Shield Batteries rather than continuing on with the attack.
If you come across a tightly packed group of Magic users such as Battlecruisers, Templars, Wraiths, Ghosts, and so on you can use EMP to reset their energy slowing down their magic use.
from a very reputable source