This unit is based on the height of the viewport. The VH stands for "very high". It refers to the interrupt rating (AIC), which is 22 kAIC, as opposed to the standard 10 kAIC. Do we offer fire alarm breaker lock offs for QO and/or Homeline breakers? Is there an available QOB280GFI breaker? Do you need to use QO-HID breakers for lighting loads at a. An Electrician must know Electrical Abbreviations and Full Forms to read a electrical drawings. If you don't know you can't work with SLD drawings. You can be a. Let's first look at what these units mean. Set different length values, using px (pixels): Try it Yourself » Note: A. The viewport is a moving target, and CSS gives us a unit designed specifically for that movement: vh. If you want hero sections that feel full-screen, panels that scale with device orientation, or typography that responds to screen height instead of just width, you need to understand vh clearly.