# the joule

How come surge protectors are measured in joule-ratings and not ampere/volt/watt-ratings? The sources i loooked at (techweb, wiki, howstuffworks, sdmb archive) weren’t much help. Wiki was talking about force and work, and barely touch base electrically. So could a more electrically-inclined doper shed some more light on here?

A Joule is a watt second.
5 joules = 5 watt seconds

You might find the answer here.

Force, work and all those other units apply to electricity. A volt is the basic unit of force in electricity. An Amper is the basic unit of current flow. A Watt is one amper of flow at a force of one volt which is a unit of power. A watt of power for one second is one joule of energy. Of course if you get a surge that number of joules will be dissapated in a fraction of a second so the power level can be extremely high, enough to destroy cicuitry that isn’t protected.

The volt is not a unit of force. Volts are joules per coulombs, that is, energy divided by charge. Typically, voltage is measured between two points, and the voltage is the amount of work it would take to move a standard charge (a coulomb) from one of those points to the other. Voltage may seem like force, since a voltage difference “pushes” charged particles, but the units are different.

I looked up surge protectors at How Stuff Works . There are three important ratings: clamping voltage (the voltage at which the protector activates), response time (the amount of time it takes for a protector to respond to a surge), and energy absorption. The last one is the joule rating, and it tells how much energy the surge protector can absorb before it fails. When the voltage exceeds the clamping voltage for a time longer than the response time, the surge protector will start routing current to ground until either the surge ends or the joule rating is exceeded. The reason the rating is given in joules is that a short, high-powered surge and a longer, lower-powered surge can have the same amount of excess energy. It’s not a particular level of current or voltage that causes the surge protector to fail - it’s an amount of energy dissipated.