rms = root mean square. For alternating current, this is the value of dc current that gives an equivalent heating effect in a resistance. For a pure sine wave, the rms current value is 0.707 times the peak value. For power systems, any current anyone gives you will be rms, unless otherwise stated, or shame on them.
symmetrical = means that the ac sine wave is centered at zero - the top half looks the same as the bottom half. This is normally the case, except for transient conditions, such as faults, when the current wave can be offset in one direction or the other.
Probably a Google search will yield some nice graphics illustrating this.
Hi Steve.
You have been asking about fault currents so I will take a chance and assume that you are asking about RMS Symetrical fault currents.
There are two types of fault current. Symetrical fault currents which can be easily calculated and Asymetrical fault currents or transient fault currents. The calculations for asymetrical or transient fault currents are more involved than the calculations for symetrical fault currents.
The interupting capacity of circuit breakers (Not the rated current but the current that they are able to interupt without exploding or doing other serious damage) is given in symetrical current. The breakers are actually capable of interupting a current several times the symetrical rating, so as to be able to safely interupt the Asymetrical currents that will be present during a fault.
Example:
A normal 15 amp circuit breaker may have an interupting rating of 10,000 amps.
It will actually be capable of interupting a current of well in excess of 20,000 amps.
This allows the simple calculations for symetrical current to be used while having the capacity to safely interupt the greater asymetrical current associated with a symetrical current of 10,000 amps.
There was a recent discussion concerning the relationship between the symetrical rating of circuit breakers and the actual asymetrical capacity.
respectfully
