# True RMS for circuit breaker

Electronic Circuit breaker

On page 9 of the breaker above, they take a sample of the full cycle and calculate the true rms. That would mean for a 50 Hz wave, a time of at least 20 ms before it trips. If a short circuit occurs, the breaker should trip instantly with little time delay. What sort of time delay is acceptable for instantaneous tripping ?

You are misinterpreting what they are saying. The averaging period does not "start over" on every complete cycle of the line current; instead, it is computed over a "sliding window" and gets updated on every sample (roughly 2 kHz). This gives a minimum trip time of about 0.5 ms.

• So it has a buffer of values used for calculation ?, when the buffer gets full, it will just shift the first entry out and add the new one ? Commented Oct 2, 2017 at 20:42
• Yes, that's what sliding window means. Commented Oct 2, 2017 at 22:07

Rather than some $I(t)^2*t$ typical electromagnetic response curve, it uses $I_{rms}(t)^2 *t$ energy curve , it more accurately measures pulse currents or harmonics together.

It has a similar response but more accurate with a full-function variable threshold that is logically defined by a boundary for more precise settings.

e.g. >~600As for long pickup, >~80As for long delay, >4AS for short delay >0.5As and 5ms for instant pickup. (normalized to 1A for rated current)

These are user-selectable curves starting at p18.

• Why are we multiplying by time ? Commented Oct 2, 2017 at 20:37
• power *t=energy= heat damage Commented Oct 2, 2017 at 20:42
• I(t) and Irms samples are stored in memory for over 10 minutes to accumulate history so if repetitive surges occurred that elevate breaker temp, but may not trip breaker, this can be estimated. This is more accurate and simulates actually sensing contact temperature,. the factor that accelerates permanent breaker failure (or Tfmr failure), so an algorithm computes the trip levels based on user settings. Commented Oct 2, 2017 at 20:58
• It might also have the ability to disable instant trip for more reliable supply from transient shorts within the breaker's capacity and possibly trip during a CT sensed zero crossing to reduce remanence. Commented Oct 2, 2017 at 21:04