CB Opening time
CB Opening time
(OP)
For the 5 cycle breaker, I see in IEEE 242 a 0.20 sec CTI, coordination time interval between static relays. I also see arc flash calculations using 0.083 (5 cycle) for the same equipment. Shouldn't these numbers be the same?






RE: CB Opening time
RE: CB Opening time
If your CTI is ever LESS than the breaker clearing time, you've got a problem.
RE: CB Opening time
RE: CB Opening time
RE: CB Opening time
RE: CB Opening time
RE: CB Opening time
For arc-flash calcs, if you want to add a safety factor to the calculated clearing time for the relay/breaker combination, you can do that, but this is still not the same thing as the coordination time interval.
The 5-cycle breaker clearing time is generally conservative to begin with, since most faults will clear sooner than this.
RE: CB Opening time
3 to 5 cycles (0.083 s) is the breaker clearing time but you have to add about 7 1/2 cycles for relay tolerance and setting errors to arrive at 0.2 sec CTI.
However, there are MCCB's can have as low as 0.5 cycle clearing time. Using the same relays (7.5 cycles from fault detection to trip signal), we could end up with a CTI which is 0.133sec.
Field calibrations done on relays may slash a little on the CTI to a lower value.
IMHO, it's best to say that we will use the Buff Book as guide, but we will verify if the CTI we will be using is true in the field.
RE: CB Opening time
RE: CB Opening time
RE: CB Opening time
I think dpc, that cb operating time of 5 cycles is the mechanical operation, and it is not much different from what we have tested in the field, for a fully calibrated, serviced, maintained CB. The difference between the 0.20 CTI and 0.08 is 0.120. That seems to be logical that part is for the upstream and part for downstream, for coordination.
For coordination, the question is how quick can the upstream RELAY operate. For arc flash, the question is how long can the upstream CB take. I think at least 1/2 of the 0.120 or 0.06 to be added. So at least a total of 0.140 or maybe higher. It could even be closer to 0.20, as in coordination.
RE: CB Opening time
If as noted by EP007, the concern is at a low fault level where the relay time is in seconds, a fixed CTI may not be appropriate. Relay timing accuracy is a % of time plus sometimes a fixed time. An SEL-351S has a curve timing accuracy of ±1.5 cycles and ±4% of curve time for current between 2 and 30 times pickup. Accounting for a 10% CT error would require a time interval that depends on the slope of the relay curve.
Then again, if we're talking about seconds, the 2 second maximum arcing time rule will come into play. What is the accuracy of that?
I think that if you are conservative in your fault current calculation, it would be difficult to justify using higher fault clearing times than are required by the standards.
RE: CB Opening time
The relay must have some calculation time such as 1-2 cycles for sampling, and I assume that is additional to the setting.
jghist, what is the difference on that relay between timer accuracy and time dial (+/-5% of pickup?)
A CB rated 5 cycle has 3 cycle contact parting time, 2.5 cycle opening time, but I don't know how that enters into this.
RE: CB Opening time
The requirements in arc flash standards are not a limitation on clearing time, but the clearing time has a major effect on arc energy.
The rms level can be determined with 0.25 cycles of samples. I would assume that this is the reason for the 0.25 cycles in the instantaneous unit timer accuracy.
I'm not sure what you mean by time dial accuracy. This would be part of the curve timing accuracy that I quoted, which is the same for an SEL-351A.
RE: CB Opening time
From this discussion, it looks like the 0.20 sec CTI listed in the IEEE standard is higher than needed for definite time elements (50), and subject to more detailed examination for TOC (51). The standard referred to is ieee 242 Protection and Coordination of Industrial and Commercial Power Systems, Table 15-2 and 15-3.