CB Opening time

[EP007]

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?

 

[Zogzog]

If I understand your question correctly (possible I do not) the answer is not. The breaker operation time and the relay actuating time from the curve are not the same thing. The breaker is capable of opening in 5 cycles following a trip signal.

 

[dpc]

As zog says, these are not the same. The 0.2 sec CTI includes the breaker opening time PLUS the relay error, the CT error and all other variables.

If your CTI is ever LESS than the breaker clearing time, you've got a problem.

 

[EP007]

Shouldn't arc flash use the same as the relay coordination, 0.20 sec, in this case, as CB open time. Of course, the relay setting would be additional. What I am seeing is 0.083 for arc flash, not 0.20. What is the fundamental difference between the two that accounts for that difference?

 

[Zogzog]

The difference is that the breaker wont open until something tells it to.

 

[jghrist]

The coordinating time interval includes a safety factor for CT and relay error, setting error, etc. in addition to the breaker opening time. These errors could occur in an arc flash incident, but they are not required to be considered in the standards. You may want to be conservative and consider the additional time, however, if you consider personnel safety as important as relay coordination.

 

[EP007]

The 0.2 sec seems to be more appropriate. Reports that I have been seeing, prepared by various firms, have mostly used 0.08, directly from Ieee 1584 Table 1. I used 0.20 sec., but it has to be defended every time, against the majority practice.

 

[dpc]

The CTI is based on worst-case assumptions that the downstream device will operate at the slow end of the error band and that the upstream device will operate at the fast end of its error band. With arc-flash calcs, the error of the upstream device is not an issue.

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.

 

[burnt2x]

EP007,

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.

 

[Zogzog]

You can use any book you want but to really know you need to actually test it, all your books and standards assume the relays and breakers operate in the times they were designed to and dont account for worn springs, dirt, dried lubrication, , incorrect contact pressures, improper lubrication, no lubrication, etc....

 

[apowerengr]

Coordination and arc flash are two very different things, arguably diametrically opposed concepts. Coordination being between protective devices and arc flash based on the performance of just the nearest upstream device. I agree with zogzog and add that for certain designs of oil circuit breakers the clearing time varies in an inverse relationship to the amount of fault current available. For low level faults the magnetic fields associated with the current path don't help the arc interruption mechanism as they do at higher fault levels. So, a breaker rated at 5 cycles clearing time at its maximum interrupting rating of 20kA (nameplate ratings) might take 10-15 cycles to clear a 2kA fault. If you incorrectly model the breaker or don't take the slower clearing times into account you might misrepresent the arc flash values conservatively low and would most likely wind up with a miscoordination issue as the breaker could operate slowly enough to cause the upstream device to trip.

 

[EP007]

Is there a concensus on how much to add to the CB opening time of 0.083 sec.? The CTI calculations give some guidance on how to include for that. This is 4.16 KV, so no MCCB. Most of the issues are at higher fault levels, not at low fault like 2 KA, where relay setting may be in seconds anyways.

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.

 

[jghrist]

As noted by dpc, the CTI includes errors of both upstream and downstream relays and CTs. Including timing errors for one relay and current errors for one set of CTs might be considered for arc flash, but not both.

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.

 

[EP007]

The concern is higher current levels. I don't see requirements in arc flash standards for fault clearing time. Higher is more conservative. For SEL-351A, and 200 ms def time (50), accuracy is +/-0.25 cycles, +/-0.1% which is 4ms.

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.

 

[jghrist]

The concern is higher current levels. I don't see requirements in arc flash standards for fault clearing time. Higher is more conservative. For SEL-351A, and 200 ms def time (50), accuracy is +/-0.25 cycles, +/-0.1% which is 4ms.

If the concern is at high current levels, above the instantaneous/definite time pickup, then the 0.2 second margin seems excessive. At most, for a 200 ms definite time setting, the relay error would be 0.25 cycles + 0.1% or 200 ms, or 4.37 ms. CT error would not affect the timing unless you are within 10% of pickup, in which case the effect would be severe.

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 relay must have some calculation time such as 1-2 cycles for sampling, and I assume that is additional to the setting.

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.

jghist, what is the difference on that relay between timer accuracy and time dial (+/-5% of pickup?)

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.

 

[EP007]

I understand some other relay manufacturers base processing on rms, but in 351A, under Specs/Processing Specs/Digital Filtering, it is noted 1 cycle after cosine filter, net filtering rejects dc and all harmonics greater than the fundamental. I don't think 0.25 cycle can obtain dc. I read Time Dial Accuracy as a separate item in Specs for (51) element.

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.