This week's question is a continuation about "exceptions" found in the 2002 Edition of IEEE 1584.

If the calculated arcing short circuit current is lower than a protective device's instantaneous setting, the result can be an arc duration that is quite long according to the time current curve - sometimes seconds. It can be debated whether the time is realistic or not but the only language found in IEEE 1584 regarding using a "faster" clearing time is commonly refereed to as the "two second rule" Although technically not a rule, the specific language from IEEE 1584 is as follows:

If the time is longer than two seconds, consider how long a person is likely to remain in the location of the
arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible
and two seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who
has crawled into equipment will need more time to move away.

This week's question was asked years ago but practices evolve. Here we go.

Do you/clients/consultants ever use the "Two Second Rule"?
Yes
No
We use something different (please explain)

Jim, you did a pretty extensive paper on the subject.

As a counter argument if not the "2 second rule" then what? Is the right value 1 second or perhaps 10 seconds? If so, what's the basis for it? Is the basis any more than the 2 second rule? We use it because there's really nothing else out there even if it's little more than a passing reference, sort of like the "<=125 kVA, <=208 V" rule.

The 2 second rule was just human reaction time (well known number) as a way to address long clearing times. I believe most people react much faster and the working distance will have a rapid rate of change i.e. the person is moving away quickly. The 2 second rule assumes a person is there for the entire 2 seconds so my view it may be considered conservative. So far the 2 second rule has made the cut of the next edition of IEEE 1584.

The 125 kVA rule was (as listed in IEEE 1584) just a way to deal with a similar problem but with low current based on transformer size and voltage < 240 Volts. As some know, subsequent tests show 125 kVA was perhaps optimistic. This one will change in the next edition of IEEE 1584. We are resolving the comments right now from the vote on the new draft and are now completing a revised draft for vote - The 125 kVA rule is changing and will reference current. For now, that's all I can say.

Since 2002, we have learned quite a bit more so the standard evolves based on what has been learned.

I use the clearing time as specified by the time curves. Then I explain to the client why the incident energy is high and give them a recommendation for replacing the CB with a device that has a faster clearing time. If they choose not to replace the breaker then I label it with the calculated incident energy and move on. Having testified in many product liability cases I want to be able to show where my numbers came from and I don't want to be in a position of explaining the two second rule to a jury.

This 2s requirement for carrying a short circuit current (may be referred to as the short time current rating) is presumably selected to co-ordinate with the maximum permissible tripping delay for 4.76 kV - 72kV circuit breakers in accordance with IEEE Std. C37.04. The maximum permissible tripping delay for circuit breakers rated > 100kV is normally 1s. 2s /1s duration is also used to size ground conductors. Very conservative but readers should note that most utilities factor this into their specs. TimMol

Thanks for your comment however, the "2 second rule" being discussed here refers to a maximum duration based on IEEE 1584 that can be used in the event of a time current curve indicates a very long clearing time.

As Jim said the two seconds here refers to IEEE 1584, not The ANSI C37 series.

Generally this may be true for opening time when there is no specific delay and would apply to say the first zone in a distance relay or say differential protection but it's not true in general with time overcurrent coordination because each layer of relays needs a little more than one "opening time" delay to achieve coordination. With medium voltage the slowest breakers are around 0.2 seconds so in years past the recommended delay for coordination was 0.2-0.3 seconds although today that can be reduced to as little as around 0.1 second or less. At 115 kV+ when large air break circuit switchers are used and transient recovery voltage resistors are a must, switching can last 1 second or more so unless coordination is done with schemes other than time delays, relay operation can easily exceed 2 seconds.