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 Post subject: Mitigation techniques and labels
PostPosted: Wed Feb 04, 2015 7:29 am 

Joined: Fri Dec 05, 2014 12:23 pm
Posts: 13
Re: Arc Flash mitigation techniques (optical sensing, maintenance switches, Arc Resistant gear etc…) and labeling

Questions:
1. If I were to incorporate optical sensing (which is on 24/7) throughout medium voltage switchgear, am I able to take the opening time of that system into account when doing an arc flash study and subsequent labeling of that gear?
2. Under what standard or reference am I allowed to do so? (which reference(s) do I hang my hat on)
3. Anyone have any experience with “false trips” of optical sensing that incorporates both monitor light and current?
4. What about something like a maintenance switch – which is not on 24/7 and would need to manually be turned on – dual labels?
5. Any general comments on labeling after incorporating mitigation techniques.


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 Post subject: Re: Mitigation techniques and labels
PostPosted: Sat Feb 07, 2015 9:59 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Except for the fuse equations (which are not generally used), opening time is an input to the equations in IEEE 1584 and other incident energy calculations but it is not a part of the scope. Normally one uses the time-current curves to determine opening time but with some specific types of tripping mechanisms such as optical sensors, differential 87 relaying, and zone selective instantaneous tripping, time-current curves don't tell the whole story. It becomes necessary to first determine trip time from the protective device literature/testing (typically1 cycle or less) and then look at the opening time of the circuit breaker or with arc terminators, closing time.

Depending on how you do it, multiple ratings may occur not only with maintenance switches but also with different sources (generator power in particular), doors closed (arc resistant gear), sometimes based on cable length, etc. One size may fit all or may not.


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 Post subject: Re: Mitigation techniques and labels
PostPosted: Mon Feb 09, 2015 8:40 am 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
danocap,
I have been reading your posts and I think Paulengr hit the time issue. As far as false trips, the current and flash virtually eliminates false tripping - but it is a programmed device and subject to human error. That is why we test relays.

For the Maintenance switch we provide two labels, Normal and Maintenance.

Using a maint/sw is an effective solution provided you have a relay. If you have to add the relay it is about $5000 if done while doing breaker maintenance -per breaker. Adding arc flash relay requires the switchgear to be out of service but provides some additional benefit of the light detection - although not recognized by software.

Another possibility is the use of a differential relay. This is much more expensive (possibly) but a much faster protection scheme.

Lastly, what standard can be referenced? I do not know of any as they do not reference the newer technologies. The only reference you have is good engineering practice and the standard of care. By that I mean, what is the rest of the community doing and is it an accepted practice. This forum is probably the reference for that.

R


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 Post subject: Re: Mitigation techniques and labels
PostPosted: Tue Feb 10, 2015 1:39 pm 
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Guess I might now have explained "standard". IEEE 1584 takes an "opening time" in the equation as an input. It is not defined within IEEE 1584 itself. That standard simply predicts that if an arc happens for X seconds, what the incident energy will be.

Outside of IEEE 1584, we either have to consult fuse curves, or relay curves to determine opening time. If we are dealing with a breaker, an additional factor often ignored is the time required from the instant that the protective device starts sending a trip command to the point where current is interrupted. It is generally between 2 and 3 cycles for a vacuum interrupter. With air, oil, and SF6, there is a lot more variability among manufacturers from around 1 cycle for small molded case breakers up to 10-15 cycles for very old oil circuit breakers. So be very careful even with just time-current curves as to what the source is.

The rest of what I was trying to say is that time-current curves don't tell the whole story in some cases. They are great when the only things to consider are the fault current and the curve that the device is responding to. They are no use when dealing with tripping logic that is something other than an inverse time curve relationship.

As a real world example by the way, I have just done an inventory and found that in about 7% of the busses in my plant, the opening time is already under 65 milliseconds but the incident energy exceeds 1.2 cal/cm^2, up to 30 cal/cm^2 in one case with an internal bus in a low voltage drive. Fault currents are 30-50 kA. In these instances the solution is going to have to come from either current limiting or reducing power densities. Even if the protective relay trips in 0 ms, it's not going to be fast enough. Perhaps a very fast breaker such as an "arc terminator" combined with the fast relay will help but in general the solution will involve adding impedance to the system to reduce fault current.


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