Relay coordination

[aqua1985]

Hey,

Needed some help with relay coordination
I have a system whose arc flash study i have already conducted. However, the arc flash level is very high. I have now been assigned the task of reducing the arcing time by coordinating the devices again. I have read IEEE 242 chapter 15 on overcurrent coordination. However, i am little unsure of how to go about changing the relay settings of an already coordinated system. I need some advice.

Thanks

 

[avogel]

Hire a licensed PE consultant who knows what they are doing!

 

[dpc]

Faster tripping means lower arc-flash. Review the settings and see if you can reduce the fault clearing times.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[aqua1985]

dpc,

yes, i am trying to lower the short time settings.

thanks for the advice.

aqua

 

[davidbeach]

Short time setting probably aren't going to do much for you as the fastest short time is probably too long to make a lot of difference. You want instantaneous everywhere for arc flash. Instantaneous everywhere using overcurrent means no selective coordination. So, you need zone protection, typically differential, but that gets expensive. So, then you are left with making compromises and for that lots and lots of experience is quite helpful. Since all installations are different, you won't get more than the most general information here. avogel provided good advice if you don't have the experience or have it available on site.

 

[stevenal]

Re-coordination may be difficult to achieve. Another approach is to implement a maintenance setting. The maintenance setting is only to be enabled during live work, and sacrifices coordination in favor of safety during this time. Optical arc detecting relays are another option.

 

[dpc]

Actually, for 480 V power circuit breakers, the short time settings make a lot of difference in many situations, since instantaneous is often not used or even available in the trip unit.

Differential or instantaneous and/or maintenance settings can provide lower arc-flash, but there are a lot of 480 V systems with main breakers set to max delay and tie breakers set to intermediate. Just lowering the main breaker ST delay down to intermediate can make a big difference while you work at replacing the trip units or incorporating something better.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg