Arc Flash Mitigation

[timm333]

I am trying to figure out that which arc flash mitigation technique is better for switchgear: either maintenance-switch, or arc flash relay which uses bare fiber to detect light generated by arc flash.

The operating time of arc flash relay is around 5 ms. I heard that arc flash rely is better because it is faster than the maintenance-switch. Is it correct? Thanks.

 

[jim dungar]

Remember, a protective scheme/device cannot protect itself. They on!y work for downstream locations.

Arc flash relay schemes are somewhat complicated and maintenance intensive, so they are often better suited for MV equipment which already require relays.

 

[Jraef]

It's not a simple comparison of speed. The "Maintenance Switch" option is just a key switch input to a feature in a special circuit breaker that changes the trip threshold to be lower than normal, with the idea that the breaker BEGINS tripping sooner and thus limits the arc flash energy by the time it clears the fault. The fiber system DETECTS the arc flash via the light it creates, which also begins the process of interrupting the fault sooner. Yes, the light sensing will be earlier, but still depends on the clearing time of the breaker and it might nuisance trip on some minor arcing component that would not have resulted in a full blown arc flash event, so that risk must be weighed. On the other side, someone may inadvertently forget to reset the Maintenance Switch and leave the breaker in the lower trip threshold, which may have the same effect.

 

[David Castor]

The "maintenance switch" is generally an instantaneous overcurrent trip element in a relay or trip unit that is enabled via some type of manual switching. The instantaneous trip is not really instantaneous, but will trip with no intentional time delay. The arc-flash sensor relay may be slightly faster, but the difference in speed will generally not result in a big difference in incident energy (a few milliseconds. Regardless of the approach, it will take a few (3 to 5) additional cycles (16.7 ms/cycle at 60 Hz) for the breaker to actually clear the fault. There is also the question of the reliability of the arc flash sensing system. Will it actually detect all arcing fault in the gear? The overcurrent sensing is well-proven and reliable.

The maintenance mode approach will be vastly simpler and less expensive than installing the arc sensing within the switchgear, and is relatively simple to implement as a retrofit. However, while in maintenance mode, coordination is sacrificed.

If money is no object, you can do both.

So, deciding which is better depends on your priorities, budget and confidence level.