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Series Inductor-Limit Fault current

Series Inductor-Limit Fault current

Series Inductor-Limit Fault current

(OP)
has anyone had experience using and Impedance to limit the fault current inorder to limit the ARC Flash Cat at the main switchgear?

RE: Series Inductor-Limit Fault current

You should calculate how much you'd have to reduce the fault current to have a big impact on the arc-flash level.  The problem with inserting a reactor is that it is there all the time and creates a lot of voltage drop and heat generation even during normal operations.  They can be effective in reducing fault current levels a little in order to get within a circuit breaker's interrupting limit.  But with arc-flash you generally need a much bigger reduction in current.  

I don't think it is going to be a practical solution unless you are just trying to go from 40.1 cal/cm2 to 39.9 cal/cm2.  

 

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

RE: Series Inductor-Limit Fault current

Am I missing something here (I am having a dead brain day)? It seems to me that the more fault current, the faster the protective device (PD) operates. Reducing the fault current will slow the PDs & thus raise the arc flash hazard.

There might be more to this than I am thinking of right now!

RE: Series Inductor-Limit Fault current

(OP)
Level of fault current at some point has more affect than faster protecive device.

RE: Series Inductor-Limit Fault current

Eleceng01,

It depends on the rate of change in the clearing time as a function of fault current.  For the same clearing time, more current = higher arc energy.  

If you're on the short-time delay of a 480 V trip unit, the clearing time stays constant over a fairly wide range of current.   

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

RE: Series Inductor-Limit Fault current

The problem with faster relays is the breaker speed dosen't change. So at some point making fault interruption faster actually increases in cost at a very high rate.

However, it is possible to order transforers with a higher inpedance. But the higher impedance also develops higher losses.

The exception to the reactor loss condition is when it is placed between two load serving transformers (assuming the load serving bus is served by more than one transformer). In this case the reactor won't see load all the time, and thus whon't incure the losses.

Other schemes to reduce arc flash are to use a neutral impedance on the load serving transformer. Or current limiting fuses.

RE: Series Inductor-Limit Fault current

A neutral impedance will not do anything toward lowering arc flash as the arc flash calculations are all based on 3-phase faults and the neutral reactor does not change the 3-phase fault value.  Current limiting fuses are only of value when the fault current is high enough to be in the current limiting range.

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