This is a low voltage motor control center (LVMCC) under arc flash test conditions with the compartment door open, similar to applications found in the process industry where de-energizing the LVMCC is not practical
Five calorimeters were placed at 18” from front of open door to measure heat energy from fault through the open compartment. The configuration is bare #10 fine-strand SIS wire in a short circuit in the line side breaker terminals, simulating a fault that could occur by a technician mistake measuring incoming voltage or expulsion of a lead wire during a load side fault. The fault current for this test was 50kA.
The short-circuit arcing fault begins at the breaker terminals and within one-half cycle propagates to rear bus through the bucket stab assembly and is cleared by Main Backup Breaker in 300ms. The calorimeter readings were in excess of 40 cal/cm2. The fault caused vaporization of approximately 12 inches of copper riser bus, resulting in heavy contamination of adjacent structures.
A fault of this nature would have significant safety impact on the technician working with the door open to troubleshoot a problem and result in a long mean time to repair (MTTR) due to the damage to the LVMCC Assembly.
This is a similar test to the one above, but actually at a higher short circuit value of 65kA. The starter unit and the LVMCC assembly has been modified utilizing the MMT Services Inc. SelfEx program. This is the same line side fault condition as described above, however there was no tripping of the Main Backup Breaker and the fault was completely extinguished within the starter compartment, limiting damage to that compartment only. The fault duration was reduced to 9ms, and because the main backup breaker did not trip, the remaining starter compartments remained energized, avoiding process interruption. The average calorimeter reading was 0.32 cal/cm2 - a significant reduction from above. Utilizing the MMT SelfEx program, a technician could avoid the bulky arc flash suits required for 40 calorie equipment and use standard 8 calorie HRC2 type PPE with appropriate face shields, gloves and hearing protection. Also, NFPA 70E Risk Assessment factors for both arc flash and contact with live parts are reduced to minimum values due to the nature of the MMT SelfEx Modifications - all with the compartment door open.
The MTTR in this case would be the replacement of the starter unit - with no process interruption.
Below you will find the IEEE PCIC conference paper and a commented version of the presentation done in Calgary in September 2017. We welcome any questions or comments - please see our contact page for information
MMT Services has a tested, proven method to reduce arc flash hazards below 1 Cal/cm2 for open door access to LVMCC.
Please see the information below for information concerning the LVMCC SelfEx program.
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