Flashover in MCC 1

[splitbolt]

I maintian a Hammermill that has a control cabinet built in 1960. The supply voltage to the MCC is 500V ungrounded delta. The MCC has a 400A main breaker with 25 #10 AWG conductors per phase on the load side feeding 25 30A G.E. breakers and starters in the same cabinet. The MCC was site built to very slackend standards and the componets are snugly fit with little air space.

The problem is that when a fault occures at any motor feeder we lose 5 or 6 breakers and starters do to flashover. Is the voltage spiking to high for the close coupled components? Could it have somthing to do with the sytem being ungrounded. Any input would be greatly appreciated.

Best Regards,
ASB1

 

[gordonl]

Some free speculation:

With the ungrounded system when you get a fault phase to ground voltage rises to the phase to phase voltage on two phases. I would suspect this is the source of the flashover in the MCC when a motor circuit fails. (Leakage ground current flows initiating the flash over) (As noted speculation) A good cleaning may help.

 

[DanDel]

ASB1, when you say "we lose 5 or 6 breakers and starters do to flashover", do you mean that they actually burn up/get destroyed, and must be replaced?

 

[GordS]

Ungrounded systems are subject to voltage rises in the range of 6X during arcing ground faults. In my opinion, most faults start out as arcing as insulation begins to break down and degenerate to bolted faults. But during the arcing portion, expect high voltages.

 

[busbar]

Low-voltage ungrounded-system {transient, oscillatory or steady-state} "neutral shift" outside of nominal ø-ø voltage can be disastrous. One account is 67.115.161.42/dat/beemaIPSH6.doc

Conversion to a high-resistance-grounded system for ungrounded 240-600V distribution may be desirable, and is well documented. An example is John P. Nelson and Pankaj K. Sen, High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

 

[splitbolt]

DanDel, the breakers and starters do not burn up they carbon track past the point of safe reliable operation.

 

[DanDel]

Assuming that the voltage rating of the components are adequate for the system voltage, the spacing between components connected to different phases along with the amount of dirt/moisture/ambient humidity present becomes critical during the overvoltages already mentioned.
Sounds like you might have to rebuild the unit with better spacings and/or go to a grounded(or resistance-grounded) system.

 

[jbartos]

Suggestion: The system solidly grounded neutral 500V power supply would be a good solution. It will need a grounding transformer to create an artificial neutral for the solidly grounded artificial neutral system grounding. Zig-zag or Delta-wye grounding transformer will do.