KEGHA wrote:
We have a MCC Panel, which contains separate compartments, with separate doors, for each compartment. The energy level is different, for each compartment, because of individual sections breakers, and therefore convenient to label the doors with the actual Arc flash level. What is the requirement to the physical dividing plates, separating the compartments, to consider them as separate compartment? If an Arc occurs in one of the other locations within the MCC panel, and a service technician is carry out work in one of the separated compartments, the diving plates must withstand the Arc pressure, to protect the worker. Are where any standard describing, the requirements to the dividing plates? I guess the IEC based IP rating is not applicabel as it doesn't consider the mechanical strenght.
Yours sincerely
Kenneth G Hansen
Lead Engineer
Converter Electrical – Engineering Solutions
1. IEC IP ratings are related to environmental intrustion only, not resistance to an internal pressure rise.
2. MCC's are rated by maximum interrupting current capacity but this is a bolted fault. The limitation is with the bracing on the busbars that are subjected to sever magnetic force, not pressure from hot gases.
3. Arc blast pressure will rupture an MCC within 1-2 cycles regardless of construction unless it has a pressure relief system built in (arc resistant designs).
4. But arc FLASH is not rated on pressure and the two cannot be compared because arc last is over with after about 1 cycle (16 ms) whereas arc flash is potentially indefinite but generally modelled for 2 seconds or until the breaker/fuse trips, whichever comes first. It is radiant heat energy, not pressure.
5. A related effect is called arc tracking. On a bus, thre arc will actually travel away from the power source by magnetic propulsion (basically a rail gun) at a speed of several hundred feet per second. Once the plasma hits a "barrier" at the end of a bus or at a substantial wall in the enclosure it pools and the "arc flash" erupts from that point. Thus within a given vertical section of MCC, the actual point where the arc flash occurs may be at the bottom or top and not in the middle or near the bucket where it started.
6. If the temperature of the surrounding air is hot enough, to lower the flashover poiint low enough the arc flash can travel from one section to another. This is common inside the same enclosure such as inside a panelboard but less likely between sections of an MCC and very unlikely from different sections of switchgear.
7. Although especially very near a transformer there is a big difference if you calculate arc flash from one horizontal section to another based on the bus bar, nobody does this in practice. Generally you rate the entire MCC with one rating at the incoming main lugs if it is ain lug only. If it has a separate incoming main breaker, rate the compartment with the line side breaker equal to the line side arc flash, and the rest of the MCC based on the load side of the main breaker. There are those that suggest using the line side rating for the entire main breaker section but I haven't seen a lot of evidence for when the arc flash "jumps" (due to air heating) from lod to line side. Not saying that it doesn't happen but I can't give a hard and fast rule for when to apply this. So at most, MCC's would have two labels.
8. This is all based on bus/bucket stab failures which is worst case. As you look outside the MCC on the load side, incident energy is generally extremely low. It is rare that I've found >1.2 cal/cm^2 on the load side in practice and unless the equipment is worked on live such as in a subpanel, I don't calculate or apply stickers to the load side.
