MCCB single pole interrupting rating
MCCB single pole interrupting rating
(OP)
I've seen it stated in several places that special consideration needs to be made for corner grounded delta, high resistance grounded, and non-grounded systems regarding the single pole interrupting rating of MCCBs. I understand why this consideration must be made, however I don't understand how this is a special consideration. I haven't seen any mention of the single pole interrupting rating being an issue for solidly grounded systems.
Consider the following:
Let the subscripts 0, 1, and 2 signify the zero, positive, and negative sequences respectively. V is the pre-fault voltage), Z is an impedance. Zf is an impedance is the fault path.
The line to ground fault current is given by:
I0=I1=I2=V/(Z0+Z1+Z2+3Zf)
The line to line fault current is(I realize that the fault on page 38 of the link is a double line to ground fault with fault impedance in between the phases but I'm trying to keep this simple):
I1=-I2=V/(Z1+Z2+Zf)
For a truly solidly grounded system Z0 should be close to 0 ohms. For a bolted fault Zf is 0 ohms. Thus in a solidly grounded system with a bolted fault, the ground fault current could be 1.73 times a line to line fault. I have assumed the positive and negative sequence impedances to be equal.
Let a=e^(j*120) the A matrix = [(1,1,1);(1,a^2,a);(1,a,a^2)]. Let Z0=0, Zf=0, Z1=Z2=1.
For a line to ground fault:
[Ia;Ib;Ic]=A*[1/2;1/2;1/2]=[3/2;0;0]
For a line to line fault:
[Ia;Ib;Ic]=A*[0;1/2;-1/2]=[0;-j3^0.5/2;j3^0.5/2]
And (3/2)/(3^0.5/2)=3^0.5
Is the question clear enough? - Why is single pole interrupting rating (seem to be) only mentioned when the system isn't solidly grounded?
From the link:
I don't see how full phase voltage on a pole is really relevant. If the fault current is higher than interrupting rating, the fact that the voltage is too high is somewhat of a secondary issue I think.
As a secondary question, consider a resistance grounded system with a three phase symmetrical fault level of, say, 60kA. Assume a fault occurs as shown of page 38 of the link for the resistance grounded system (Figure 7). Lets assume the current for this fault is 87% of the three phase fault, or 52kA. Are there breakers available with single pole ratings this high? I can only seem to find some with 12120 and 8660 amp ratings. There must be some otherwise there are probably a lot of misapplied breakers out there...
Link:
http://spd.cooperbussmann.com/Documents/SPD_Sec2.p... (pages 37 and 38 are of interest)
Reference thread:
http://www.eng-tips.com/viewthread.cfm?qid=86146
Thanks
Consider the following:
Let the subscripts 0, 1, and 2 signify the zero, positive, and negative sequences respectively. V is the pre-fault voltage), Z is an impedance. Zf is an impedance is the fault path.
The line to ground fault current is given by:
I0=I1=I2=V/(Z0+Z1+Z2+3Zf)
The line to line fault current is(I realize that the fault on page 38 of the link is a double line to ground fault with fault impedance in between the phases but I'm trying to keep this simple):
I1=-I2=V/(Z1+Z2+Zf)
For a truly solidly grounded system Z0 should be close to 0 ohms. For a bolted fault Zf is 0 ohms. Thus in a solidly grounded system with a bolted fault, the ground fault current could be 1.73 times a line to line fault. I have assumed the positive and negative sequence impedances to be equal.
Let a=e^(j*120) the A matrix = [(1,1,1);(1,a^2,a);(1,a,a^2)]. Let Z0=0, Zf=0, Z1=Z2=1.
For a line to ground fault:
[Ia;Ib;Ic]=A*[1/2;1/2;1/2]=[3/2;0;0]
For a line to line fault:
[Ia;Ib;Ic]=A*[0;1/2;-1/2]=[0;-j3^0.5/2;j3^0.5/2]
And (3/2)/(3^0.5/2)=3^0.5
Is the question clear enough? - Why is single pole interrupting rating (seem to be) only mentioned when the system isn't solidly grounded?
From the link:
Quote (Cooper Bussmann)
Although not as common as the solidly grounded wye connection, the
following three systems are typically found in industrial installations where
continuous operation is essential. Whenever these systems are encountered,
it is absolutely essential that the proper application of single-pole interrupting
capabilities be assured. This is due to the fact that full phase-to-phase voltage
can appear across just one pole. Phase-to-phase voltage across one pole is
much more difficult for an overcurrent device to clear than the line-to-neutral
voltage associated with the solidly grounded wye systems.
I don't see how full phase voltage on a pole is really relevant. If the fault current is higher than interrupting rating, the fact that the voltage is too high is somewhat of a secondary issue I think.
As a secondary question, consider a resistance grounded system with a three phase symmetrical fault level of, say, 60kA. Assume a fault occurs as shown of page 38 of the link for the resistance grounded system (Figure 7). Lets assume the current for this fault is 87% of the three phase fault, or 52kA. Are there breakers available with single pole ratings this high? I can only seem to find some with 12120 and 8660 amp ratings. There must be some otherwise there are probably a lot of misapplied breakers out there...
Link:
http://spd.cooperbussmann.com/Documents/SPD_Sec2.p... (pages 37 and 38 are of interest)
Reference thread:
http://www.eng-tips.com/viewthread.cfm?qid=86146
Thanks






RE: MCCB single pole interrupting rating
On a 480/277 grounded wye system, the breaker contacts only have 277V across them or 480 Volts across two contacts in series when interrupting a phase-phase fault. For a corner grounded wye, the single pole will have 480V across it's opening contacts.
Mulitple restrikes of the arc will generate more heat inside the breaker.
A breaker's interrupting rating is also affected by the amount of heat generated in the arc. With 480V across an arcing contact there will be more heat generated than if it was 277V. (I know this comment is not quite correct. I'm sure the physics of arc interruption is a lot more complicated than my simple explanation).
RE: MCCB single pole interrupting rating
RE: MCCB single pole interrupting rating
RE: MCCB single pole interrupting rating
So to answer your question, zero sequence impedance is generally appreciably higher than positive sequence impedance in solidly grounded installations. That said the 3 phase fault is equal to the line to ground fault at the transformer LV terminals, so very close to transformer may be an issue. Also if the source has a lower zero sequence impedance (generator without NER) the line to ground fault can be larger than the 3 phase fault. LG can even be higher than 3 phase at a delta-wye LV terminals, although not by much, because the zero seq impedance of the upstream network does not have path through the transformer.
Hope this helps.
RE: MCCB single pole interrupting rating
RE: MCCB single pole interrupting rating
I agree, the 87% rating seems odd, even in common installations.
RE: MCCB single pole interrupting rating
RE: MCCB single pole interrupting rating
RE: MCCB single pole interrupting rating
ASIDE:
The (old) Westinghouse [EHD] and GE [TED] MCCB "frame books" listed single-pole interrupting ratings as 8660 amperes.