MCCB single pole interrupting rating

[wroggent]

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:

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.pdf

(pages 37 and 38 are of interest)

Reference thread:

http://www.eng-tips.com/viewthread.cfm?qid=86146

Thanks

 

[rcw retired EE]

Interrupting capability of a breaker is not just dependent on the current level, the voltage impressed across the contacts also affects the ability of the breaker to extinguish the resulting arc. If the voltage is too high, the distance between the opening contacts may not increase fast enough to prevent the arc from restriking after the first or subsequent current interruption(s).

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).

 

[healyx]

I might have this completely wrong but are you sure that Z0 is close to zero in a solidly grounded system? Zero sequence impedance is calculated by bridging the 3 phases at the source, then supplying with a single phase voltage source, so the path includes the 3 phase cables in parallel plus the return path in series. So even if the return path is zero, you'll still have the paralleled phase cables counted towards Z0.

 

[wroggent]

Good point. I don't know what I was thinking. Would you say then that the zero sequence impedance would be equal to the positive sequence impedance at minimum in a solidly grounded system?

 

[healyx]

Sorry, I wasn't sure about some of the terminology you were using, I'm in IEC land but I have looked at the bussman document now. I don,t think you are correct in saying Z0 in a solidly grounded system is zero, and not just because you need to include the phase cable impedances. The return current path impedance is usually higher than than a single phase, because the current returns primarily through the ground conductor which is a smaller cable than the phase cable (it is in IEC anyway). So roughly speaking the zero seq impedance is the Zph + 3*Zgnd, where Zph is impedance of the phase cable and Zgnd is the impedance of the ground cable. Z1 and Z2 = Zph. Line to Ground Fault current I = 3V/(Z1 + Z2 + Z0) = V/(Zph + Zgnd). 3 phase fault current = V/Zph. I have massively simplified this to show that the return path (ground conductor path) is the difference between line to ground and 3 phase faults in a solidly grounded system. Determining the reactance part of the sequence impedances is tricky (need to use cable geometry) and I have not included the path through the mass of earth (very hard to calculate).
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.

 

[wroggent]

I agree with you on all of that healyx. Any insight into my original questions?

 

[healyx]

A line to ground fault close to the transformer could be of the same magnitude or greater to a 3 phase fault at the same voltage. So having a single pole rated at 87% seems like it would be under rated even for a solidly grounded installation. I couldn't find any mention of these single pole ratings in some of the IEC device data I have, and I'd not heard of it before. I have always assumed IEC devices could take the 3 phase rated fault through a single pole. Can any IEC people confirm this?

I agree, the 87% rating seems odd, even in common installations.

 

[dpc]

In a solidly grounded system, the single pole only has to interrupt a single line-to-ground fault. In an ungrounded or high-res G, the single-pole may need to clear a double line to ground fault.

 

[wroggent]

So you assume a single line to ground fault will never exceed the single pole interrupting rating?

 

[busbar]

ASIDE:
The (old) Westinghouse [EHD] and GE [TED] MCCB "frame books" listed single-pole interrupting ratings as 8660 amperes.