Equipment zero sequence fault current sizing

[Tainted]

If an equipment is rated to withstand 100kA symmetrical rms,

does this mean the equipment will have to be sized to also withstand zero sequence fault currents?

For example lets say the symmetrical fault current is 88kA, but my zero sequence fault current is 110kA,

can I still use the 100kA symmetrical rms rated equipment?

 

[topgone]

The zero sequence fault will always be lesser or at least equal to the three-phase fault. So if you base your circuit breaker size on the symmetrical RMS fault, your CB will be able to withstand the zero sequence fault also. Here's a quote from a web search I did:

The ratio of (Z0/Z1) will rarely be close to unity, and often much greater than unity. In most cases, the magnitude of the single-phase to-earth-fault will be smaller than the three-phase fault. This is because of the involvement of the cable impedance in the fault impedance. The zero sequence impedance of a cable will be much greater than the positive sequence impedance of the same cable. Hence the ratio of the system (Z0/Z1) at the point of fault will be much greater than 1. This will cause the single-phase to-earth fault to be always less than the three-phase fault.

Rules:

1. Your breaker's close and latch rating should be = asymmetrical fault current
2. The bus bracing rating ~ should be equal to or greater than the rms value of the fault at that location. (withstand rating)
3. The continuous current rating of the circuit breaker should be about 80% of the continuous current expected to be passing through the breaker.
4. Zero sequence current can only be greater than the positive sequence current when the breaker is located very near the generator.

Hope that helps.

 

[jim dungar]

If an equipment is rated to withstand 100kA symmetrical rms,

does this mean the equipment will have to be sized to also withstand zero sequence fault currents?

For example lets say the symmetrical fault current is 88kA, but my zero sequence fault current is 110kA,

can I still use the 100kA symmetrical rms rated equipment?

It must be rated for the maximum fault, even if it is Zero sequence.
Zero sequence values tend to fall off relatively quickly as the point of the fault moves away from the system source, be it a generator or transformer. About the only time I have seen a problem is with close coupled equipment.