Phase to Phase rating of cut-outs

[mbrooke]

I semi often see distribution substations with paralleled secondaries or large power transformers with low relative Z. The secondary neutrals are always grounded through a 2, 3 or 5 ohm reactor limiting L-G current to 10ka or less. However 3 phase fault currents calculate higher with typical cutouts employed out on the line starting outside the substation. Are ratings higher when considering a phase to phase fault? Can I rely on cutouts being in "series" during a 3 phase fault?


For those in the NEC world I'm open to your input as well as this could technically apply to any fuse installation.

 

[Tony S]

For a Ph→E fault you’re relying on the earthing system to clear the fault with all the inherent problems that go with it. The NER limits the current avoiding problems such as step and touch voltage.

Ph→Ph you have (or should have) the Ph conductors to clear the fault.

 

[mbrooke]

For a Ph→E fault you’re relying on the earthing system to clear the fault with all the inherent problems that go with it. The NER limits the current avoiding problems such as step and touch voltage.

Ph→Ph you have (or should have) the Ph conductors to clear the fault.

Ok- however I assume the cutouts will not share and clear the fault equally?

 

[Tony S]

Have a look for Merz Price protection and its derivatives.

 

[mivey]

You need cutouts rated for the fault current, 3-phase or otherwise. You can't assume equal sharing of interrupt duty between cutouts.

Current limiting fuses may help.

 

[GoldDigger]

Ok- however I assume the cutouts will not share and clear the fault equally?

For a single phase to phase fault the two cutouts are effectively in series, not parallel, so what you are asking is, in effect, whether two cutouts in series can clear a higher current just because the effective voltage is lower.
Since one failure mode of the cutout is mechanical damage from magnetic forces, I would definitely not count on being able to exceed the current rating.
If the manufacturer gives different current ratings depending on system voltage, you might be able to leverage the lower voltage rating, but only if actual series tests were done by the manufacturer.
JMO

 

[mbrooke]

You need cutouts rated for the fault current, 3-phase or otherwise. You can't assume equal sharing of interrupt duty between cutouts.

Current limiting fuses may help.

Are you thinking of this scenario?

 

[mbrooke]

For a single phase to phase fault the two cutouts are effectively in series, not parallel, so what you are asking is, in effect, whether two cutouts in series can clear a higher current just because the effective voltage is lower.
Since one failure mode of the cutout is mechanical damage from magnetic forces, I would definitely not count on being able to exceed the current rating.
If the manufacturer gives different current ratings depending on system voltage, you might be able to leverage the lower voltage rating, but only if actual series tests were done by the manufacturer.
JMO

Yup, my exact question. Had no considered magnetic stress, the would certainly limit max current a cutout should be subjected to.