Does the Slash rating on a breaker (120/240 480Y/277) only apply when the load is connected to a neutral?

[jim dungar]

I am still not understanding. Consider what Jaggedben said in post #8. Now that makes inuitive sense. I guess I am not seeing how a breaker would see any difference in a L-L fault on a corder grounded vs a L-L fault on a center grounded system.

Say you have a corner grounded B phase system with a 2 pole breaker that has a line to ground fault on A phase. The fault is only being cleared by the A phase contacts which sees the full L-L voltage because in this system L-G = L-L voltage.

But in a center grounded system the same condition has 1 pole of the breaker clearing the fault to ground on line A, but this time it only sees the L-G voltage and not the L-L one.

 

[electrofelon]

Say you have a corner grounded B phase system with a 2 pole breaker that has a line to ground fault on A phase. The fault is only being cleared by the A phase contacts which sees the full L-L voltage because in this system L-G = L-L voltage.

But in a center grounded system the same condition has 1 pole of the breaker clearing the fault to ground on line A, but this time it only sees the L-G voltage and not the L-L one.

Ok, now go back and read my post #15 and your response in post #17. Would you like to amend your response?

 

[Julio Chadrone]

Say you have a corner grounded B phase system with a 2 pole breaker that has a line to ground fault on A phase. The fault is only being cleared by the A phase contacts which sees the full L-L voltage because in this system L-G = L-L voltage.

But in a center grounded system the same condition has 1 pole of the breaker clearing the fault to ground on line A, but this time it only sees the L-G voltage and not the L-L one.

So trying to visualize it. I drew a picture of how I think it works from all your explanations. The first one is why Slash rating works on WYE and and second one is why it doesnt work on delta?

 

[jim dungar]

Ok, now go back and read my post #15 and your response in post #17. Would you like to amend your response?

No.

The situation of slash ratings is more complicated than simple 1 pole interruption as found with corner grounded systems.
The issue of fault clearing byproducts and L-G voltages is still an issue even if the fault is being cleared by multiple poles.

 

[LarryFine]

 

[Julio Chadrone]

Okay I think I understand where I was getting confused. I was treating line voltages and phase voltages interchangeably. I forgot that line voltage and phase voltage are different from delta and wye. In delta line voltage = phase voltage. In why line voltage = sqrt3phase voltage. The site I was reading about slash rating was talking about how they are rated in terms of phases.

"Slash-voltage rated breakers are tested to interrupt fault current with a higher voltage between phases and a lower voltage between phase-ground."
" Slash-voltage rated breakers are not rated to open faults at phase-phase voltage across only one pole."

Since the neutral is grounded in WYE a L-G fault will always go through the neutral.

 

[electrofelon]

No.

The situation of slash ratings is more complicated than simple 1 pole interruption as found with corner grounded systems.
The issue of fault clearing byproducts and L-G voltages is still an issue even if the fault is being cleared by multiple poles.

Ok, can you explain what the breakers sees differently on a L-L fault on a wye grounded system vs a L-L fault on a corner grounded system? Assuming AFC is the same, why would there be a difference?

 

[winnie]

Ok, can you explain what the breakers sees differently on a L-L fault on a wye grounded system vs a L-L fault on a corner grounded system? Assuming AFC is the same, why would there be a difference?

Draw out the entire fault current circuit for a L-L fault in the two systems. You start at the source, go through a breaker, through some wire, to a fault, through some more wire ( remember, L-L fault ), and back to the source.

On a wye system the return path to the source almost always involves another breaker. So in a wye system you are very likely to have two breakers in series handling the fault.

On a delta system the return path for a L-L fault has a much greater chance of not having a breaker. On a delta system a single pole will likely handle a L-L fault.

By 'fault clearing byproduct' I think @jim dungar is talking about the plasma/arc from the contacts opening. This can create as new connection to surrounding grounded metal. The voltage across that connection is obviously different for the wye and delta system.

 

[jim dungar]

Ok, can you explain what the breakers sees differently on a L-L fault on a wye grounded system vs a L-L fault on a corner grounded system? Assuming AFC is the same, why would there be a difference?

The interior of the slash rated breaker is built differently than that of the fully rated breaker.

 

[kwired]

If you have a delta supply, it is almost certainly a corner ground. So your "phase to grounded conductor' voltage would be at 480 volts. That exceeds the 277 rating of the breaker

BTW "grounded conductor" includes neutral (if Needed) and any kind of "ground wire" or maybe in the case of metallic raceways the raceway itself, everything.

so would the ~416 volts if it happened to be a high leg delta, or if you have ungrounded system you potentially could still have up to 480 volts needing interrupted once one line becomes grounded.

However on the more common 240 volt high leg delta systems you still can use 120/240 slash rated breakers if not connecting to the high leg. Is done all the time on those systems with single pole loads or two pole loads not utilizing the high leg of the system.

 

[jim dungar]

By 'fault clearing byproduct' I think @jim dungar is talking about the plasma/arc from the contacts opening. This can create as new connection to surrounding grounded metal. The voltage across that connection is obviously different for the wye and delta system.

The plasma/arc is a problem at the arc chute area of the breaker, as the gasses cool and are vented from the breaker the soot and melted metal may still be conductive as they exit.

The problems of single pole interruption exist for all breakers used on corner grounded and delta systems even those rated for full voltage ones. The L-G voltage issue is primarily associated with breakers designed for use only on grounded wye systems.

 

[jaggedben]

The interior of the slash rated breaker is built differently than that of the fully rated breaker.

Not an answer to his question. The breaker is built differently to address a different condition, right? What is the different condition in an L-L fault.

I think Jon already answered, but are you contending there's something more?

 

[jim dungar]

I think Jon already answered, but are you contending there's something more?

I have been saying a large part of the issue is the internal construction of the Slash Rated breaker. Others have gotten into the characteristics of the faults themselves.

The internal construction of breakers, such as inter phase barriers and gas exhaust paths, is important. For example: some manufacturers require breakers rated for Grounded B Phase systems be built using 3 pole housings even though only 2 poles are used and the breaker is rated for full voltage be it L-L or L-G.

 

[wwhitney]

On a delta system the return path for a L-L fault has a much greater chance of not having a breaker. On a delta system a single pole will likely handle a L-L fault.

Not following this. Let's say that on the corner grounded system, either the L-L fault is between the two ungrounded conductors, or the grounded conductor is also protected by the breaker. Then I don't see any difference in the fault clearing path between the corner grounded system and the wye system (ignoring the internals of the supplying transformer). In both cases it goes through two poles of the breaker.

By 'fault clearing byproduct' I think @jim dungar is talking about the plasma/arc from the contacts opening. This can create as new connection to surrounding grounded metal. The voltage across that connection is obviously different for the wye and delta system.

Are we saying that the breaker behavior/enivronment during a fault will be different if it is mounted in a grounded metal enclosure vs mounted in a non-conductive enclosure?

That is an explanation that I can understand, that in an extreme event any breaker in a metal enclosure effectively sees the L-G voltage that way,
and so the L-G voltage always matters.

Of course, that raises the question of using a slash rated breaker on an ungrounded system, where the L-G voltage is undefined. How are the conditions that the breaker has to deal with to clear a fault any different than on a grounded neutral wye system?

Cheers, Wayne

 

[jim dungar]

Are we saying that the breaker behavior/enivronment will be different if it is mounted in a grounded metal enclosure vs mounted in a non-conductive enclosure?

It is not just the grounded enclosure. There is also the issue of other equipment in the area.

This is not unlike the discussion of using 300V rated equipment and fuses on 480Y/277V systems.

 

[wwhitney]

It is no just the grounded enclosure. There is also the issue of other equipment in the area.

Other equipment that may be grounded, like a grounding bar in the breaker enclosure? If all the grounded equipment is say at least 12" away from the breaker, can the presence of that grounded equipment still affect how/whether the breaker clears a fault?

Cheers, Wayne

 

[tortuga]

Of course, that raises the question of using a slash rated breaker on an ungrounded system, where the L-G voltage is undefined.

It would always be the line - line voltage, there are (or were) actually 'Hybrid' high resistance grounded systems where the line-to-ground voltage can actually increase on the ungrounded phases to a value equal to the previous line-to-neutral voltage multiplied by √3.

1000V and under this is considered obsolete.

 

[wwhitney]

It would always be the line - line voltage

I understand that the NEC defines the "Voltage to Ground" to be equal to the L-L voltage for ungrounded systems, but physics-wise it's undefined. Whatever voltage you might read due to capacitive coupling or whatever is not going to be able to pass thousands of amps from a fault (unless you have a L-G-L fault due to two faults or a multiwire fault).

Cheers, Wayne

 

[Joethemechanic]

416 volts if it happened to be a high leg delta,

I know they exist, but I've never seen a 240/480 high leg delta in the wild. Are they common out your way?

 

[winnie]

Not following this. Let's say that on the corner grounded system, either the L-L fault is between the two ungrounded conductors, or the grounded conductor is also protected by the breaker. Then I don't see any difference in the fault clearing path between the corner grounded system and the wye system (ignoring the internals of the supplying transformer). In both cases it goes through two poles of the breaker.

I agree that a L-L fault between fused circuit conductors will be very similar to a L-L fault on a wye system. The fault current flows through 2 breakers.

But how many faults are between circuit conductors vs a fault between a single circuit conductor and bonded metal such as a conduit or junction box or enclosure. You might put a breaker on a grounded conductor. You certainly would not put a breaker on an EGC.