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

[Julio Chadrone]

I've been reading up on slash rating and maybe I'm overthinking it but some of the word is vague to me. It states it only applies when the lower value is path between line and ground and higher value is between phases. But also, any one pole does not see more than the lower value.
To me this reads as for breakers the load must always be connected to gound. So, if I have a 2-pole breaker I can't connect both to a different leg of line/hot wire.

But also, i that's the case why not just rate it to the lower value because It can't see more voltage than that. Is the 480Y/277, just there to help the guy spec'ing this from doing mental math or am I missing something. Because how I am currently reading this, Slash ratings make it sound like 2 pole breakers, they are just 2, 1 pole breakers that have a shared trip.

 

[jim dungar]

To me this reads as for breakers the load must always be connected to gound.

NO.

Slash ratings have nothing to do with loads. There are about the system Line - Ground voltage. They do not have enough internal insulation to interrupt a significant fault when their L-G voltage is too high.

 

[Julio Chadrone]

NO.

Slash ratings have nothing to do with loads. There are about the system Line - Ground voltage. They do not have enough internal insulation to interrupt a significant fault when their L-G voltage is too high.

So like if I have a 480Y system and a load tapping 2 hot legs so it sees 480VAC single phase. Can I protect it with a 2pole breaker slash rated 480/277?

 

[Joethemechanic]

So like if I have a 480Y system and a load tapping 2 hot legs so it sees 480VAC single phase. Can I protect it with a 2pole breaker slash rated 480/277?

Sure, it's done all the time. Think of it this way. The 480 is the "phase to phase" rating. The 277 is the "phase to grounded conductor" rating

 

[Julio Chadrone]

Sure, it's done all the time. Think of it this way. The 480 is the "phase to phase" rating. The 277 is the "phase to grounded conductor" rating

Yeah thats the part That confuses me, if I can connect it phase to phase that means each leg is seeing 480VAC. So if each leg can handle 480VAC whats with the slash rating. Literature I've read stated you cannot use slash ratings on delta 3 phase. But connect wye 3 phase phase to phase then the breaker doesnt see the difference between wye and delta connection. Like if it can handle 480Y phase to phase, then it shouldnt need to be slash rated no? because to the breaker its all the same

 

[winnie]

My understanding is that a L-L fault will be handled by _two_ poles in series, and L-G faults are more common than L-L faults. A single pole of the breaker will not have to handle 480V, or at least the fault conditions that would lead to 480V on a single pole are much much less likely than the other circumstances.

 

[Joethemechanic]

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.

 

[jaggedben]

So like if I have a 480Y system and a load tapping 2 hot legs so it sees 480VAC single phase. Can I protect it with a 2pole breaker slash rated 480/277?

Yes, because in a 480Y system we are assuming the neutral is grounded as required. You could *not* use the slash rated breaker in a 480 delta system because the line-to-ground voltage may be 480V or unknown.

Yeah thats the part That confuses me, if I can connect it phase to phase that means each leg is seeing 480VAC.

No, not really because the breaker poles are in series. So in a line-to-line fault each is only really handling 240V. In a line-to-ground fault, a single pole needs to handle 277 in a grounded wye system. But in a corner grounded or ungrounded system each pole needs to be rated for 480V because it may need to handle that entire voltage by itself in a line-to-ground fault.

Literature I've read stated you cannot use slash ratings on delta 3 phase.

Correct. Delta not being wye.

Like if it can handle 480Y phase to phase, then it shouldnt need to be slash rated no?

No for the reason stated above.

A breaker never needs to be slash rated. It needs to be *not* slash rated when used in a delta or ungrounded system where the voltage to ground is higher than the slash rating. *Non* slash rated breakers are the higher rated, typically more expensive type. You could use non-slash-rated breakers in a grounded wye system but you might be wasting money (especially on 1 or 2 pole breakers).

 

[jim dungar]

if it can handle 480Y phase to phase, then it shouldnt need to be slash rated no? because to the breaker its all the same

No.

The issue is the fault clearing capability of the breaker. It's fault clearing path is only rated 277V L-G.
As long as a voltmeter reads The slash voltage L-G, it does not matter if a neutral is present or not.

 

[jim dungar]

My understanding is that a L-L fault will be handled by _two_ poles in series,

This is the problem with corner grounded systems and breakers.

It is similar, but not the same, as slash ratings which have to do with fault clearing paths for the arcing by products.

 

[Julio Chadrone]

No, not really because the breaker poles are in series. So in a line-to-line fault each is only really handling 240V.

Ah okay I've never heard of this. Is there some where I can read up on it. I understand how voltage drop works across a resistor, but breakers would have no resistance until they trip then they would open. So, the slash rating is incase there is line to ground fault which would make them see full 480 across one phase. And when the breaker trips L-L they work as to resistors closing splitting the voltage evenly amongst themselves. And when they trip L-G they work independently as only load

 

[Julio Chadrone]

Slash ratings have nothing to do with loads. There are about the system Line - Ground voltage. They do not have enough internal insulation to interrupt a significant fault when their L-G voltage is too high.

Okay with another response as context I think I understand this explanation

 

[jim dungar]

So, the slash rating is incase there is line to ground fault which would make them see full 480 across one phase. And when the breaker trips L-L they work as to resistors closing splitting the voltage evenly amongst themselves. And when they trip L-G they work independently as only load

Don't confuse Corner Grounded and Grounded B Phase issues with slash ratings. These conditions deal with how many poles clear the fault.
Slash ratings deal with fault clearing byproduct, like hot gasses, exhaust path insulation and clearances.

 

[don_resqcapt19]

So like if I have a 480Y system and a load tapping 2 hot legs so it sees 480VAC single phase. Can I protect it with a 2pole breaker slash rated 480/277?

only if the system voltage is 480Y/277

 

[electrofelon]

Don't confuse Corner Grounded and Grounded B Phase issues with slash ratings. These conditions deal with how many poles clear the fault.
Slash ratings deal with fault clearing byproduct, like hot gasses, exhaust path insulation and clearances.

But isnt the issue just for L-G faults? Say that you somehow knew that there would never be a L-G fault, only L-L, then would it be ok to use a slash rated breaker on, say, a corner grounded system?

Also, I wonder why manufacturers always have a different part number for straight rated breakers. You would think that maybe a 22k or 65k rated breaker would also be NOT slash rated.

 

[don_resqcapt19]

Say that you somehow knew that there would never be a L-G fault, only L-L, then would it be ok to use a slash rated breaker on, say, a corner grounded system?

The majority of faults are line to ground, so how would you know that.

 

[jim dungar]

But isnt the issue just for L-G faults? Say that you somehow knew that there would never be a L-G fault, only L-L, then would it be ok to use a slash rated breaker on, say, a corner grounded system?

No.

The issue is with the L-G clearances during moderate to significant fault clearing. The byproducts produced while clearing faults may continue the arcing if the L-G voltages is exceeded. As I recall, this first came to light in the 80s when some wiseacre engineer decided to test breakers at a fault level below their AIC. The faults were not cleared because the byproducts were not sufficiently expelled causing conductive gasses to buildup around the panel board bussing eventually engulfing the entire panel.

 

[Jraef]

Think of it this way. Current flowing creates electro-magnetic mechanical stresses on the things that are carrying that current. In addition, the higher the voltage, the more mechanical power (think VA) there will be in that current flow. In a fault situation, where there can INSTANTANEOUSLY be 10s of thousands of amps trying to flow in that fault, those mechanical forces become huge. That’s why breakers must have an “interrupting capacity” measured in thousands of amps, or “kAIC”. If you look at breaker AIC ratings, you will also notice that if a breaker is rated for one value at 480V, it is rated for a much HIGHER value at 240V, because there is more POWER in that current at 480V.

So if a breaker is rated for 65kAIC, that means that PHYSICALLY, It will hold itself together and not become shrapnel when 65,000 amps of current is suddenly trying to flow through it as it reacts and tries to open to stop it.

If that 65,000 amps of current is flowing in a “symmetrical” or “bolted” fault, meaning from phase A to phase B, the mechanical forces are split between the 2 poles of that breaker. So EFFECTIVELY, the mechanical forces that each pole of the breaker mechanical components have to handle, is half of the total.

But if there is an “asymmetrical” or “grounded fault”, where ALL of that 65,000 amps of current is attempting to pass through ONE pole of that breaker, then the mechanical forces are concentrated and twice as strong as they would be on a symmetrical fault.

So, bringing it full circle; if a breaker is only “slash rated”, that means that MECHANICALLY, its design can only handle an asymmetrical fault (remember, everything in one pole) if the POWER in that fault is based on 277V, not 480V, because the breaker is not built as robustly (translate: smaller and cheaper).

What the breaker is FEEDING is irrelevant in this issue, what matters is the potential energy in the SOURCE, the upstream transformer. So if the source transformer is 480V delta, of any type (corner grounded or ungrounded), any asymmetrical (grounded) fault is going to be at 480V, so any one pole gets the whole enchilada of mechanical force. The breaker must therefore be “straight rated” for 480V. If the breaker was slash rated, it would become a bomb. But if the source transformer is a 480Y277, that means that ANY asymmetrical fault is only carrying the potential power of the fault current at 277V, so substantially less destructive power.

 

[electrofelon]

The majority of faults are line to ground, so how would you know that.

Don, its just a thought expirement for discussion puposes.

 

[electrofelon]

No.

The issue is with the L-G clearances during moderate to significant fault clearing. The byproducts produced while clearing faults may continue the arcing if the L-G voltages is exceeded. As I recall, this first came to light in the 80s when some wiseacre engineer decided to test breakers at a fault level below their AIC. The faults were not cleared because the byproducts were not sufficiently expelled causing conductive gasses to buildup around the panel board bussing eventually engulfing the entire panel.

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.