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

wwhitney said:
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.
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If you start looking at the physics of ungrounded systems you might find the L-L voltage might actually be a too low number, the maximum value of some could be 480*1.732 or 832 volts or even more.
Look at the eaton paper I linked above in post 37 and this one:
 
winnie said:
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.
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Not disputing that, but the question was specifically about the (apparently minority) case of L-L faults.

Perhaps a good way to put your point above is that the "headline" number on a 480/277 slash rated breaker should be considered 277V, as most faults are L-G, and such a breaker is rated for at most 277V L-G.

Cheers, Wayne
 
Fair enough. Maybe: 480/277V breaker is more properly called a 277V breaker that is rated to handle 480V when two poles are in series with the fault as long as that 480V is limited to 277V L-G.

-Jonathan
 
winnie said:
Fair enough. Maybe: 480/277V breaker is more properly called a 277V breaker that is rated to handle 480V when two poles are in series with the fault as long as that 480V is limited to 277V L-G.
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Which still leaves the question of why that 480V needs to be limited to 277V L-G. The whole "products coming out the arc chute can connect to a ground metal case to make a ground reference" idea made sense to me. But Jim indicated it was more than that.

It just seems like if two poles are part of the fault circuit as expected, and nothing grounded is part of the circuit, and there is no ground reference sufficiently close to the breaker, it should not matter what the L-G voltage is. As there is no connection to ground, and 480V is 480V.

Admittedly an academic question, as most likely there is a grounded metal case.

Cheers, Wayne
 
winnie said:
Fair enough. Maybe: 480/277V breaker is more properly called a 277V breaker that is rated to handle 480V when two poles are in series with the fault as long as that 480V is limited to 277V L-G.

-Jonathan
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I agree with Jonathon it would have saved everyone much confusion, or if there were an engineering wiki/thesaurus
 
wwhitney said:
It just seems like if two poles are part of the fault circuit as expected, and nothing grounded is part of the circuit,
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I was confused with that too earlier. And take anything I say with a massive grain of salt. But the way they explained it is because of the path it takes when they are connected L-L in series. Path it takes is source, first pole breaker load, 2nd pole breaker source. They act like 2 resistors in series. Example 10V source, with equally sized resistors. The voltage drop across the resistors is half. So each resistor is only seeing half the voltage. Thus each pole is only seeing the phase voltage. Which in WYE is less than than the line voltage. sqrt3

Crux is why this doesnt work in delta. Is because phase voltage is the line voltage. The breakers are built to handle worst case scenario. In delta if there is L-L fault, No problem each pole is seeing same as WYE Line-Line, but there is a possibility of L-G fault. And for that (I dont ahve my drawing capaibilites on me) that fault could have a return path that does not include going through the 2nd breaker.

Actually thinking about it I have a hard time visualizing what a L-G fault would look like. Cuz it would have to be Source -> breaker Load ->ground. Bu the ground would have to be a different return path that by passes the 2nd pole and goes to a phase.

But I digress that, I believe is the logic behind it. I guess I dont fully understand it because I cant draw it
 
A interesting and related question I had recently, had to do with converting a 240 delta service to 240/139 wye,
and Ill direct this to @jim dungar (and the other experts that know how breakers are made)
why is the slash rating mixed between single phase and three phase?

Seems the Line-Neutral voltage rating of UL 489 should be the maximum line voltage / √3 for consistancy?
For example a slash rated 480V two pole breaker is marked 480y/277 or 600y/347 yet a 240V slash rated two pole breaker is only rated 120/240.
I think the 240V two pole slash rated breakers should really be marked 240y/139
 
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Julio Chadrone said:
Actually thinking about it I have a hard time visualizing what a L-G fault would look like. Cuz it would have to be Source -> breaker Load ->ground. Bu the ground would have to be a different return path that by passes the 2nd pole and goes to a phase.
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A L-G fault does not involve a load at all. There is a single Line, therefore there is a single pole breaking the fault.
If a load was involved you would have an N-G fault.


winnie said:
Maybe: 480/277V breaker is more properly called a 277V breaker that is rated to handle 480V when two poles are in series with the fault as long as that 480V is limited to 277V L-G.
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Wow. 26 words rather than an NEC and ANSI standard voltage rating of 480Y/277V.
 
winnie said:
Just wait until I'm done with 120/208 'single phase'.... :)
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But 120/208 follows the ANSI voltage format for single phase.
We need to stop using this as our slang for what the standard calls 208Y/120V

The way I was taught is to count the number of Line-Line voltages, so I don't have a problem with it.

But this discussion should really be in a separate thread
 
Birken Vogt said:
There was somebody on another forum who just ran into one at a feed mill or grain elevator or something.
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With the 120/240 high leg it's done to eliminate the need for another transformer for your 120 loads. 240/480 high leg seems like you'd need another transformer anyway. I wonder if it's just an alternative to corner grounding because people are freaked out by corner grounds
 
winnie said:
Just wait until I'm done with 120/208 'single phase'.... :)
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Yeah, that one bugs me too. I saw a 3 phase motor on a saw hooked up to that 120/208 "single". It ran fine until they tried to make heavy cuts and tripped the overloads. Tested the voltage at the motor starter and one phase was "dead" relative to ground, and spent 15 minutes scratching my head wondering how something with a dead phase could be starting
 
I think the best term for 120/208 is 'open wye', if the NEC defined that (and open delta) they could save probably a 1/2 page of words.
 
tortuga said:
I think the best term for 120/208 is 'open wye', if the NEC defined that (and open delta) they could save probably a 1/2 page of words.
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Open wye? is that just delta? Im guessing when they say open it means ungrounded?
jaggedben said:
240Y would be 139V line to ground so I take it that exceeds the rating of a 120/240 breaker.
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so the 120/240 breaker is that specifically for the. I forget the term but "single phase hi leg" wher there's a neutral stuck in the middle of a winding to provide 120 power. I dont know how common that is but it seems like a really specific scenario to build a breaker around
 
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