Multi wire circuit?

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elecold

elecold

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277 volt circuit feeding 2x4 lights. two phase sharing a neutral. Half lights on a phase half lights on b phase. Bad splice on neutral approximately middle of branch circuit.


The leds after the bad splice were FRIED. is this because I had 480 coming back on the neutral because the neutral was disconnected??
 
elecold said:
277 volt circuit feeding 2x4 lights. two phase sharing a neutral. Half lights on a phase half lights on b phase. Bad splice on neutral approximately middle of branch circuit.


The leds after the bad splice were FRIED. is this because I had 480 coming back on the neutral because the neutral was disconnected??
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After the splice failed the neutral played no part in the circuit at all, the circuit became a series circuit.

Roger
 
roger said:
After the splice failed the neutral played no part in the circuit at all, the circuit became a series circuit. ...
Click to expand...
That doesn't explain it. In a two-out-of-three-phase circuit, losing the white wire (it's not "neutral" in an unbalanced three phase circuit) would put 240 volts on each luminaire instead of 277. (assuming there was a reasonably equal number on each phase)
 
drcampbell said:
That doesn't explain it. In a two-out-of-three-phase circuit, losing the white wire (it's not "neutral" in an unbalanced three phase circuit) would put 240 volts on each luminaire instead of 277. (assuming there was a reasonably equal number on each phase)
Click to expand...
Huh? It's a 480V series circuit and I'm sure that LED drivers don't act like baseboard heaters so I have no doubt they saw more than 240V.
 
drcampbell said:
(it's not "neutral" in an unbalanced three phase circuit)
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Ok, I'm open for your explanation, what would it be? (I know it would be carrying the same current as the other ungrounded conductors in the OP's scenario)


Roger
 
Makes sense theoretically: 480 series circuit so each bank of lights sees 240. However, not sure I trust led drivers to divide the voltage equally, plus we dont know if there were the same number on each leg of the mwbc.
 
electrofelon said:
Makes sense theoretically: 480 series circuit so each bank of lights sees 240. However, not sure I trust led drivers to divide the voltage equally, plus we dont know if there were the same number on each leg of the mwbc.
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where are you getting the 240 volts from? If there are two 277 volt feeds
Trying to figure out why all the led drivers were fried and how that is related to the bad splice in a multitude 277 volt circuit. Thanks for your help
 
elecold said:
where are you getting the 240 volts from? If there are two 277 volt feeds
Trying to figure out why all the led drivers were fried and how that is related to the bad splice in a multitude 277 volt circuit. Thanks for your help
Click to expand...
There is no 240V, ignore that for now.b

You only have two 277V circuits when you have a neutral, A to Neutral- 277V, Neutral to B- 277V. After loosing the neutral you have one 480V circuit A to B. That's what smoked your led's.
 
elecold said:
where are you getting the 240 volts from? If there are two 277 volt feeds
Trying to figure out why all the led drivers were fried and how that is related to the bad splice in a multitude 277 volt circuit. Thanks for your help
Click to expand...

When you lose the neutral, you have a 480 volt circuit with two lights (or two banks of lights) connected in series. Thus each light, assuming equal resistance, would see half of that or 240.
 
If the circuits are poorly balanced, then the higher-wattage side could have been subjected to WAY more than 277V.

Sent from my SM-G900V using Tapatalk
 
electrofelon said:
When you lose the neutral, you have a 480 volt circuit with two lights (or two banks of lights) connected in series. Thus each light, assuming equal resistance, would see half of that or 240.
Click to expand...

Obviously the OP didn't have equal resistance. There is a great chance he had over 277V on one circuit and less than that on the other. You can't assume equal resistance, especially since he said he "fried" the LEDs on one circuit.
 
elecold said:
I meant to say every lights from the bad splice to the last light was fried. They alternated between one circuit to the other sharing the neutral
Click to expand...
If you started out with same amount of load on each line, then voltage to the open neutral point should have shifted to 240 (half the 480 applied). If those loads started to fail because they could not operate at 240 volts, then you start to get load balance changes as they fail, leaving varying voltage depending on what the balance is at the time.
 
roger said:
After the splice failed the neutral played no part in the circuit at all, the circuit became a series circuit.

Roger
Click to expand...

elecold said:
I meant to say every lights from the bad splice to the last light was fried. They alternated between one circuit to the other sharing the neutral
Click to expand...


I agree with Roger that the bad splice created a series circuit.

The best way to understand what is happening or could have happened is to draw out the circuit.
 
Many LED drivers operate over a wide range of voltages a constant power devices. This means that if the voltage is reduced the current consumption increases.

If you put two constant power devices into series, then voltage will drop on one device and will rise on the other device.

I expect that when the neutral was lost, you ended up with two constant power loads in series with a 480V supply. Whichever side was the rated _higher_ power side was trying to draw more current, and so saw relatively lower voltage, which drove the devices on that side to draw even more current. Half the system was running at higher than rated voltage, half at lower. Either condition can destroy drivers.

My guess is that half the system saw excessively high voltage and burned out, and then some component acted as a short and burned out the other half.

-Jon
 
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