an Open in one of the lines feeding Wye connect loads - Electrician Talk - Professional Electrical Contractors Forum
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Old 11-11-2018, 09:09 PM   #1
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Default an Open in one of the lines feeding Wye connect loads

If an open developed in line C, the voltage measured across the open would be 170V?

Now if the supply was a 120V/208V three wire supply without a neutral, and an open occured at line C, the voltage measure accross the open would be 180V?

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Old 11-11-2018, 09:20 PM   #2
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104v?
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Old 11-11-2018, 09:27 PM   #3
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Is this a homework question?
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Old 11-11-2018, 10:45 PM   #4
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It's a trick question. How you get 120v without a neutral? I'd say the voltage would be floating.
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Old 11-11-2018, 11:12 PM   #5
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This is likely drawn wrong. Is the voltage across the C coil actually 0?

If it is actually 120, then the voltage across the open would depend on what was connected to the downstream side of the open.
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Old 11-11-2018, 11:33 PM   #6
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the 120V/208V four wire supply is feeding lines A, B, C, the 3 resistor in the wye formation are the loads. Sorry I should have clarified that earlier.

So the open in line C is like when a fuse in line C blows open.

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Old 11-12-2018, 12:08 AM   #7
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Quote:
Originally Posted by Wire Tags View Post
the 120V/208V four wire supply is feeding lines A, B, C, the 3 resistor in the wye formation are the loads. Sorry I should have clarified that earlier.

So the open in line C is like when a fuse in line C blows open.

Change the fuse!
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Old 11-12-2018, 12:37 AM   #8
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I just realized, between line A and neutral its 120V, between line B and neutral its 120V, so between line C and neutral it should also be 120V, but because the open is where it is, the 120V will no longer be across the load, but across the open. Is that right?
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Old 11-12-2018, 08:38 AM   #9
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Originally Posted by joebanana View Post
It's a trick question. How you get 120v without a neutral? I'd say the voltage would be floating.
He shows a neutral this is a wye connected load. So he has an open neutral?
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Old 11-12-2018, 08:47 AM   #10
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Quote:
Originally Posted by Wire Tags View Post
the 120V/208V four wire supply is feeding lines A, B, C, the 3 resistor in the wye formation are the loads. Sorry I should have clarified that earlier.

So the open in line C is like when a fuse in line C blows open.
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This is likely drawn wrong. Is the voltage across the C coil actually 0?

If it is actually 120, then the voltage across the open would depend on what was connected to the downstream side of the open.
If it's a resistor, then there should be no need for a neutral. Are you thinking like on a 3 phase 208 volt heater that blows a fuse on one leg? Or is it 3 - 120 volt loads and a fuse blows?
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Old 11-12-2018, 09:04 AM   #11
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Originally Posted by glen1971 View Post
If it's a resistor, then there should be no need for a neutral. Are you thinking like on a 3 phase 208 volt heater that blows a fuse on one leg? Or is it 3 - 120 volt loads and a fuse blows?
He/she does not say the resistance, so if the loads are of different resistance values, there will be additional issues.
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Old 11-12-2018, 10:35 AM   #12
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I don’t think you would get 0V across that Load C except in a perfect world where there was no hysteresis (not sure if that is the right term (current and voltage loss due to heat and other magnetic properties)). The voltage between Wire C and Load C should be 120V or whatever the voltage is on Wire C.

As @brian john says, since there is no R value we don’t know what the voltage is on Wires A, B or C is; we just know that it is 120 V across the Loads A and B. Since it is WYE configuration, you can assume that the voltage on Wire C is 120V because the common point and the other two wires are 120V before the load. We also know that if this is balanced system we would take 120V and multiple by 1.73 to get phase to phase voltage (~208V)

Am I out to lunch?

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Old 11-12-2018, 10:46 AM   #13
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If it was single phase connected to a resistive load, you would read both lines across an open. Like if it was 240 volt, 120 to ground and you blew one fuse, you would read 240 across the blown fuse.

In a three phase situation it is different. You would definitely read voltage across the blown fuse ( or open) but how much Im not sure 🤔
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Old 11-12-2018, 12:25 PM   #14
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I ran into something similar recently. We cut in a new 3 UD xfmr, and did phasing at our NO as another guy closed at the NC. Normally we dont phase with equipment in between switches but we made an exception for some reason or another.

When our guy closed SW1 we read 7620v on A to ground, but ~4kv on B and C to ground. Then when he closed SW2 we got 10kv from C to ground. On a 13.2/7.62 system for anyone who wants to figure out the math of that one for me.
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Old 11-12-2018, 02:39 PM   #15
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Originally Posted by brian john View Post
He shows a neutral this is a wye connected load. So he has an open neutral?
He shows a neutral, but states it has no neutral, or is that a ground? He didn't mention if it's grounded or not. No mention of what the other loads are, or if it's balanced, either. If so, it would be 120v, no?
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Old 11-12-2018, 02:59 PM   #16
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Quote:
Originally Posted by Wire Tags View Post
If an open developed in line C, the voltage measured across the open would be 170V?

Now if the supply was a 120V/208V three wire supply without a neutral, and an open occured at line C, the voltage measure accross the open would be 180V?

I just tested this in the lab. I used three 7.5 watt light bulbs for the loads and connected them in a Wye. My incoming voltage is 121 volts to ground/ 211 phase to phase.

When I opened the circuit with a neutral connected to the center of the Wye, I got 121 volts across the open.

When I opened the circuit without having a neutral connected to the center of the Wye, I got 182 volts across the open.
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Old 11-12-2018, 08:10 PM   #17
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Quote:
Originally Posted by eddy current View Post
I just tested this in the lab. I used three 7.5 watt light bulbs for the loads and connected them in a Wye. My incoming voltage is 121 volts to ground/ 211 phase to phase.

When I opened the circuit with a neutral connected to the center of the Wye, I got 121 volts across the open.

When I opened the circuit without having a neutral connected to the center of the Wye, I got 182 volts across the open.
1212=60.5
121+60.5=181.5
There's your 182 volts.
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Old 11-12-2018, 09:42 PM   #18
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hey @Chops146 isnt the 180V accross the open found by taking the square root of [(208x208)-(104x104)] = 180.1V
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1212=60.5
121+60.5=181.5
There's your 182 volts.

Last edited by Wire Tags; 11-12-2018 at 09:45 PM.
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Old 11-12-2018, 10:04 PM   #19
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Originally Posted by Wire Tags View Post
hey chops isnt the 180V accross the open found by taking the square root of [(208x208)-(104x104)] = 180.1V
That was actually a joke. Without a neutral you have an open Wye configuration. VL = 2 * VP however VL is 1.73 * VP in a standard WYE configuration. In an unbalanced WYE which is the area I can't find the info I was looking for, IL is 1.73 IP and IIRC that effects this case. Anyways, your 211 VL * 1.73 = 365.03. 365.03 2 = 182.515. Or 211 2 = 105.5. 105.5 * 1.73 = 182.515. I'll try to upload some pictures to clarify this tomorrow.
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