800V phase to phase?

[emahler]

Had a guy yesterday get the following readings at an RTU
Fused disconnect, 3PH 480V unit 25A circuit

Line side of disconnect
L1-G 280V
L2-G 280V
L3-G 280V

L1-L2 480V
L1-L3 480V
L2-L3 480V

No problem so far...load side of disconnect
L1-G 280V
L2-G 280V
L3-G 280V

L1-L2 480V
L1-L3 800V
L2-L3 800V

when we bypassed the disconnect, voltage was normal. Issue is in the disconnect

Any guesses how we got 800v between phases? I'll have the disconnect on my desk Friday to look at it, but I have no idea what would cause 800V without it blowing up

 

[hbiss]

He doesn't know how to read his meter is the only thing that comes to mind. A disconnect is only a switch. I'm assuming that he checked the fuses and they are all good.

What do you mean by bypassing the disconnect? Remove the lines and loads and splice them together?

-Hal

 

[jeremysterling]

I would guess the L3 fuse is blown, and your meter has no reference causing the nonsensical reading. .

 

[hbiss]

L3 is open and motor is acting as a generator.

-Hal

 

[LarryFine]

L3 is open and motor is acting as a generator.

Or as a rotary converter.

 

[ptonsparky]

Your tech needs to polish his/her skills.

I’ve never taken voltage readings of a single phasing 480 v motor be it a blown fuse or open disconnect pole. Never thought it wise to wait that long

 

[hbiss]

What amazes me is he couldn't tell it was single phasing, took those readings and never checked for an open fuse, then took the whole disconnect off the wall because he thought it was defective. :happysad:

-Hal

 

[Cow]

L1-L2 480V
L1-L3 800V
L2-L3 800V

when we bypassed the disconnect, voltage was normal. Issue is in the disconnect

Any guesses how we got 800v between phases? I'll have the disconnect on my desk Friday to look at it, but I have no idea what would cause 800V without it blowing up

Does the disconnect have an issue or is their just a blown fuse on L3?

 

[hbiss]

Guess he'll find out when he finds it on his desk.

-Hal