low voltage measured across any phase and neutral
low voltage measured across any phase and neutral
(OP)
Hi,
I would like to know the possible causes of a low voltage measured across any phase and neutral. Voltage between phases however, remains normal. Causes please??????
Has something to do with the neutral but can not get my head around it.
I would like to know the possible causes of a low voltage measured across any phase and neutral. Voltage between phases however, remains normal. Causes please??????
Has something to do with the neutral but can not get my head around it.






RE: low voltage measured across any phase and neutral
RE: low voltage measured across any phase and neutral
Next draw the "normal" neutral - it's final point should be the midpoint of the equilateral triangle. In this normal condition the resultant phase to neutral vectors have equal magnitude, equal to the phase to phase magnitude divided by root 3.
Now try to draw a neutral vector such that all of the phase to neutral magnitudes are lower, as you describe in your first statement. You'll find it is impossible. Moving the neutral closer to one phase will make that phase to neutral lower but it will increase the other two.
As davidbeach suggests, you can't have one without the other. Could there be a measurement mistake?
RE: low voltage measured across any phase and neutral
My guess: An old analogue meter and metering error.
Note: The distortion and the error usually went away when some load was placed on the transformer bank.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: low voltage measured across any phase and neutral
Really need some practical response other than theoretical. This problem occurred at a customers end - Voltage measured between RED/BLUE phase for example, gives abt 400V. Was expecting voltage btn RED/Neutral to give abt 220V but it gave 50V.
This was the customers complaint; " When they switch on all the loads, Voltage goes very High"
RE: low voltage measured across any phase and neutral
Regards
Marmite
RE: low voltage measured across any phase and neutral
Regards
Marmite
RE: low voltage measured across any phase and neutral
You only gave Red-Neutral, how about measure White-Neutral and Blue-Neutral?
RE: low voltage measured across any phase and neutral
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: low voltage measured across any phase and neutral
RE: low voltage measured across any phase and neutral
It could mess up the readings if one use the old analog pin-point type meter as there are different reading scales!
RE: low voltage measured across any phase and neutral
This was the customers complaint; " When they switch on all the loads, Voltage goes very High"
is this a misprint?
is this a 3 phase customer? If so, the statment may make sense if they switch loads on on 2 phases (essentially pulling the neutral towards those phases) but measure volts on the other phase to neutral.
high resistance or open circuit neutral are certainly causes of the shifting neutral point as stated above.
one way to check is to measure all the currents in their switchboard, including in their earth cable (neutral to earth connection). If this current equals the resultant of the 3 phase currents, then it is probably open circuit. If this earth current increases proportionally with the load, but does not equal the resultant of the 3 phases currents, then there is a high resistance neutral.
if they are a single phase customer, the same problems may be determined by this method, just comparing single phase, neutral and earth currents.
as stated, the problem may be local to the customer's service neutral, or may be (less likely but possible) further upstream in the utility's mains.
ausphil
RE: low voltage measured across any phase and neutral
RE: low voltage measured across any phase and neutral
Which voltage goes very high. line to neutral?? line to line?? which phases??
Have you metered all 3 line to neutral voltages at the same time? If not, then you can't really say all 3 read low at the same time.
Has anything been changed on the power system very recently?
As already posted, start with the simple causes first. In this case, it's likely a break or poor connection in the neutral so someone who actually knows how to troubleshoot just needs to go find it.
RE: low voltage measured across any phase and neutral
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: low voltage measured across any phase and neutral
Simple 40W incandescent lamp on one phase of 240/120 service, 150W on the other. Break neutral. Compute voltage divider and all currents.
Good on ya,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: low voltage measured across any phase and neutral
RE: low voltage measured across any phase and neutral
I never noticed that before, must have done it late at night. I found the old source file, fixed one is attached.
Good on ya,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: low voltage measured across any phase and neutral
Rule those things out first.
RE: low voltage measured across any phase and neutral
If the customer has a circuit breaker pole in the neutral I'd check it as well.
It's done in Oz with multi-phese supplies on occasions, often when RCDs are fitted to the incoming supply at a main switchboard. As has already been shown here open-circuit (or bad contacts) in this neutral pole will have a disatrous effect. For example an electric kettle on one phase will cause a TV set on the other phase to start smoking.
RE: low voltage measured across any phase and neutral
Thanks a lot guys. Soo much info to digest.
Some customers can very much confuse you.
Yea, customer is on a three phase supply