low voltage measured across any phase and neutral 3

[ekwaffo]

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.

 

[davidbeach]

High voltage to neutral on a different phase is the usual cause of low voltage to neutral on one phase.

 

[LiteYear]

Draw three vectors in two dimensions to represent the three phases. If by your second statement (voltage between phases is normal) you mean that they are equal, then the final points of the three vectors must be the vertices of an equilateral triangle. There's no other way to get equal phase to phase measurements.

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?

 

[waross]

Three transformers connected in wye but with no load. Look at the line to line voltage and the line to neutral voltage with a scope. You may see one much more distorted due to the non linear magnetizing current. If you measure these voltages with a d'Arsonval type analogue meter it will give erroneous results. The d'Arsonval meters rectified the AC to DC and measured the DC. The result an average measurement, not an RMS measurement. The meter scale was adjusted by the ratio of average to RMS for a sine wave. With other than a sine wave or a distorted wave, the ratio was different and the reading was in error.
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

 

[ekwaffo]

Thanks guys for the quick response.

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"

 

[Marmite]

There is a fault on the neutral which has either gone open circuit or partially open circuit.
Regards
Marmite

 

[Marmite]

I apologise.Partially open circuit doesn't make sense. There is either an open circuit fault on the neutral or a high resistance fault on the neutral.
Regards
Marmite

 

[pwrtran]

ekwaffo -
You only gave Red-Neutral, how about measure White-Neutral and Blue-Neutral?

 

[waross]

That's more than meter error!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PHovnanian]

It sounds like an open or high resistance connection in your neutral someplace. Start with the source (service entrance or transformer bank) and measure the phase to neutral voltages. Unless the problem is in the service (utility side) or transformer bank, the voltages will suddenly go from balanced to unbalanced as you move toward the load in the circuit.

 

[pwrtran]

In short, it is impossible. You can have two phase-neutral voltages that are less than the nominal value but the third one MUST be greater than the nominal.

It could mess up the readings if one use the old analog pin-point type meter as there are different reading scales!

 

[ausphil]

the customer's complaint as stated doesn't make sense as to the orignal "low volts" statement.

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

 

[davidbeach]

It is also entirely possible to have an unstable neutral. I've seen a case where a 480V system had a phase to ground voltage greater than 277V on all phases. (There's a thread in this forum from late 2006 about it.) Short answer was that 277V primary VTs were used on a delta system that should have had 480V primary VTs and the partial wave saturation of the VTs had the neutral moving around at 60 Hz so that any meter would record a high phase-ground voltage (on the VT secondaries) while phase-phase voltages were fine. Other perverse system conditions could create a situation where any measured phase reads a low phase-ground voltage. Measure all three in succession and all three read low. Can't measure something without affecting that which is being measured, see "Heisenberg".

 

[LionelHutz]

Your question is so vague that you shouldn't expect anything other than theoretical causes....

This was the customers complaint; " When they switch on all the loads, Voltage goes very High"

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.

 

[waross]

A heavy load causing a high voltage on another phase or line is a classic symptom of an open neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

I've had to explain the broken-neutral thing so many times, I made a picture (attached).

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

 

[pwrtran]

Should it be 96 ohm on the first one DRWeig?

 

[DRWeig]

Yes, pwrtran, good catch! Of course, the powers and voltages on the broken-neutral case were also in error.

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

 

[DTR2011]

If I understand correctly, the customer is measuring the voltages. What are they using? Are they old panel analog meters? Has this been re tested with something reliable like a fluke 87 or equivalent? Have the customer take the measurements and document them in a simple spreadsheet (which also defines the process).

Rule those things out first.

 

[purplepete]

Agree with all before.
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.