Under normal conditions in a residential setting, when an appliance motor starts, you get the brief dimming of lights. What would cause lights to become brighter when a motor starts? All the connections in the panel and meter socket are fine. This only occurs on one of the two hot legs.
voltage spike
- Thread starter ajn
- Start date
- Status
- Location
- Massachusetts
- Location
- Massachusetts
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
It is affecting both: when one line has extra current on it, its L-N voltage drops and the other line's L-N voltage rises.
Current can only cause voltage drop. The only way for a voltage rise is for another voltage sharing the neutral to sag.
The entire house is a shared-neutral MWBC (multi-wire branch circuit), and is behaving like one from your description.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
110409-EDT
iwire and larry are giving the correct analysis.
Your problem is to find where the problem is located. It may not be in the house. If there is a metalic water line coming into the house, then initially use this as a voltage reference point, using the street side of the meter. Relative to this reference point measure the neutral voltage on a circuit that does not include the motor and preferably somewhat near the main panel. If you can not be close to the main panel use the EGC wire in a long extension cord to extend your meter lead. If you have a Fluke with a min-max mode use that mode.
At this point we can only guess that the voltage change will be at least several volts. About 3 V change to an incandescent lamp is the minimum that most people detect.
Based on your description, if the neutral to water pipe measurement is very small, then your neutral connections in the main panel are probably OK.
Next question. Is the power company transformer close by? There will be a ground rod at the transformer and it is connected to the neutral terminal of the transformer. How it connects may influence the effectiveness of the next test.
This test is to measure the change in voltage between the transformer ground rod and your water pipe reference point.
If the transformer ground rod wire, this is what you want to connect to, is solidly connected to the transformer neutral, then a large voltage change between the transformer ground rod wire and your water line reference point when the motor starts would indicate a high resistance between the transformer and your main panel neutral.
If there is little change in the above experiment, then measure from the transformer ground rod wire to a hot phase in the house that is not the one with the motor load. If this voltage changes by several volts, then the problem is likely in the transformer, or at the termination to neutral at or near the transformer. Exactly what would happen depends upon how the wires are connected at the transformer.
If you can not get to the transformer ground rod, then drive a long screwdriver in the earth near where you expect the ground rod to exist, and use this as the test point.
.
iwire and larry are giving the correct analysis.
Your problem is to find where the problem is located. It may not be in the house. If there is a metalic water line coming into the house, then initially use this as a voltage reference point, using the street side of the meter. Relative to this reference point measure the neutral voltage on a circuit that does not include the motor and preferably somewhat near the main panel. If you can not be close to the main panel use the EGC wire in a long extension cord to extend your meter lead. If you have a Fluke with a min-max mode use that mode.
At this point we can only guess that the voltage change will be at least several volts. About 3 V change to an incandescent lamp is the minimum that most people detect.
Based on your description, if the neutral to water pipe measurement is very small, then your neutral connections in the main panel are probably OK.
Next question. Is the power company transformer close by? There will be a ground rod at the transformer and it is connected to the neutral terminal of the transformer. How it connects may influence the effectiveness of the next test.
This test is to measure the change in voltage between the transformer ground rod and your water pipe reference point.
If the transformer ground rod wire, this is what you want to connect to, is solidly connected to the transformer neutral, then a large voltage change between the transformer ground rod wire and your water line reference point when the motor starts would indicate a high resistance between the transformer and your main panel neutral.
If there is little change in the above experiment, then measure from the transformer ground rod wire to a hot phase in the house that is not the one with the motor load. If this voltage changes by several volts, then the problem is likely in the transformer, or at the termination to neutral at or near the transformer. Exactly what would happen depends upon how the wires are connected at the transformer.
If you can not get to the transformer ground rod, then drive a long screwdriver in the earth near where you expect the ground rod to exist, and use this as the test point.
.
brian john
Senior Member
- Location
- Leesburg, VA
Tightening a bad connection does not necessarily improve the connection.
In addition the issue could be in the meter, the weather head or at the transformer.
To not fix this problem could result in a fire, and that is not a scare tactic.
In addition the issue could be in the meter, the weather head or at the transformer.
To not fix this problem could result in a fire, and that is not a scare tactic.
- Status