dimming/brightening lights with voltage fluctuations

[nadprosper]

Here is one I have not seen before. Kind of exciting really...here goes. A client is complaining about dimming lights when other loads come on. When I turn on the microwave...the lights dim and the MW seems to bog. Testing the outlet shows 112V with MW off and 104V with it on. The dimming seems to be housewide. I go to the panel and have, coming into the main bkr, 112V on one leg and 126V on the other...with MW on I have 104V and 130V. Pop the meter and all connections are tight. With main bkr off...I have 120/120 ( problem not from the street ). Turn main back ( with all others off ) and get 120/120. Turn on bkrs one at a time and identify 2 single pole bkrs ( on different phases ) and a 2 pole 50 that all seem to tweak the voltage to 112/126. The 2 pole 50 is too an older fuse box ( still fused ). I had to leave before I could get any further. Any ideas? Neutral maybe....but it seems to be on at least 3 independent cables.....and then why would it affect the whole house? Incoming neutral? then why only the few brks? Any words of wisdom appreciated in advance.

 

[brian john]

Check for issues with the neutral this is one of two issues that can result in light flicker the second is listed below.

Light flicker (dimming and restoration of light to near preflicker level) of lights from motor inrush) is a byproduct of several factors and generally is difficult to over come.

AC motor loads (in particular compressors) but all motor loads have inrush currents. These inrush currents result in voltage drop in the branch circuit, panel bus, service laterals, utility transformer and possibility the utility HV feeders.

Flicker is in addition user dependent; some people are more susceptible to flicker. For some reason flicker seems to bother women more that men (based upon personal experience).

A Voltage Drop (VD) as little as 3 volts on a nominal 120 VAC system is noticeable (by me), 2.5% VD.

We have had cases were the VD was all a byproduct of primarily single phase 120 VAC loads, in these cases we were able to put the lighting on L1 and all the motor loads on L2, the HVAC compressors were something the homeowner had to live with. This minimizes the flicker.


To test for this VD and resulting flicker to determine the source of the inrush, use a min max amp clamp and min max multimeter at the main service watch the lights(it may take two workers) and correlate to the meters.

 

[highendtron]

Had a similar problem a few days ago...

Had a similar problem a few days ago...

Nuetral bar/bus had corrosion between the lug and bus...arching caused pits and higher resistance. Interesting but easy to fix.

 

[charlietuna]

You removed the meter and had 120/120 volts means nothing without a load and without data logging equipment you are guessing at this problem. If the power company has a problem on their neutral or possible just a voltage drop problem due to the distance back to the transformer, you can surch all you want inside the customer's home and find nothing. I always start at the power source and work in! If the service has the correct voltage under load with the problem noticable i start working towards the branch circuit wiring to identify first those using a common neutral.

 

[LarryFine]

This sounds like a combination of several things, in no particular order:

Voltage drop along the service drop: Old drop with upgraded panel and new loads.

The 50a 2p feeds the whole house's original circuits, now relatively overloaded.

The microwave, if built in, was connected to the lighting circuit (as was the exhaust hood it replaced.)


Fix: new micro circuit directly from the panel, place on whichever phase causes the least dimming.

 

[gar]

090303-2333 EST

nadprosper:

You have a neutral problem. Unloaded you read 120-120, the sum is 240, putting some kind of load on the system you get 112-126, the sum is 238. This is not an unreasonable drop in the total. The measurements you made clearly indicates the neutral at the house shifted relative to the transformer center tap.

If these voltages are being measured at the bus bars in the main panel. then at least some of the problem is between the transformer center tap and the neutral bus in the main panel.

If the transformer is in the customer's yard, then put a 10" or so screwdriver in the ground next to the pole. There should be a ground rod at the pole where the transformer is located. From the screw driver probe run an insulated wire, an extension cord is a good means, back to the house and measure the voltage between this pole probe and the neutral in the main panel.

Be careful just in case there are some large neutral to ground voltages.

With a problem between the pole transformer and your main panel neutral you should see something in the range of 6 V for the voltages you have indicated under load. If the voltage is much lower and there is a ground rod at the pole, then the problem would most likely be at the neutral connection to the transformer center tap.

.

 

[hurk27]

If you have the sea saw effect that exceeds 5% of the normal voltage higher or lower then your problem is possably a weak neutral or lost neutral or one about to fail. the test for this is easy.
First turn off all breakers on one phase, in most panels just turn off every other breaker and make sure the same breakers straight across from the ones off are also turned off as these share the same phase, older panels like zenco's don't work like this so you would have to use a meter to see which breaker are on the same phase, this loads one phase down and forces the return load all onto the neutral or any other path back to the transformer such as a water pipe bond.

using a clamp on amp meter check to see how much current is on the water pipe bond if there is any after doing the above, I have found many times that the neutral had failed but the water pipe, gas, or other grounding electrodes would return current back to the transformer, hiding the problem. a good water pipe connection can sometimes handle quite a load before it shows up. if you have current on the water pipe bonding conductor that is high then you have lost the neutral connection somewhere between the panel and the transformer. and if it is possible to be there when you call the PO-CO you can unbalance the panel as showed above so the service man from the PO-CO will see it at the meter, many times when they come out they don't do or know to do this and just think your wrong. so this can save some time in getting a lost neutral fix the first time. the reason the water pipe can mask this problem is when we bond to the water system the house next door is also bonded to the water pipe which serves as the return path back to the transformer, sometimes there might be a few houses sharing the same transformer or the MGN which will provide a return path for the neutral.
but in any case you will have a high level of current on this connection that can be as high or close to the current of the ungrounded conductor that is still loaded when you turn off all the opposite phase breakers.

hope this helps with your trouble shooting
Wayne

 

[hurk27]

Just to add (because I couldn't edit in time)

Never remove the water ground bond if you suspect a lost or weak neutral connection!!!

removing the water pipe bond could result in a high potential between earth and anything bonded to the service neutral I.E the meter can, ground rods, the alum. siding, and other grounded metal items. also if there is cable TV, this can put a high amount of current on the coax between the house and pole, which can cause the cable to overheat and cause a fire. this is because the coax shield is bonded at both ends.

Wayne

 

[nadprosper]

Fixed

Fixed

Thanks everyone for your help. So the problem turned out to be in the neutral splice ( between street and house ). I have been doing this for ten years and had never seen those symptoms before....although I felt strongly that it was a neutral problem. My conclusions were hindered by the fact that the first power company tech that came out said that there was no problem on their end. I scratched my head for a while and then called them back and had someone come out and give me a second opinion. Then new guy was more on the ball....( plus I made sure I was there )and he made fun of me for never having seen a "high/low". Now I have....what a great learning experience. Oh.....and some of the T/S techniques you guys recommended were great...i hope I get a chance to use them.

 

[LarryFine]

My conclusions were hindered by the fact that the first power company tech that came out said that there was no problem on their end.

Many of them don't understand that a load is required to make the imbalance appear.

Then, once they get that, they blame the ground rods for not compensating for the neutral.

A sagging voltage is usually a line conductor; a rising voltage always involves the neutral.

 

[JohnJ0906]

My conclusions were hindered by the fact that the first power company tech that came out said that there was no problem on their end.

Many of them don't understand that a load is required to make the imbalance appear.

Then, once they get that, they blame the ground rods for not compensating for the neutral.

I agree with Larry. The POCO here always denies responsibility 1st, then blames improper grounding. :roll:

 

[charlie]

. . . So the problem turned out to be in the neutral splice ( between street and house ). . .

Nearly every time this happens, it is an open neutral or a loose connection at a neutral connection. For some reason (normally lack of training), the linemen or service linemen are not familiar with this type of problem. However, the trouble men and power quality men are all over it. Whenever you run into this type of problem, call the serving electric utility first an be there when they are there.