Voltage surge

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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Of course. Either turn off the lights or the AC or both :)

Seriously: an answer would require details of the entire electrical system, including the supply transformer, the service entrance conductors, the branch circuits, in particular the lighting circuits and the AC supply circuit, and the details of the AC unit. The causes are numerous and varied as are the solutions. No reasonable answer can be given without more details, and even then you will likely only get good guesses and recommendations.

-Jon
 

AV ELECTRIC

Senior Member
If this is in a common neighborhood maybe check with the neighbors to see if they have the same problem.This can be common with high draw equipment during startup . Check with owner to see if this is a recent problem or has it been happening since they lived in the house. How old is the ac unit Age and use may be creating a harder start and the unit may have reached the end of the line.
 
there is a possibility the utility may have a transformer too small or the conductors to the property are too small.

there is also the possibility that you have undersized the conductors, possibly due to length (voltage drop).

There is also a possibility both situations have occurred.
 

StephenSDH

Senior Member
Location
Allentown, PA
there is a possibility the utility may have a transformer too small or the conductors to the property are too small.

there is also the possibility that you have undersized the conductors, possibly due to length (voltage drop).

There is also a possibility both situations have occurred.

Pierre is right. The available current largely depends of the utility transformer and feeder. My sisters house they actually upgraded the transformer and moved it a pole closer because of voltage sags.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
091022-2317 EST

I conjecture that in most residential cases that you will see some flicker in an incandescent lamp when a whole house air conditioner turns on.

The real question is -- is the amount of momentary voltage drop excessive?

You need some instrument that can record voltage with a response time of probably 1 millisecond. Even 1 second may be sufficiently good in some cases.

I think most people will detect a momentary voltage change of 3% with an incandescent lamp. My sensitivity is somewhere between 1 and 2%.

If the voltage drop is not excessive for the transformer and wiring impedances, then try GE dimmable CFLs and see if the problem goes away. These have moderately little light change with voltage change down to about 100 V where they drop out.

Keep in mind that dimming lights may imply wiring problems. Especially if some lights momentarily brighten. This is not likely a test in the case of a 240 V air conditioner.

.
 

glene77is

Senior Member
Location
Memphis, TN
Gar,
You are right. Always interesting posts.

I've also done this with my clamp-0n Analog Amp meter.
Had similar results.

I use it one time tracing a bad neutral connection (in the meter base) and was able to detect the bass audio on the owners stereo
at about 1 Amp. Seems that was enough to trip his computers UPS to kick in. I had to replace the meter socket.

This was the same customer who called me at 1:00 AM, complainging that his ceiling fan had fallen out and was hanging by some wires. Turns out that he had aluminum house wire, and had bigger problems than just his ceiling fan.
 

brian john

Senior Member
Location
Leesburg, VA
AS little as 3 volts drop with incandescent bulbs.

There are several issues here,

Possible loose neutral which needs attention IMMEDATLY. This can be from the transformer to your branch circuits.

Flicker< which can be something you are dingf (motors starting or possible a neighbor issue, depending on the utility distribution in your area.


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.
 

480sparky

Senior Member
Location
Iowegia
Undersized service.
Undersized serivce conductors.
Undersized AC circuit conductors.
Loose connections.
Capacitors in AC is bad.
Motor in AC is bad.
AC needs cleaning & servicing.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Voltage Drop Measurement

Voltage Drop Measurement

091022-2317 EST

I conjecture that in most residential cases that you will see some flicker in an incandescent lamp when a whole house air conditioner turns on.

The real question is -- is the amount of momentary voltage drop excessive?

You need some instrument that can record voltage with a response time of probably 1 millisecond. Even 1 second may be sufficiently good in some cases.

I think most people will detect a momentary voltage change of 3% with an incandescent lamp. My sensitivity is somewhere between 1 and 2%.

If the voltage drop is not excessive for the transformer and wiring impedances, then try GE dimmable CFLs and see if the problem goes away. These have moderately little light change with voltage change down to about 100 V where they drop out.

Keep in mind that dimming lights may imply wiring problems. Especially if some lights momentarily brighten. This is not likely a test in the case of a 240 V air conditioner.

.
Gar, as you pointed out, it has to be determined if the VD is excessive before you proceed with corrective action.
I suggest using a Fluke 87 multimeter with the min/max feature set on voltage. At the meter box it is fast enough to record a 100 millisecond RMS sag on one scale and a 1 millisecond peak to peak change on the other scale.
This will determine whether you are within or way beyond the 3% range. If within then live with it; if beyond then start looking for undersized wiring, transformer or loose connections.
 
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