Is it normal for lights to get brighter when a heavy load turns off?

I'm currently in a rehab facility (Don't worry it is not for substance abuse) and their main AC broke so they installed crappy single hose portable units (better than nothing I suppose) in the patient rooms.

They are doing OK however I have been noticing the lights getting brighter for a second or so after the compressor shuts off either by the thermostat or the main power button being pressed is this normal or to be expected when a heavy load shuts off?

I believe they have three phase service with most convenience receptacles being NEMA 5-20R's and a few 6-20R's in the hallway for the old floor machines (No longer used as they have battery machines now)

They also have some red generator backup receptacles scattered across the facility as well as in the patient rooms.

This is just an I'm curious question as even if it is a problem I can't do anything about it as I'm not an electrician or a worker here just a patient

Anyway Thank You in advance for any insight



AC Data label

 

It's a balance change in the three phase supply. Perfectly normal.
Some areas probably respond in reverse.

 

Well known when you go for a lo cost solution. Many ears ago (40?) we had a cabin with about 1200 yds to the transformer, we shared the line with several others, and when the neighbor turned on their stove for cooking eggs at Easter morning the lights became dim for an hour. Their eggs did not cook at all, so we were happy to use a propane stove ourselves. That may be the reason to never change that. (New transformer closer to us has solved that and other problems.) Power outs are more common on the coutry side so we will keep tha propane range "for ever".

 

What is meant by that?
I'm not really familiar with three phase supplies other than they can supply 277V/480V as I have a few Metal Halide ballasts with a 277V tap and one high pressure sodium fixture with a 480V tap.

You know real lights not the LED garbage that passes as lighting now)

PS: Why do flammable refrigerants exist I know one (R-290) is just Propane without the scent LoL whoever thought that was a good idea!

These portable AC units use R-32 which is just Difluoromethane a flammable gas

 

3Φ voltages are based on the square root of 3 as in '3 phase'. The value of √3 is equal to 1.732.

If we do a little math and divide by the square root of 3 you can see the relationship of how the different common voltages come about to be available.

If you take the 480 volts 3Φ ÷ √3 or 1.73 you get the 277 volts, commonly referred to as 277/480 3Φ.

If you take 208 volts 3Φ ÷ √3 or 1.73 you get 120 volts, commonly referred to as 120Y/208 in a WYE configuration and the line to neutral (L-N) voltage is 120V.

The WYE configuration is the makeup of how the 3 windings of the power transformer and the center tap form what is known as WYE.

 

The lights that got brighter are on the same phase as the air conditioner that shut off. The lights are a load where Ohms law states the load current is voltage divided by impedance. The brighter the light, the higher the current. Since impedance isn't changing, the only way current was increasing is the voltage was increasing to satisfy Ohm's law. My guess is the load is highly inductive and the changing current with time, when the air conditioner shut off, resulted in a momentary rise in voltage locally.

 

The neutral would have to be common to both loads in some way - the load of the bright burning lights and the load that causes the lights to burn brighter - and the neutral has to have a high resistance connection to the mains for the following to happen:

The bright burning lights and the heavier load (which could be a highly inductive motor load but not necessarily so) actually form a voltage divider between the two branch circuits. If both the bright burning lights and the heavier load were drawing nearly the same current then they would each see the same voltage, and the neutral would have little or no current. However, when the heavier load is being used and thus draws more current, the voltage divider is therefore changed, the heavy load sees less voltage and the lights see more voltage - and the lights then burn brighter. This can only happen if the neutral is common to both branch circuit loads and is a high resistance due to a poor splice or poor connection such as with daisy-chained outlets, or in a junction box, the main panel's neutral/ground bond or between panels, etc.