Microwave suddenly dims house lights -- but nothing else does as severely

[Axel114]

A 6-year-old Whirlpool microwave oven has started significantly dimming lights throughout the house when the magnetron kicks on. I know some minor dimming can be normal when a refrig/AC/washer/vacuum starts, but this level of dimming was not typical of the oven, and it stays dimmed until the magnetron goes off. I've plugged it into a different circuit but the result is the same. I connected it to a 12-guage extension cord with an LED-lighted plug and the plug dims severely and even flickers. A 1500 watt electric heater on the same cord will not affect it this way. Does anyone know what's causing this? Is this dangerous? The oven is still heating food.

 

[c_mitra]

You need to test the current taken using a clip-on AC current meter. Something is seriously wrong and you should not continue to use it on a regular basis.

 

[BradtheRad]

A similar thing started happening in our house due to a miswired lamp socket. It caused lights to dim in the next room. However it did not blow a fuse.

Your condition is dangerous whether or not it blows a fuse. The oven may be faulty in the sense that it draws intense pulses at a low duty cycle. Thus it does not overload your house circuits however it does cause brownout (low voltage). Something inside the oven could overheat and catch fire.

 

[Axel114]

I don't have an AC current meter to test. I just wondered if the symptoms pointed to something definite. Knowing very little about electricity I just assumed that the dimming might mean it was drawing some outrageous amount of current that could cause wires to melt. But the current has to be used/released in some way, right? Is a 1200 watt appliance capable of drawing more than 1200?. It's on a circuit with a 30 amp fuse, so it would take a lot to blow it. Because of the 30 amp fuses, we carefully monitor and restrict load on each circuit for safety reasons so that the total doesn't go over about 1700 watts, but this presents a new issue. With some googling I can't find anything exactly like this. Everything is about overloaded circuits and such, which is not the case here. It dims if nothing else is on the circuit.

 

[c_mitra]

There could be multiple defects: particularly worrisome is that the dimming is not transient, it stays on as long as the magnetron is on. Usually they about 700-900W (total may be 1200 if there are other options) and that should not cause a brownout.

It is a serious fire hazards because it is taking less current (than necessary to trip the circuit breaker) but is high enough to dim the lights. Please do not use on a regular basis.

I cannot guess what is wrong; it may be as simple as a microswitch or as complex as a PFC circuit.

 

[RobertFalbo]

This could be a loose connection on the circuit breaker.
After years the connection can become loose and you get some voltage drop with high current loads.
Outdoor boxes are prone to this because of hot to cold cycling.

One question, Do some lights get brighter ?
That would be a open neutral wire back to the pole.

 

[betwixt]

I agree with RobertFalbo, it's unlikely the microwave is still functioning normally while drawing an unusually high current. The problem is almost certainly an unwanted extra resistance in the wiring which under normal load is causing a voltage drop.

The safety aspect isn't the microwave itself, it's where the heat from the voltage drop is occurring. As wiring itself is very unlikely to deteriorate, the connection points have to be prime suspects, this leads back to a common point where both lights and microwave currents are flowing. Almost certainly if the building is wired properly, that will be the fuse box or circuit breakers, whichever is fitted.

Do an experiment, pull the fuses or switch the circuit breakers off one by one to find which is for the lights and which is for the microwave, they should be on different circuits. If one switch turns both off, you need an electrician to do some rewiring urgently! If they are on different switches, check the switches or fuse holders themselves and for a common connection between them. In Europe, standard wiring would be a bank of circuit breakers with a copper bus bar feeding power to their input side with the output side going to the individual circuits but depending on your location and the age of the installation, your situation may be different.

Brian.

 

[c_mitra]

This could be a loose connection on the circuit breaker....

Possible but unlikely. The OP says that "A 1500 watt electric heater on the same cord will not affect it this way"- that suggests that the problem is with the load and not the supply.

Common magnetrons are all less than 1kW and with fancy grills and other features, they will hardly draw more than 1.2 kW.

The oven is 6 years old; it has come close to its design life. My microwave gave up after 8 years and the fault was in the microswitch door interlock that I could not get a replacement.

If he is having a 30A circuit breaker, it is very likely that the supply is single phase and other people share the other phases.

Microwave ovens are rather robust devices and a little care in the design can be very useful. I get angry when people have to throw away an otherwise perfectly good machine just for a bad capacitor or a switch.

 

[schmitt trigger]

Very simple test:
Take the oven to the home of a friend or relative.
Turn on the oven....if the lights dim, then the oven is defective.

 

[Axel114]

This could be a loose connection on the circuit breaker.
After years the connection can become loose and you get some voltage drop with high current loads.
Outdoor boxes are prone to this because of hot to cold cycling.

One question, Do some lights get brighter ?
That would be a open neutral wire back to the pole.

If that's the case, wouldn't it affect other appliances that draw a lot of current? Only the microwave does this. No, the lights don't get brighter (though they do recover when the magnetron goes off).

 

[Axel114]

The safety aspect isn't the microwave itself, it's where the heat from the voltage drop is occurring. As wiring itself is very unlikely to deteriorate, the connection points have to be prime suspects, this leads back to a common point where both lights and microwave currents are flowing. Almost certainly if the building is wired properly, that will be the fuse box or circuit breakers, whichever is fitted.

Do an experiment, pull the fuses or switch the circuit breakers off one by one to find which is for the lights and which is for the microwave, they should be on different circuits. If one switch turns both off, you need an electrician to do some rewiring urgently! If they are on different switches, check the switches or fuse holders themselves and for a common connection between them. In Europe, standard wiring would be a bank of circuit breakers with a copper bus bar feeding power to their input side with the output side going to the individual circuits but depending on your location and the age of the installation, your situation may be different.

Again, this unit has been plugged into different locations within the house with the same result. Air conditioners and electric heaters plugged into the same circuit don't do this (though ACs do create an initial dimming that recovers).

We mapped the house's circuits to the outlets several years ago and have a diagram for what links to each fuse, so I know that I've tested it on a circuit with nothing else.

Could the "unwanted resistance" you speak of be inside the microwave?

 

[betwixt]

Could the "unwanted resistance" you speak of be inside the microwave?

No, it has to be between the source of power and the load (the microwave). When current passes through a resistance it drops the voltage, more current causes more voltage drop and more resistance causes more voltage drop. The lights dim because the voltage reaching them has reduced. The voltage drop is expressed as heat so wherever it is occurring is going to get heated up. The problem with blaming the microwave is the only way it can draw excessive current while still operating is if something else in it is producing the heat. It sounds like in your case, it would have to produce as much extra heat as maybe that 1.5KW electric heater and as the magnetron isn't capable of such high power it implies some other part of the oven is running red hot. That is a very unlikely scenario, the whole oven would be blisteringly hot, possibly glowing red, and I'm sure you would notice that!

Brian.

 

[Axel114]

Thanks. Unfortunately, all this leaves me confused. The above sounds like the culprit should be in the house wiring. Yet the microwave dims things more than anything else. I did more experiments. This time the electric heater did dim the lights until it was turned off. But it still didn't do it quite as badly as the microwave. The mw sometimes even causes the LED at the end of the extension cord to go out entirely. I'm just trying to make a decision whether to buy a new microwave. I don't want to spend that money only to find out the new one does the same thing. As I said, the severity of this is something new, but of course it's possible a change could have occurred outside the microwave too.

 

[andre_luis]

I connected it to a 12-guage extension cord with an LED-lighted plug and the plug dims severely and even flickers.

Are you sure that it isn't misconnected the Neutral with Earth wirings on the cord extension ? I would dare to say that these cheap ones may have no efficient manufacturing quality control process after assembled.

 

[c_mitra]

... This time the electric heater did dim the lights until it was turned off. But it still didn't do it quite as badly as the microwave....

The microwave may take a high current for a fraction of a second but it would be barely noticeable. But if the electric heater is also affecting the voltage (brownouts) then the house wiring may be the real culprit. It is common that old wiring slowly degrades because of local heat (high current through an under-capacity conductor) and subsequent degradation of the insulation.

You also need to test the house wiring with a "megger"- it is widely used to test insulation resistance at a high voltage.

 

[Mattylad]

ON a sunny day, turn off the power to your fuse box at the main switch, test for no power and use an insulated screwdriver and tighten all your connections, both live, neutral and earth.
Do this with the screwdriver in one hand, your other hand in your back pocket.

Turn the power back on and try it again, you have nothing to lose if you do it safely.

 

[Axel114]

andre_teprom: The extension cord is connected properly and it is a rather expensive 12-guage model we got to use for outdoor 12-amp motor equipment. Also, it's been tested with other high current appliances.

Megger? Don't have anything like that. Sounds like I'd be in need of an electrician if all these tests are really necessary.

The power test recommended by Mattylad scares me a bit since there are multiple "main" switches here and I'm unsure which controls what. There's two boxes outside and one inside. The inside one has two "main" looking dual fuses that pull out and 4 screw fuses. The screw fuses are the ones that were mapped to the outlets.

For those who've mentioned possible wiring issues that could cause lower voltages, is this dangerous? How concerned should I be about this? I'm really considering getting a new microwave, and if it doesn't dim the lights as badly, are things likely to be OK? I know you can't guarantee this sort of thing, but I'd like to know the likelihood and odds of things. I really appreciate the input so far, it's just that nothing seems to be definite enough at this point for a conclusion.

 

[andre_luis]

Ok, if you say that the cord is Ok, it is likely having some issue at the Neutral connection of the house. There wouldn't be any possibility of a recent intervention in the installation so that there could have been some misconnection ? The dimming that you are noticing sounds like due to the Neutral being unbalanced by the load rather than due to an induced interference.

 

[dick_freebird]

We had a bad connection (corrosion) at the pole, which
made the same symptoms. An "inverter" microwave tries
to get its 1200W output regardless of line sag so if the
line sags, it pulls more current rather than less. Don't
know whether yours is this newer sort.

Power company came and tightened up the pole
connections and it's been OK since. You might take a
look at the exterior wiring at night, see if you have
any sparking at the pole under heavy loads.

 

[c_mitra]

If the problem is not confined to the microwave oven, it shows up with other power appliances too, then the problem lies outside the microwave oven.

It is very likely your house wiring is divided (or made up of) several circuits. Each circuit is protected by its own fuse or circuit breaker. It is easy to identify: remove each fuse and see which lights or power points go off.

The question I am asking is whether the effect is seen only in the circuit in which the microwave oven is plugged in OR all lights are getting affected (effect is seen in other circuits too).

If the dimming effect is seen in all the circuits, the cause is beyond - it is close to the meter box or the connection from the meter box to the inside.

You need to track the source of this "loose connection" that is failing under high current. If you have an air-conditioner, turn it off during these tests.