Microwaves spark if you plug them in when the power to the circuit is on. They are not intended to be unplugged and plugged in every time you use it, or there would be a switch in the unit to shut ALL the circuits off. The clock would not keep very good time if you keep unplugging it. If you put a wire from the positive to negative posts on your car battery, you will get the same sparks. It is just what electricity does between the time the conductors get close together, and the time they are securely connected. If you take an ordinary lamp and plug it into the wall, it will not spark as long as the switch on the lamp is in the off position. If the switch on the lamp is on when you plug it in, you will get a spark. Fiddling with the tamper resistant receptacles just makes the spark worse, and for a longer duration. You just don't usually get a spark because most appliances are OFF when you plug them in. A microwave is NOT entirely off when you plug it in, and I always recommend turning off the circuit breaker when plugging in a live appliance like a microwave.

If you examine a standard light switch, you will notice that it is actually called a "snap" switch, because it snaps into the open or closed position quickly to avoid, or at least lessen the spark effect. You can not accomplish a quick connection with a microwave while plugging it into a tamper resistant receptacle, THERFORE, it will spark as long as it takes to plug the microwave in all the way, unless you turn off the breaker.

It is the tamper proof. I was thinking of that as child proof.

I think that the way I was inserting the plug was a problem. The time I did finally get it in I was very careful about making sure it was straight on and not angled at all. (It was still very difficult and still sparked.)

The second time in a month that I successfully plugged it in I left it plugged in because of the difficulty. It has been working fine.

Fred, the inspector did say something about the microwave clock being "on" all the time, so maybe he was saying what you are explaining. It is so that MWs aren't often plugged in/out, but I have had a few MW's over the years and never had this issue. I've also only had 15A outlets before though. This is a 20A outlet as well as a 20A dedicated circuit. I don't know if that makes a difference.

I've never heard of flipping the breaker off before plugging in, but it is certainly good advice that makes a lot of sense with your explanation. Thank you Fred and Mike. I am reassured that the sparking (which I still think is strange not to be the subject of cautions) does not mean something defective is going to start a fire at that outlet/line.

15 or 20 amp doesn't make a difference. The big change is that you are trying to plug it into the PITA tamper resistant receptacle. I have melted off half the prong before from the sparking when things aren't quite right, thus the suggestion to turn off the breaker, especially with the new tamper resistant receptacles. Your other microwaves sparked too when you plugged them in, but it was just such a quick and smooth motion that it wasn't much of a spark to notice with a standard receptacle. If you had plugged them in while in a dark room, you would have noticed a spark.

Older microwaves with mechanical timers are not the same as newer microwaves with digital timers as far as any potential arcing when first plugged in

Older microwaves and also many toaster ovens or indeed toasters have a mechanical timer that serves to operate like a switch/timer combination. The digital timers can not be turned "on" with the device unplugged. The mechanical timers can be turned on with the device unplugged.

The current draw to operate an electric clock or display is next to nil and arcing should never be observed when plugging in the device.

Microwaves should have their own dedicated circuit IMO. They use a lot of amps. No need for that circuit to have tamper resistance either..... or any of your other kitchen outlets not within reach of a child on the floor. If an adult sticks a knife into an outlet they deserve the pain.

No need for that circuit to have tamper resistance either..... or any of your other kitchen outlets not within reach of a child on the floor

It should be noted that the above is NOT what the code states since the 2008 NEC. All required 120V 15/20a NON-LOCKING receptacles in dwelling units are required to be TR.

The exceptions provide for receptacles providing appliance in a dedicated space. A countertop microwave even with a dedicated circuit for it still requires a TR receptacle. Further, "within reach of a child from the floor" has no meaning in the code either. The actual requirement is 5 1/2 feet (anything above that escapes the TR requirement). Most kitchen countertop outlets are not that high.

In general, the voltage difference is what causes a spark, not how much current is allowed through by the clock circuitry. The clock circuitry (resistance) is in fact what keeps the spark going instead of tripping the breaker when the spark occurs. In this instance, there IS enough current going through "normally on" circuits in the microwave to cause sparking BECAUSE IT IS SPARKING. PERIOD.

I don't have any TR receptacles in the kitchen but I just unplugged/plugged a "modern" microwave and no sparking of any kind.

Just as aside, it's tamper resistant, not tamper (or child) proof. They make it more difficult to be shocked by sticking something that doesn't belong in a receptacle but not impossible.

"In this instance, there IS enough current going through "normally on"
circuits in the microwave to cause sparking BECAUSE IT IS SPARKING.
PERIOD."

Well that solves it.

Say we plug an extension cord into the 120V receptacle. In the first case nothing plugged into the extension cord. No sparking. In the second case we short the extension cord prior to plugging it in, result is sparking.

Voltage 120 V in both cases.

Is no one going to ask the OP if other devices, such as my un-shorted extension cord, spark?

What is the impedance of a magnetron when first plugged in on the old style microwave?

Well, if you want to be pedantic about it, the three things determine an arc:

Voltage, Dielectric constant of the material, and distance. The dielectric constant is that of air (typically 1) is for your tests constant. Voltage is constant. What is changing is how far the blade is from the conducting part of the socket and how long it is held there.

Perhaps the newer TR outlets dont act up as much. I know most people loathe them. I also wonder what other purpose TR outlets serve other than child protection. They are a nuisance anywhere but outlets near the floor, since they serve no other real purpose than child protection.

SaltiDawg said:

"And of course, the actual arcing is caused by current flow and can occur at even low voltages"

The spark plugs in your car run at thousands of volts, but less than 4 amps. I won't take the time to look up the exact numbers because it is irrelevant other than the point that it is the voltage that the engineers increase from the standard 12V battery to get your car to run.

.

"Is no one going to ask the OP if other devices, such as my un-shorted extension cord, spark?"

There is no voodoo going on here. An un-shorted extension cord would be no different than the bird on a single wire scenario. (The bird doesn't get electrocuted because there is no voltage difference) The bird only receives a shock if it were to touch two wires or the ground at the same time. The only way an un-shorted extension cord could cause a spark is if it were actually shorted, or if it were so long as to have it's own voltage potential and then cause a Single spark from Static electricity.

If the receptacle sparks simply by sticking anything at all in the slot, even if it is a nonconductive material, then the receptacle would be sparking all the time, and would have to be physically falling apart to the point that you would not be able to plug anything into it anyway.

"What is changing is how far the blade is from the conducting part of the socket and how long it is held there."

In other words, the TR feature makes the blades stay at a relatively short distance away from the hot contact for a considerably longer period of time than a normal receptacle. Therefore, creating an arc!!!

Sometimes, the simplest answer is the correct answer and there is no reason to try to find some other off the wall scenario. If the OP indicated that the microwave was working improperly, then there may be a reason for further guesses and hypothetical fantasy.

Fred S. It really seems that you have no idea what you are talking about. Try googling on something like electrical arcing definition .

As to the situation the OP is trying to address, your midnight post started a train of posts by you that are not helping the OP one bit.

To the OP, I tried but I'll move on and I do hope that someone weighs in and gives you the advice/answers you seek.

SaltiDawg, I don't need to Google it. I took several college courses relating to this with an average test score of 99%, not to mention 25+ years experience as a licensed electrician

You are the only one trying to confuse the issue with impedance nonsense. The microwave obviously allows enough electrons through to cause a spark, or it wouldn't be happening.

The problem has been solved since the last time the OP chimed in. The microwave is plugged in and works fine. Even the inspector agrees with me, so do move on.

Just revisited this.

Will not reinsert the MW plug. I fear dislocating my shoulder to get in back in. The MW is a small Panasonic with inverter technology if that matters to any of you. I don't know if that means it doesn't have a magnetron. It's 950 watts.

I now have a Cuisinart CSO-300 steam mini-oven on the same 20A circuit, same outlet. It is 1680 watts. It has one of those flat space saving plugs. It was quite difficult to plug in when I first did it but not nearly as difficult as the MW with a "regular" plug. I couldn't remember what happened with the sparking, so I have just unplugged and replugged it. It did go in more easily than the first time. There was one spark, not a shower of sparks.

I think the excessive MW sparking is related to the length of time that the prongs are sort of in place as I try to force the plug in. (Would DW-40 help?)

(I don't intend to use CSO-300 and the MW at the same time. They were to be in different locations, each with its own 20A dedicated. Unless you haunt the Kitchens forum you won't care about why they aren't separated as planned.)

Thanks all for your comments. My father (EE) would have loved joining in - too bad knowledge and experience isn't genetically passed on.

"I think the excessive MW sparking is related to the length of time that the prongs are sort of in place as I try to force the plug in."

Exactly, no complicated theories required here...

"(Would DW-40 help?)"

I would not suggest that. You may want to see how flamable it is before you put a spark to it! ;D

Egads! I do read all labels before buying and using products BUT you have just taught me that I must reread before using on any new applications I may have in mind. I was just thinking that it wouldn't rust or corrode metals so it might be a good lubricant for those tough plastic shutters in the TR outlet.