Is Polarized Plug enough to prevent Appliance Electronics Fire Hazard?

[zerocross]

A short time back my wife complained that her new Vacuum Cleaner started smelling funny and that the "Clean/Dirty" led lights had stopped working. It was over it's warranty period for a few months so I took a peek inside to see if it was easily correctable. It turns out that two electronics modules containing low voltage dirt sensing and led driver components were severely burned (and surrounded by lots of charred plastic parts). Curious, I traced the circuit to see how the boards received power. Turns out that the "Meter Ground" (it's actually labeled that on the PCB) side of the low voltage board is connected directly to the Neutral side of the two prong line cord plug. My wife evidently plugged the vac into a heavy duty extension cord that upon inspection happened to be one that was manufactured with the neutral/hot leads reversed. That put 120 volts A.C. onto the low voltage circuit boards and I suspect that caused the failure. BTW, there is a circuit breaker and thermal cutout used but they only protect the main 120 volt motor.

I know the faulty extension cord is to blame but the question I raise is: Should appliance manufacturers take extra steps to isolate or otherwise protect low voltage components from potentially catastrophic failures like this?

 

[DanZ]

I know the faulty extension cord is to blame but the question I raise is: Should appliance manufacturers take extra steps to isolate or otherwise protect low voltage components from potentially catastrophic failures like this?

Yes, they should. That extension cord company should have paid more attention!:wink:

Seriously, yes, they should. But you won't pay for it, so they don't. Their liability usually extends to their equipment only, so that's all they care about.

 

[Lcdrwalker]

How far should a manufacturer go to protect his product from unforseen circumstances? If the product was protected against every possible anomaly we probably couldn't afford it.

 

[ironman3452]

was vac ul approved, the cord should have been polarized, but if the extension cord was wired wrong, that could be a very big problem.

 

[DanZ]

was vac ul approved, the cord should have been polarized, but if the extension cord was wired wrong, that could be a very big problem.

But, the extension cord is the extension cord manufacturer's problem, not the vacuum manufacturer's problem.

And, yes, it should have been polarized.

 

[don_resqcapt19]

With a two wire supply cord, there is no reference to ground so the equipment should not know that the hot and neutral were reversed. It should not have been the cause of the failure.

 

[charlie b]

With a two wire supply cord, there is no reference to ground so the equipment should not know that the hot and neutral were reversed. It should not have been the cause of the failure.

I agree absolutely. All that changes, when you switch the two wires within the extension cord, is that the point within the appliance at which the power is turned off (i.,e., when the vacuum's switch is off) becomes associated with the neutral wire, not the ungrounded wire. That does not cause anything within the appliance to change, since there is no completed circuit with the switch off. When you turn the switch on, the appliance does not care which wire had the switch leg; it just runs.

My opinion is that this appliance exceeded its useful life, and merely died. All things do.

 

[zerocross]

I agree absolutely. All that changes, when you switch the two wires within the extension cord, is that the point within the appliance at which the power is turned off (i.,e., when the vacuum's switch is off) becomes associated with the neutral wire, not the ungrounded wire. That does not cause anything within the appliance to change, since there is no completed circuit with the switch off. When you turn the switch on, the appliance does not care which wire had the switch leg; it just runs.

My opinion is that this appliance exceeded its useful life, and merely died. All things do.

Thank you for the responses everyone! While I agree with the explanation of the wire swap results internally, at least as it would apply to the AC motor, I should clarify that a 5 volt DC circuit board used to drive the LED indicators has it's minus side tied to the neutral line on the cord. Seems that putting 120 VAC on that line would not be healthy for those components. Maybe there was another cause I'm not aware of. I'm going to look again at how the second controller board derives it's DC power which feeds signals to the one mentioned above that fried. BTW, The unit is just over a year old with little hours on it. Thanks!

 

[mxslick]

With a two wire supply cord, there is no reference to ground so the equipment should not know that the hot and neutral were reversed. It should not have been the cause of the failure.

I have to somwewhat disagree with this. I have personally experienced problems with some electronic equipment of poor design where reversing line and neutral triggered catastrophic failures. A film transport system popular in the 70's had this issue with the motor control boards. Part of the low-voltage (24vac ac) circuit, specifically the center tap of the transformer, was connected directly to the neutral of the incoming AC line. That center tap also referenced the control board, and the rectified DC output to the drive motors. When the line/neutral was reversed, all three control modules would fail, violently. Replace the modules and correct the incoming line and all was well.

All it would take is the overvolage as referenced to ground of any kind, directly or through capacitve coupling (which I believe is what happened with the vac in this thread) to impress an overvoltage on some component(s) leading to the failure.

Strange but true. There is another thread on this forum discussing "Polarity" that gives some insight to this issue.

 

[don_resqcapt19]

mxslick,
Did that equipment have an EGC?

 

[G0049]

With a two wire supply cord, there is no reference to ground so the equipment should not know that the hot and neutral were reversed. It should not have been the cause of the failure.

Yup. With only a two conductor suppy cord, being fed from an Alternating Current source, the only thing that happens when the supply conductors are reversed, is that the directional flow that you think may have caused this failure, got there a half cycle sooner. Or maybe later.

You're going to have to look somewhere else for the cause of this "failure". From the maunfacturers postition, it was a success. The vacuum lasted long enough to barely make it through the warrenty period.

 

[LarryFine]

From the maunfacturers postition, it was a success. The vacuum lasted long enough to barely make it through the warrenty period.

Welcome to planned obsolescence. :roll:

 

[mxslick]

Some insight...

Some insight...

mxslick,
Did that equipment have an EGC?

Yes sir it did, However, out of eleven failures of that kind in the field, four happened with the grounding pin on the cords cut off. Of those four, three were sitting on their metal feet directly on a concrete floor slab, so I'd say that would have made a fair path to ground, one was on carpet layed (lain?) over vinyl tile, so maybe a so-so path there.

Edited to add: No part of the control circuit or low voltage supply was connected in any way to the egc.

I used to repair those control modules, and have the power supply, one platter arm and motor as a test set-up (the complete system has one power supply and three arms/motors.)

For giggles I set up a test and sacrificed a few modules. One test was with hot/neut reversed and egc connected, the burnout happened within 10 seconds.

In the second test I lifted the EGC with a ground lift adapter. The failure happened, but it took about 30-45 seconds with some rather strange noises from the motor.

The entire rig was sitting on rubber pads on my wood workbench.

Unfortunately I did not think to measure voltage from the egc to ground in those tests.

My conclusion from those tests and results was that it was a very poor design, but the actual failure mode was difficult to track, as the modules used LEDS and photocells, with a moving cam to regulate the light (and thus the speed of the motors), and the control arm had to be fully enclosed and light-tight to do any testing. The failures always resulted in almost total destruction of all the components, so it was difficult to tell what failed first and what followed in what order.

Goo49:

I can agree partly with this part of your post:

Yup. With only a two conductor suppy cord, being fed from an Alternating Current source, the only thing that happens when the supply conductors are reversed, is that the directional flow that you think may have caused this failure, got there a half cycle sooner. Or maybe later.

Again, I stand by my supposition (and the results of my "no egc" test above) in that the issue is not when the half-cycle happens, but the fact that a part that is supposed to be referenced to neutral in whatever form was subjected to the line voltage instead, and capacitive coupling to earth created enough voltage rise to kill it.

Ask any transformer expert what happens to a transformer designed with "graded insulation" (most single-bushing distribution transformers were made with it) when the line and grounded terminals are reversed.

** Graded insulation, accoring to my reference book, is windings with more insulation materials at the line end, and less insulation used/applied as the winding progresses to the grounded end. (Often the end closest to the core) This was done supposedly to save money, weight and give a very slight efficiency boost.

At the other end of this spectrum, the film projectors at a screening room I run are designed for 208-240 volts line-to-neutral, yet are running quite nicely (after 11 years) running 208 line-to-line. (The machines are German). Studying the schematic very carefully, there are NO components in it where HV and LV share any common reference. I think in the case of my platters and this vac, that common reference between HV and LV is the key issue to the problem.

 

[pfalcon]

I'm a little out of my element on PC board design but last I talked with my specialist:

Conversion from AC to DC in small electronics is normally done by tapping the positive half of the AC signal from the HOT lead; and the negative half of the AC signal from the Neutral lead. One of the signals is then flipped in reference to what it thinks should be the signal ground. If the system is hardened from polarity then it rejects the reversed power as though disconnected. If it is not hardened then the DC flow is reversed causing component failure.

Most electronics come with polarized plugs. If you somehow bypass the mechanical protection then the warranty is void - such as with the bad cord.

I could use a little help with this explanation.

 

[pfalcon]

I guess I forgot to add. Since this is a known problem with these circuits; something should be installed to blow on reverse current flow instead of cooking the electronics; even if it means servicing the device. Better service than fire.

 

[brother]

I asked this question about 'ungrounded' systems (like Isolation panels for hospital O.R.)that had reversed polarity.

It appears that this would cause a problem even if no referenced to ground

 

[ultramegabob]

My wife evidently plugged the vac into a heavy duty extension cord

sounds like you need to add a receptacle somewhere in your house if shes using and extension cord for a vacuum cleaner.

 

[zerocross]

I checked the other board and indeed that one derives it's low voltage DC through a couple of diodes capacitively coupled to what normally would be the hot side of the line, the PCB ground plane normally ties to neutral. Reverse those two lines and poof! Sadly, right next to the LED module is a full wave bridge rectifier that is used only to drive the DC power head motor. That would have made a polarity reverse "hardened" source for the DC but probably would have costed more to filter. The bad extension cord got used to vacuum out a car in the driveway. I have three of these extension cords, all look identical but one had the plug molded on backwards. I believe there have been a few recalls on this type of cord.

So, I go back to the original question of whether or not the mechanical protection of two conductor polarized plugs is adequate to prevent these potentially hazardous failures. Considering the millions of older homes still with two conductor outlets, ground-lift adaptors and spongey extension cord receptacles that allow the wider spade to be pushed in on the wrong side, etc, etc, maintaining adequate product safety and reliability still seems an issue to me. Similarly, does anyone here know what codes and safety standards manufacturers must presently comply with in this regard?

Thanks for the insights everyone.

 

[DanZ]

So, I go back to the original question of whether or not the mechanical protection of two conductor polarized plugs is adequate to prevent these potentially hazardous failures. Considering the millions of older homes still with two conductor outlets, ground-lift adaptors and spongey extension cord receptacles that allow the wider spade to be pushed in on the wrong side, etc, etc, maintaining adequate product safety and reliability still seems an issue to me. Similarly, does anyone here know what codes and safety standards manufacturers must presently comply with in this regard?

Thanks for the insights everyone.

So, auto manufacturers should figure out a way to keep you from putting diesel in you unleaded fuel only tank? Or put only the right plugs in your engine?

Tools makers should figure out a way to keep you from shorting out two wires with your screwdriver or pliers? Or using a screw driver as a chisel?

Manufacturers still rely on your common sense, and good manufacturing practices of other companies, mostly to keep costs down. If they planned for everything, your vacuum would cost as much as your car!

On a side note, that's why vacuum's have instruction books. I glanced through the book for the last one I got, and it actually said "Use a polarized outlet only!" and explained what they were, and why they are around, then said something to the effect of "If you don't have one, or somethings not right on your end, call a professional to fix your problem."

 

[zerocross]

DanZ, I'll distill that to a "No". You focus on the acts of idiots and you're right, no manufacturer can completely outwit them without outrageous consequences. Then there are inadvertent actions, ones taken unwittingly by smart people and idiots alike because they just "didn't know". If conditions in the imperfect world arise frequently enough that these inadvertent actions pose a public safety concern then standards and manufacturing practices need to change. I posted this topic because I was surprised at what I found inside this thing. I realize this is not the forum to resolve consumer safety issues or check for product recalls.

I saw this Safety forum and wanted to get opinions of other professionals because I felt that a common household appliance like this might be subject to inadvertent misconnection by any number of people whether they read the owners manual or not. Heck, there were responses from knowledgeable people right in this thread that suggested reversing the polarity in this case wouldn't make any difference. But it does.

So perhaps the question needs refining - how high do you feel the conditions are for inadvertent polarity reversal of ungrounded polarized plugs by consumers? Should consumer electronics and appliance manufacturers design products to remain safe when this happens?