Noor, they were probably trying to save money on the switch. Not taking into account the failure mode you ran into, they figured that the 110 volts on the element were not as problem. Not very smart, and how did they pass UL testing?
It's been a while since I've done an oven, but a double line break was not required in the US, however was required in Canada. We used to have two seperate to handle the destination and save money when it was domestic.
The correct way to switch power, at least by JIC standards for industrial equipment, is to break all the non-neutral lines. I think that is also required by the NEC. So a control switch in one side of the 240 volt lines is a poor excuse for a control method in the US. In the UK the power is a basic 230 volts with one side as neutral, which is usually also grounded. So in those systems only breaking the "hot" side would be quite adequate, and would not constitute any hazard in a polarized-connector installation. But with the basic voltage being 230 instead of 120, I am far more likely to disconnect the power before opening anything. But that is a separate issue.
tekochip, that company's policy is an invitation to an unwinnable lawsuit. At least, that's the argument posed by most companies selling industrial safety components. The plaintiff's argument would question why a less safe product was sold in one country and a more-safe product in another.
I haven't worked on an oven in almost ten years so this may have changed, but the products I worked on went through all agency approvals for US and Canada. Only Canada required a double line break. The operational standard was to use one relay as a "safety" and two other relays to perform the actual cycling for the bake and broil elements. Both relays were single line break and this was common for Whirlpool, Kenmore, KitchenAid, Maytag...... well, every brand I ever saw.
Would it be possible at least for the author to tell us what make and model oven this occurred in? A manufacturer who not only makes such shoddy products but won't reveal basic information that the author requested is one I want to avoid.
Yeah, I would have guessed that it was one of Whirlpool's products... But it sounds like they are all doing this kind of crap...
And they wonder why Samsung, Lucky Goldstar, Bosch, Miele and Electrolux are taking market share away from them them... And Fisher & Paykel are getting more popular. Not that these companies all make great products... But they all make OK products... (Well, in my opinion Miele makes some great products!)
Whirlpool is to appliances like Fram is for car parts... They are a brand I will never buy again...
I can remember a time when they both made decent products... But I'm pretty old.
I heard a rumor that GE was making slightly better quality than the junk they hawked a few years ago... But I have not seen any hard evidence yet that it's true... I bought a 12 year guaranteed water heater at Home Depot (Labeled "Best" on the sign...) and it flooded my garage 14 months after I installed it. (At least I had it in a pan that dumped towards the garage... Otherwise it would have flooded my house...) That left a sour taste in my mouth...
Noor, thank you for posting the company name. It seems that the quality of once-great brands has really dropped way, way down. I bought a Sears Kenmore oven and fridge and a fairly low-end GR washer and dryer about 12 years ago. I still have them, and the Sears machines, in particular, still work great. I've grumbled here occasionally about the GE knobs and membrane keypads, but the basic machine parts still work well. I dread the day when I have to replace them.
I just found a Whirlpool oven schematic online that does the same thing - switches L2, but not L1. So I think that it is not required by UL, and that most, if not all, all manufacturers do it. Admittedly, you don't pull the element out for cleaning like a stovetop burner. Still, I feel nervous when I can come into contact with an element that has one side hot just by cleaning my oven, now that I've seen what can happen when the element breaks down.
Yes, I've seen it be dangerous. But what really irked me was how the manufacturer practically went out of their way to NOT give me any help on this. They were so polite and so utterly useless. They must think it's dangerous for them just to talk about it.
I'm with you on that, which is why I'm asking about UL. There are supposed to be legal and/or monetary consequences for dangerous electrical products/components. That makes me wonder if the Whirlpool machine with the same switching pattern may also be violating UL standards. Or did something happen to consumer product oversight?
I searched out some more schematics, and the majority of them only switch one side of the oven heater element. I'm sure that some of these models had to be UL approved, so I guess it's just the industry's dirty little secret. And you really can't get electrocuted touching a live heater element if it is dry and in good condition. But the insulation layer is constantly subject to high temperatures, so I still think it's a bad design for a consumer appliance. I'm glad that I got such a good response to the article here. Maybe things will change.
Cabe, Toolmaker - the mfg in this case was Amana, but I found schematics from others, such as Whirlpool, who do the same thing: only switch one side of the oven element. And I'm located in the USA, where UL approves such things, apparently.
A single line switch would not normally be a problem when the mains supply is 230V, at least not in the UK or in the country where I live. But in many EU countries they use the Schuko type plug, which is reversible, meaning you get the live mains on the right hand pin on Mondays, Thursdays, and Saturdays, and on the left pin any other day of the week you plug it in.
Most heating elements in an oven in the US are 220V, which means both hot sides are connected (110V each, opposite phase). If only one side is switched, the element is still live because it is connected to the other hot line. I'm surprised this design passed UL.
The answer is that, because of their frantic hyperactivity (which is interpreted by CEO's and MBA's as "productivity"; and their low demanded salaries, many workplaces at UL had been asigned to Monkeys.
Usually, appliances have a diagram/schematic on the back or on the inside of a panel. So, if you think flying sparks are fun then check out the electric dryer for a shocking good time.
I have worked on a few and was stunned. They use 220Vac as well. But, the motor is not 220V,...it is 110V! So, on three wires, where do they get the tap for 110V? GROUND!!! Thats right. They use the ground wire; the same one that is connected to the chassis. Turns out this is not true in all cases. Some dryers have the option for 4-wire plugs (which provides the neutral). But even they have the option to be wired for 3-wire plugs. My research showed that mobile homes requires 4-wire but a regular house does not.
Safety seems to be suffering in the name of profit; or is simply being ignored.
I addressed this topic in another similar blog last week, but will repeat it for information purposes.
You ARE correct about the 3-wire & 4-wire services to electric ranges & clothes dryers. Up until recently, the NEC allowed the connection of either an electric range OR a clothes dryer with 3-wire in the application of SINGLE FAMILY DWELLINGS only!
In the case of the clothes dryer either 10/2 ROMEX or BX w/GND were allowed, the GND conductor ALSO serving as the "neutral" for the power to controls, solenoids, motors. The NEMA codes for these devices is 10-30 (R) & (P), where the (R) & (P) stands for Receptacle & Plug, respectively.
And, it is true that in MOBILE HOME installations, the feeds MUST have been w/ 10-3 /w GND cable. The NEMA dodes for these devices is 14-30 (R) & (P).
The latest NEC code however has changed the requirement so that the frame of the appliance is connected to the GROUNDING conductor (bare, green or yellow/green), and the two lines & neutral (white) are connected to the terminal block provided on the rear of the appliance.
This change also affects single unit electric cook ranges, except the NEMA designations are 10-50 (R) & (P) for the old-style connections and,
14-50 (R) & (P) for the new code revision requirement.
One final note: Since both electric ranges AND clothers dryers are considered "permanent" appliances from the code's perspective, it is now & has always been that the feed cable may be directly connected to the appliance. However, some installers prefer the added convenience of using a cordset w/ a permanently installed receptacle device, either surface mounted OR recessed in an appropriate device box.
It seems that some of us have made assumptions that may be flawed. First, not all appliances have a wiring diagram included, but I sure do appreciate this when a manufacturer is nice enough to include one. It's not required! Second, we should not assume that the author is located in the US! This person has an Arabic (assumption) last name! Third, UL listing is not required everywhere, so there is no reason to assume that the oven passed UL testing! Last, we don't know where the manufacturer is located, so we should not assume that it should behave like most US manufacturers!
I work in industry and it is common practice on heater control circuits to switch only one of the hot legs. In most three phase heater applications only two out of the three phases are controlled leaving the third continuously hot. Most of these designs also include another control element in the line controlled by an over-temperature sensor as a safety in case the control element fails closed. The safety contactors are usually conventional relays as opposed to solid state contactors. The reason this is done is cost and I would assume that's the reason they've done it on the oven.
Excellent post Noor. I retired from the appliance industry before starting my company and remember the very same issues from earlier times. Now, UL requires "double line break" for 220 VAC heating elements and circuits. At one time, one leg remained "hot" even after an element or switch malfunctioned. One of the greatest problems with electric elements, bake and broil, is entrained moisture. Moisture enters and when heated, expands causing ruptures in the tubular element. The MgO acts as a "sponge" thus causing issue when water is present. In the newer ranges, DLB systems will solve this problem.
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