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maxrexx
09-12-2005, 10:42 AM
http://www.straightdope.com/classics/a2_389.html

For sure the UL guy is way off base. In order to use holes to improve cooling, the holes must be configured in a way that increases surface area. The formula for this is that the diameter of the hole must be less than twice the thickness of the material. If it is not, a hole will actually make cooling worse, not better. for plug prongs, this would result in quite small holes.

lightingtool
09-12-2005, 11:07 AM
I always thought the holes were there to aid people who are trying to test the cable, or the appliance the plug is attached to. If you are using a meter with probes, you can put one probe through the hole and let go; the probe will hang on the hole and stay in contact with the prong leaving both hands free to position the other probe.

What Exit?
09-12-2005, 11:09 AM
Simple logic tells me that the GE guy is nuts. Prongs of electric plugs used in other countries, with sometimes higher voltages (e.g., 220 volts in Germany), do not have holes. Hope I have made your day. --Sibylle A., Tempe, Arizona

I'm surprise Cecil didn't call her on this.
Higher voltage on an appliance = lower amperage = less heat.

tim314
09-12-2005, 12:06 PM
I'm surprise Cecil didn't call her on this.
Higher voltage on an appliance = lower amperage = less heat.
Um, what? For an Ohmic device, V = IR. If V goes up and R is fixed, I must also go up.

09-12-2005, 12:16 PM
Um, what? For an Ohmic device, V = IR. If V goes up and R is fixed, I must also go up.
He's talking about a case where the overall power is held constant. Then I (generally) decreases as V increases.

What Exit?
09-12-2005, 12:28 PM
Thank you Una.
This is typically why much transmission is high voltage is you can send the same wattage at lower energy loss due to high amperage heat generation.

This is also why Dryers & Ovens are typically 220V and not 110V.

Overall 220V is more effiecient than 110V.

tim314
09-12-2005, 01:11 PM
He's talking about a case where the overall power is held constant. Then I (generally) decreases as V increases.Ah, that makes sense. :cool:

illflux
09-12-2005, 01:33 PM
I always thought the holes in the prongs were there to keep the plug secure in the recepticle. Inside the recepticle slots, there is a tensioned tab of metal with a little dimple in it that each prong slides past when it is inserted. The dimple rests inside the hole in the prong, and since there's a little tension being applied to it, it keeps the plug from falling out of the wall.

Can't believe it took 20 years to get the right answer.

Uncommon Sense
09-12-2005, 02:13 PM

The holes once were quite important though. Decades ago, the contacts inside electrical outlets contained small bumps that slipped into the prong holes and kept them from slipping out of the wall.

Today's plugs fit more securely into outlets, and the bumps have been abandoned. Most prongs still are made with holes, though, because they still have a place in the manufacturing process; they're used to grasp and manipulate the plugs as they move down the assembly line.

Besides that, say the General Electric engineers, people are used to seeing them.

Some of the newest plugs are being made without the holes, however, a change that won't be welcome in all quarters. Some people, for instance, parents trying to wean their kids off of television, use the holes to affix a tiny padlock, which prevents youngsters from plugging in the set and watching without permission.

Uncommon Sense
09-12-2005, 02:17 PM
Oh, and some outlets have locking devices within the outlet that can 'grab' the holes and prevent the cord from being pulled out.
From here. (http://www.everything2.com/index.pl?node=Why%20electrical%20cords%20have%20holes%20in%20the%20prongs)

09-12-2005, 02:41 PM
The holes also reduce the amount of material needed to make the prongs by roughly 10%

If you're stamping out a million plugs a week, 10% less brass is going to add up to a tidy amount of money saved.

As for higher-voltage items not having holes in their plugs, I've got two that do have holes. My (recently replaced with a gas version) electric dryer runs on 240 volts at 30 amps, and has a hole in one of the blades. The twist-lock plug for my generator (also 30 amps at 240) has a hole as well. The twist-lock means that there's no need for tension-increasing dimples - the hole is purely there for lockout purposes. In both cases (I'm going by memory) the neutral blade is the one with the hole. In a 240-volt device, the neutral wire/blade/prong is usually carrying less current overall, so less material there shouldn't have an adverse effect.

Chronos
09-12-2005, 04:52 PM
In a 240-volt device, the neutral wire/blade/prong is usually carrying less current overall, so less material there shouldn't have an adverse effect.Just to be clear, this is some sort of three phase system, with two hots and a neutral?

Dancing Fool
09-12-2005, 06:10 PM
In Australia and New Zealand, where all standard appliances are 240V and most are three pronged (Live, neutral and ground), holes are a definate exception to the rule. I think I've seen one or two. But in Pakistan, where I lived a few (cough) years ago, they had the most messed up collection of voltages, plug shapes, and so on I've ever seen. Some of the plugs had a locking device inside, and if it couldn't lock into the holes, no current would flow.

09-13-2005, 10:30 AM
Just to be clear, this is some sort of three phase system, with two hots and a neutral?
Nope. Plain old North American household power. In the case of my electric dryer, if the 240-volt heating element draws 20 amps and the 120-volt motor draws one amp, there will be a one-amp current flow in the neutral as one of the 120-volt legs that comprises 240 will be carrying 20 amps and the other will be carrying 21 amps.

The heating element can actually be ignored in this particular dryer as it has no connection to neutral at all. The only thing connected to neutral is the motor that spins the drum and the timer. Fancier dryers may have a 120-120 split element to allow lower-heat drying.

MrAlpen
09-13-2005, 11:40 AM
Nope. Plain old North American household power. In the case of my electric dryer, if the 240-volt heating element draws 20 amps and the 120-volt motor draws one amp, there will be a one-amp current flow in the neutral as one of the 120-volt legs that comprises 240 will be carrying 20 amps and the other will be carrying 21 amps.

The heating element can actually be ignored in this particular dryer as it has no connection to neutral at all. The only thing connected to neutral is the motor that spins the drum and the timer. Fancier dryers may have a 120-120 split element to allow lower-heat drying.

Surely this isn't correct - unless I am missing something more complex. I would expect that, at any instant (and neglecting minor charge build up effects) the current flowing at the live pin is exactly equal and opposite to the current in the neutral pin. There should be no current flowing in the earth pin.

This is correct, isn't it? Or do you have a (current) 'eater rather than a heater?

Fear Itself
09-13-2005, 12:45 PM
The holes also reduce the amount of material needed to make the prongs by roughly 10%

If you're stamping out a million plugs a week, 10% less brass is going to add up to a tidy amount of money saved.Very few brass plugs being made today, so the savings would be small and almost certainly eaten up in reprocessing the scrap for reuse. The dots punched out of low grade steel have no reclaimable value, IMHO; probably for brass as well.

What Exit?
09-13-2005, 01:15 PM
Very few brass plugs being made today, so the savings would be small and almost certainly eaten up in reprocessing the scrap for reuse. The dots punched out of low grade steel have no reclaimable value, IMHO; probably for brass as well.

I work for a Molding & Stamping Factory. We still make all of our plugs with one blade Brass. The holes are hardly 10% however. I will ask the stamping Engineers their opinions on the holes, but I am afraid the blades in question have not been changed in at least 25 years and no engineer has been here that long.

Chronos
09-13-2005, 01:16 PM
I was incorrect to use the term "three phase", but this is apparently a system with two hots and a neutral. It is not the typical household plug with two parallel flat prongs and a round ground pin. There are two hot pins at 120 volts, out of phase with each other, and a neutral pin. Some parts of the appliance (the heating element, apparently) run on 240 volts, and get their current up one hot leg and down the other. Other parts (the motor) run on 120, and take current up one leg and down the neutral. Total current in is the same as total current out.

Uncommon Sense
09-13-2005, 01:17 PM
On a single phase circuit, which is what the 120 volt motor is essentially on, the load on the nuetral will be whatever the current draw of the device is. So, in this case the 1 Amp motor will have 1 Amp on the hot and one Amp on the nuetral. If two single phase motors are sharing the same nuetral and the motors are on different legs of the service then you subtract the two loads from each other. A 5 Amp motor and a 3 Amp motor sharing the same nuetral will produce 2 Amps on the nuetral (assuming your service legs are 180 degrees out of phase with each other).
In the case above you have the two heating elements canceling each other out (plus they aren't referencing ground in this case) and the only load left is the motor load of 1 Amp. This 1 Amp will be on the nuetral conductor as well.

Fear Itself
09-13-2005, 01:22 PM
I work for a Molding & Stamping Factory. We still make all of our plugs with one blade Brass. The holes are hardly 10% however.
Interesting. What do you do with the scrap brass that is punched out of the holes?

What Exit?
09-13-2005, 01:33 PM
Interesting. What do you do with the scrap brass that is punched out of the holes?

All Brass scrap from everything we stamp is recycled and sold back to the Metal dealer. We get giant rolls of Brass Sheet metal cut anywhere from 1" to 8" wide.
The rolls are maybe 5' across.

According to our resident long term stamping Engineer; the holes are used by the stamp press to align the stamping of the blades. These are large, very noisy automated machines.
So to answer the original question: The holes are part of the stamping operation itself and have no special electrical or mechanical design.

(I work in the office as I am a Programmer/Analyst and we share the same slab as the Stamping floor. When some of the stamp machines are going the whole building shakes)

What Exit?
09-13-2005, 01:39 PM
Function may be more appropriate then design.