Help me understand grounding

mbrooke

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United States
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Why do metal frame small appliances like toaster ovens, radiators and mixer come with a 2 prong plug? Doesn't code require an EGC for all none double insulted equipment? What about UL?
 

paulengr

Senior Member
Why do metal frame small appliances like toaster ovens, radiators and mixer come with a 2 prong plug? Doesn't code require an EGC for all none double insulted equipment? What about UL?
There are two approaches to shock protection. One is enclose everything in a grounded conductive enclosure. Thus faults are contained and hopefully the grounding has a much lower resistance.

The second approach is purely using insulation to reduce the current to a nonhazardous level even if contact is made. Doubly insulated just means it’s twice as thick as Code requires in which case grounding is not required.

Of the two doubly insulated systems are obviously safer because they do not rely on external bonding, grounding, and breakers that may or may not be adequate. So less likely to get consumer law suits over faulty grounding that is not the manufacturers fault but might be impossible to prove otherwise;

So Q1: metal frame yes but doubly insulated internally. The paneling is insulated from the conductors internally. Q2: no that’s why it’s doubly insulated. Q3: See UL 1097, Double Insulation Systems for Use in Electrical Equipment.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
From distant memory, I believe that toasters are an exception to the general rule. The thought is that it would not be uncommon for someone to try sticking a fork into the toaster to get something out, potentially contacting the exposed heating wires, and that in this specific case the grounded chassis would create more of a risk than it would improve safety.

Like I said, distant memory.

-Jon
 

mbrooke

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There are two approaches to shock protection. One is enclose everything in a grounded conductive enclosure. Thus faults are contained and hopefully the grounding has a much lower resistance.

The second approach is purely using insulation to reduce the current to a nonhazardous level even if contact is made. Doubly insulated just means it’s twice as thick as Code requires in which case grounding is not required.

Of the two doubly insulated systems are obviously safer because they do not rely on external bonding, grounding, and breakers that may or may not be adequate. So less likely to get consumer law suits over faulty grounding that is not the manufacturers fault but might be impossible to prove otherwise;

So Q1: metal frame yes but doubly insulated internally. The paneling is insulated from the conductors internally. Q2: no that’s why it’s doubly insulated. Q3: See UL 1097, Double Insulation Systems for Use in Electrical Equipment.
I've taken both to bits, and I'm not seeing the added insulation to be honest.
 

ActionDave

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Durango, CO, 10 h 20 min without traffic from wing
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wire pulling grunt
If you think about it hard enough you could make a case that bringing an equipment ground into a house might be the most dangerous thing you could do..

I'm in my living room in a wood frame house built over a crawl space. Go ahead and energize every metal thing in there.... as long as everything is plugged into good ole' fashioned two pronged recpts I'm never going to get shocked until some do gooder comes in and properly wires up a three prong recpt
 

Hv&Lv

Senior Member
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-
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Engineer/Technician
Here is a perfect example, two wire cord, no insulation:

That isn’t double insulated.
double insulated simply means that one failure doesn’t result in a fault that would cause harm.
if the inside of that box was coated in plastic it would meet the guidelines.
in the case of your picture, if the wire burns off the terminal, it can touch the case, so not double insulated.

IMO
 

mbrooke

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If you think about it hard enough you could make a case that bringing an equipment ground into a house might be the most dangerous thing you could do..

I'm in my living room in a wood frame house built over a crawl space. Go ahead and energize every metal thing in there.... as long as everything is plugged into good ole' fashioned two pronged recpts I'm never going to get shocked until some do gooder comes in and properly wires up a three prong recpt
Well, if that were the reasoning behind it I think we wouldn't have grounding or bonding to begin with. I mean isn't the whole purpose of bonding to eliminate remote earth in a building and bring everything to a closer electrical potential when the MGN rises in voltage?
 

mbrooke

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If you think about it hard enough you could make a case that bringing an equipment ground into a house might be the most dangerous thing you could do..

I'm in my living room in a wood frame house built over a crawl space. Go ahead and energize every metal thing in there.... as long as everything is plugged into good ole' fashioned two pronged recpts I'm never going to get shocked until some do gooder comes in and properly wires up a three prong recpt
You bring up a good point actually. However to address your concern: if the voltage ends up being high enough to harm a person, or high enough to puncture 600 volt insulation with a proper grounding and bonding system within the building in place, MV and LV can be permanently separated (y) Or the POCO neutral can isolated from the building's grounding system though this is not permitted in NFPA 70.





However the risk (Id think) is mitigated sufficiently (at least outside the NESC) such that a greater risk is present in not having an EGC IMHO.

Remember that the code requires large appliances have an EGC regardless of the flooring in place.
 

mbrooke

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That isn’t double insulated.
double insulated simply means that one failure doesn’t result in a fault that would cause harm.
if the inside of that box was coated in plastic it would meet the guidelines.
in the case of your picture, if the wire burns off the terminal, it can touch the case, so not double insulated.

IMO

Thats what I'm thinking. 2 prongs, yet nothing stops a burn terminal from falling onto the metal frame. About 2/3 of all toaster ovens are like this.
 

paulengr

Senior Member
A lot of power tools are double insulated. 15 mils is the minimum at 15 A (#14). 30 mils (XHHW thickness) is all they do.
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
Why do metal frame small appliances like toaster ovens, radiators and mixer come with a 2 prong plug? Doesn't code require an EGC for all none double insulted equipment? What about UL?
The code rules do not apply to the construction of appliances. As you suggest, you would have to look to the product standards.
 
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