- Location
- Massachusetts
So post it here so we can all 'know' what you know.
- Status
So post it here so we can all 'know' what you know.
I agree that the controversy remains unresolved and it will likely continue to be so regardless of postings here. Sounds trollish
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
There is no controversy. You are simply mistaken in any belief that there is a safety risk in using an MWBC to serve kitchen countertop receptacles. It is no different at all from using two separate circuits to serve receptacles that are two feet away from each other.
Please address my earlier comments, by answering the following question: How can a person be shocked (either at 120V or 240V) by touching a kitchen appliance, unless something inside one or more appliances fails in such a way as to allow an energized wire to contact the external case? My answer is that it cannot happen, that it is a physical impossibility, and we don't use the NEC to safeguard against impossible situations. Please note also that if an appliance does experience an internal failure, then one of two things will be true. Either it will have a ground wire, which means it will trip the breaker and prevent injury, or it will not have any external metal parts (i.e., it is double-insulated), which means that the person cannot get shocked.
T
T.M.Haja Sahib
Guest
I made my statements based on 'Hand book of Electrical design by Mc Partlands'.If you have a copy of it,please go through its second chapter'lighting and appliance branch circuits.You will find support for my statements.
Why do you think that the insulation of a kitchen appliance can not become defective?
Here is a question to make you aware of how complex the situation really is.
Suppose the house wife is touching a metallic grounded kitchen appliance at the moment it is becoming alive due to insulation breakdown.Will she get a shock or not?
To T.M. and everybody else.
Much of the world including China (the most populous country in the world), Europe, Russia and previous USSR countries, Ozland, South Africa, etc. uses 220/230V single phase.
Shouldn't we be seeing droves of housewives all over the place suffering from electrocution in the kitchen on a routine basis?
Are we? Should we all be taking legal action against our utilities for supplying nasty, dangerous 240V??
This argument by TM is just plain daft. He must know that.
And I believe India, like other ex colonial states, has or had the same voltage as UK.
From Wikipedia:
- Location
- Massachusetts
Please post at least some of these statements if you wish us to comment on them.
Personally I don't think that, but to worry about a 240 volt shock two appliances would have to;
- Fail at the same time
- Be on opposite legs of the 240 volt service
- Both be in contact with the housewife.
The chances of the above are so remote it is not practical to try to keep it from happening.
Yes, 120 volts until the required GFCI protection opens the circuit.
I don't see the complexity here.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I don?t have a copy.
It can. If it does, then that is the time to throw the appliance in the trash, and buy a new one. That said, if a double-insulated kitchen appliance (e.g., toaster) does experience a failure in its insulation, and also experiences a failure of some internal part that allows the user to get a shock (please note that this is a double failure, and is therefore very unlikely), then the GFCI device into which the toaster is plugged will trip, and the user will not be injured.
She will get a shock. But then the breaker serving that circuit will trip, terminating the event. The breaker trip will happen so fast that the housewife will likely not feel the shock. Also, the trip will happen so fast that the shock will not cause any damage to her body.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
I'm sure it can- cracks, heat damage, etc. I have trouble with the concept that the insulation will start conducting enough to become a shock hazard. We also have "double-insulation", so that if one insulating element is compromised, the other will still protect. The likelihood of both becoming defective is quite small, and the goal is to minimize risk using acceptable resources and with acceptable conditions, not to completely eliminate it. The latter is usually either too costly or removes the needed functionality.
I find this whole argument drifting into abstruse arguments akin to "how many angels can dance on the point of a needle" (ref. Thomas Aquinas).
The same housewife has the same chance of being subject to a 240v shock if she is handling two appliances plugged into adjacent duplex receptacles if they are on opposite phases. The fact that the two phases are present in the same duplex receptacle rather than two adjacent receptacles doesn't change the scenario. The chance of adjacent receptacles in the kitchen being on opposite phases is quite high. The only way to reduce this possibility would be to require receptacles on opposite phases to be 12 feet or more apart. The code chooses instead, to require equipment grounding conductors as a way to reduce the chance of electric shock.
LISHAJI
Member
- Location
- Albany, NY
Does it really matter 240V or 120V. Imagine the internal resistance of the body between the two arms, about 200Ω, protected by the skin whose resistance may vary upto 2kΩ. But can the skin protect for ever? Skin breakdown may start occuring under 10 V AC 60 Hz due to electro-osmotic breakdown and its just a matter of time it becomes useless. A 250mA exposure for 5 s can cause Fibrillation threshold up to 99.5%. The voltage required for this is 250mA x 200 Ω = 50V
Be save, use GFCI as per NEC and respect any voltage above 48V or may be all voltages
Be save, use GFCI as per NEC and respect any voltage above 48V or may be all voltages
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maghazadeh
Senior Member
- Location
- Campbell CA
The Risk Evaluation Process
Michael Fontaine
Risk evaluation is the subject of F.4 of Informative Annex F of NFPA 70E-2012. Evaluating risk is really determining a risk priority number. The process of risk (risk priority number) evaluation begins with an initial risk assessment that is based upon the actual or planned conditions. If the level of risk (or the risk priority number) is unacceptable, additional or modified safety controls (protective measures) can be applied; then the impact of those safety controls on the probability of occurrence of harm (Po) -- and thereby on risk (risk priority number) -- is considered. The cumulative effect of safety control measures at each stage is used to estimate the residual risk (or final risk priority number) at a given stage of the evaluation process. The necplus article Risk Priority Number Assessment explains this process in more detail.
Michael Fontaine
Risk evaluation is the subject of F.4 of Informative Annex F of NFPA 70E-2012. Evaluating risk is really determining a risk priority number. The process of risk (risk priority number) evaluation begins with an initial risk assessment that is based upon the actual or planned conditions. If the level of risk (or the risk priority number) is unacceptable, additional or modified safety controls (protective measures) can be applied; then the impact of those safety controls on the probability of occurrence of harm (Po) -- and thereby on risk (risk priority number) -- is considered. The cumulative effect of safety control measures at each stage is used to estimate the residual risk (or final risk priority number) at a given stage of the evaluation process. The necplus article Risk Priority Number Assessment explains this process in more detail.
What exactly is your solution? Go to 230V L-G single phase? What if a housewife was touching a metallic appliance and the insulation failed with the 230V 50Hz supplied in other parts of the world? Wouldn't the available fault current be less with the 120V supply we have adopted?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
There are facts and there are opinions. It is a fact that the relevant Standard, NFPA 70 (NEC) accepts multiwire branch circuits; it is an opinion whether that is a good idea or not. It is also a fact that they are acceptable as the consensus opinion of the relevant Code Making Panels, the McPartland?s comments notwithstanding.
I confess I don?t have a copy, but as I recall, the McPartlands? latest Handbook of Practical Electrical Design was based on the 1999 NEC. I stopped using Joe?s material long before that. It became obvious he often ridiculed the NEC but did nothing to correct what he felt were its shortcomings. That is, to my knowledge he never once submitted a Proposal for anything ? and it?s very easy to do especially for a self proclaimed NEC expert. There?s been a lot of time since 1997-98 (when the 1999 edition of the NEC was developed). It may be unfair, but I?m afraid I?ve painted Brian with the same broad brush.
kwired
Electron manager
- Location
- NE Nebraska
Chances of sudden insulation failure of an appliance are not that likely without some catastrophic event either, like a big surge, physical abuse, or just plain wear and tear. Those two appliances that are both in reach of the housewife that just happened to have an insulation failure (at the same time on top of all that) probably had signs of damage and needed repaired or disposed of long before it got to the insulation failure situation.
This housewife was doomed to have bad luck no matter what for all of those events to be able to happen just right. She may be safer driving down the freeway in the wrong direction.
This housewife was doomed to have bad luck no matter what for all of those events to be able to happen just right. She may be safer driving down the freeway in the wrong direction.
T
T.M.Haja Sahib
Guest
I can't help it.But please be assured that my statements are replicas of those in that authoritative and well-received book.(I do not want to risk creativity in places like this.
)I am afraid you missed one important point regarding shock hazard.The severity of shock she would experience depends not only on the duration of exposure but also on its magnitude.Sure the time would be limited by the operation of OCPD.But how are you going to limit the magnitude of shock?
david
Senior Member
- Location
- Pennsylvania
210.6 Branch-Circuit Voltage Limitations.
The nominal voltage of branch circuits shall not exceed the values permitted by 210.6(A) through 210.6(E).
(A) Occupancy Limitation. In dwelling units and guest rooms or guest suites of hotels, motels, and similar occupancies, the voltage shall not exceed 120 volts, nominal, between conductors that supply the terminals of the following:
(1) Luminaires (lighting fixtures)
(2) Cord-and-plug-connected loads 1440 volt-amperes, nominal, or less or less than ? hp
The NEC did give consideration to the unique circumstances and the un- qualified individuals working close to electric equipment. It seems to balanced the potential risk of voltages with the use of appliances in a dwelling
The nominal voltage of branch circuits shall not exceed the values permitted by 210.6(A) through 210.6(E).
(A) Occupancy Limitation. In dwelling units and guest rooms or guest suites of hotels, motels, and similar occupancies, the voltage shall not exceed 120 volts, nominal, between conductors that supply the terminals of the following:
(1) Luminaires (lighting fixtures)
(2) Cord-and-plug-connected loads 1440 volt-amperes, nominal, or less or less than ? hp
The NEC did give consideration to the unique circumstances and the un- qualified individuals working close to electric equipment. It seems to balanced the potential risk of voltages with the use of appliances in a dwelling
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- Location
- Illinois
- Occupation
- retired electrician
If the person is touching the grounded metal part when the ungrounded conductor comes in contact with the metal part, the magnitude of the shock will be limited. The voltage to ground will equal the voltage drop on the EGC.
T
T.M.Haja Sahib
Guest
But still more can be done in the way of safety which may be still within the capability of an Engineer.Watch the discussion with Charlie as it develops,if you really enjoy it.
T
T.M.Haja Sahib
Guest
Whether it is safe or not,that is the question.:roll:
david
Senior Member
- Location
- Pennsylvania
Sure an engineer can design above the NEC standard. But when his design takes his project to far out of a reasonable cost projections whether it be material cost or labor cost, owners and other customers will look elsewhere for a more affordable design.
- Status