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| The US electrical system |
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| AlbertL:
--- Quote from: bdunham7 on June 27, 2020, 04:47:27 am --- --- Quote from: cliffyk on June 27, 2020, 03:14:22 am --- If you draw 200 A from 120 V phase "A", and 100 A from phase "B", the "neutral" wire will be carrying 100 A--enough to be a BIG surprise for anyone believing it to be "neutral", and mucking about with it while the full phase load is unbalanced. --- End quote --- That is exactly what a neutral is supposed to do. What big surprise would the neutral have? 3 volts? --- End quote --- It depends on the nature of the mucking. In a properly installed, intact system, touching the neutral and ground (earth) shouldn't yield any great surprise. But getting in series with a neutral, or between an open neutral and ground, could expose one to essentially the full line voltage. And opening a current-carrying neutral could produce a significant arc. This leads into another interesting aspect of US distribution: the multi-grounded neutral. The utility substation typically supplies three-phase distribution feeders from the wye-connected secondary of a step-down transformer. The neutral of the wye is grounded at the substation, travels along with phase conductors, and is grounded at intervals along the line. Transformers along the feeder make the final step down to customer delivery voltage, which is typically 120/240V single-phase (i.e. a center-tapped 240 volt secondary) or three-phase 120/208V or 277/480V wye. The secondary neutrals of these transformers are connected to the feeder neutral, and grounded at the transformer location. Finally, the neutral is carried to the customer's main service equipment, where it is again grounded, often at more than one location on the premises. The two key points are (1) the neutral is common to both sides of the distribution transformers, and (2) the neutral is grounded (earthed) at multiple locations, some of them widely separated. |
| Monkeh:
--- Quote from: AlbertL on June 29, 2020, 01:34:06 am ---The two key points are (1) the neutral is common to both sides of the distribution transformers --- End quote --- This seems unwise. Also unlikely. HV and LV earths are normally kept separate for good reason. --- Quote ---(2) the neutral is grounded (earthed) at multiple locations, some of them widely separated. --- End quote --- And this is nothing unusual. |
| AlbertL:
--- Quote from: Monkeh on June 29, 2020, 01:46:30 am --- --- Quote from: AlbertL on June 29, 2020, 01:34:06 am ---The two key points are (1) the neutral is common to both sides of the distribution transformers --- End quote --- This seems unwise. Also unlikely. HV and LV earths are normally kept separate for good reason. --- Quote ---(2) the neutral is grounded (earthed) at multiple locations, some of them widely separated. --- End quote --- And this is nothing unusual. --- End quote --- Using the same wire as the common grounded neutral for both sides of the transformer is standard US practice. This brochure shows typical pole-mounted distribution transformers: https://www.eaton.com/content/dam/eaton/products/medium-voltage-power-distribution-control-systems/cooper-power-series-transformers/single-phase-overhead-transformers-catalog-ca201001en.pdf. Note the ground strap connecting the center tap of the secondary to the transformer tank. And note that some of the transformers have just one primary terminal - in these models, the other end of the primary winding is internally connected to the tank, which has an external lug for connection of the ground wire. The models with two primary terminals are intended for delta connections, but can be used in wye by grounding one of the terminals. My neighborhood in fact has a mix of both types on its wye-connected feeder. I think one reason for the common neutral is to limit the voltage rise on the secondary side if the primary and secondary circuits accidentally become connected; for example by a fallen wire. The multiple ground points have the benefit of redundancy and lowered resistance, but can also hide the failure of one or more grounds, and cause undesirable "stray" ground-current paths. |
| Monkeh:
--- Quote from: AlbertL on June 29, 2020, 03:13:15 am ---Using the same wire as the common grounded neutral for both sides of the transformer is standard US practice. --- End quote --- Well, that's.. nice. We don't like bringing the primary into people's houses in the event of an earthing fault, so.. we don't do that. To quote one technical spec on the subject: --- Quote ---i. All pole-mounted substations shall be designed with separate HV and LV Earthing Systems and shall be separated by at least 20 m. ii. The HV Earth Electrode earth resistance shall not exceed 20 Ω in order to provide reliable protection operation. (Where surge arresters are installed, the HV Earth Electrode earth resistance shall not exceed 10 Ω). iii. The LV Earth Electrode earth resistance shall not exceed 20 Ω to comply with ENA Engineering Recommendation EART-01-002. iv. The HV Earth Electrode and Earthing Conductors shall be of sufficient size and surface area to safely carry fault current at that site (see sections 14.1 and 14.4). v. EPR on pole-mounted steelwork can approach system phase-to-earth voltage, which in some situations might be close to 6.33kV. All LV equipment shall be suitably insulated and separated from HV equipment to prevent flashover during HV fault conditions. --- End quote --- That said, subject to certain conditions, ground-mounted substations may have an HV earth only (which is then shared). They're a little less damage prone, for a start.. |
| tooki:
--- Quote from: tom66 on June 28, 2020, 10:23:44 am ---Are you sure? The EU standards (Annex IV): https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32012R0932 state that the 'standard' of each dryer should consume (@8kg capacity): Vented - 4.1 kWh/cycle Condenser - 2.30 kWh/cycle The ratings get better at lower capacities, with the energy consumption of a 5kg vented dryer and 5kg condenser dryer only differing by about 20% or so, although the condenser still wins. I'd be curious why there isn't a linear relationship, it might relate to the extra energy required by the condenser system, pushing the hot air through the internal condensation system might consume extra energy and give more opportunity for heat loss inside the machine perhaps. The energy consumption of the tumble motor shouldn't be that high (100W or so.) I have not seen a heatpump dryer for under £400 (~$500USD) here but then again, my current tumble dryer was a road-side find (it had a blown fuse...) I suspect that it may be more expensive to buy these types of dryers here as regulations for refrigerants may be more strict. I would be surprised if propane is an acceptable refrigerant in such a device, which is common in cheap A/C units and refrigerators here. There's probably the other factor that as you mention, condenser dryers are preferred as there is limited ventilation requirement, with no exhaust to atmosphere, especially important for flat dwellers. --- End quote --- Well, I was sure, since until this thread, I’d never heard any claims of non-heat-pump condenser dryers being more efficient than vented. When I googled just now, the first result that had any statements about energy consumption agreed with me, hence why I posted it. As for refrigerants, Switzerland is at least as strict as the EU, if not more. (Switzerland has arranged for itself the cushy position of enjoying most EU benefits while being subject to comparatively few of the responsibilities.) I know Switzerland follows EU appliance energy efficiency rules 1:1. I have no idea what’s in mine, but you have me curious now. I’ll see if I can find out. (Surprisingly to many outsiders, the US is also really strict on a lot of air quality stuff. In some areas, stricter than the EU, like vehicle exhaust. Unless the guy squatting in the White House gets his way, in which case we can soon expect 70s era smog there again.) |
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