Which Residential Voltage & Frequency Arrangement Is Best?

Yes, this business of AC loads being more inductive than DC loads is a red herring. In a DC magnet, the current is limited only by the resistance, so typically coils of many, many turns are used to get the resistance and ampere-turns up and the current consumption down. The inductance is wickedly high, but the high series resistance keeps the time constants manageable. A moderately large DC machine may have field coil inductances of hundreds of henrys. The quick-break air switch causes all kinds of problems on DC circuits. The quick-break makes the switch smaller but that is the only benefit. Switches for large motor fields have auxiliary contacts to connect a resistor across the coil as the current is interrupted to prevent damage to the coil. Unless you have worked in an old steel mill that ran on 250 VDC you have no concept of how delightfully simple AC control is.

I was involved in the development of vacuum contactors for MV motor control applications. Our first generation vacuum bottles had pure tungsten contacts. We ruined motors left and right, because upon opening in a vacuum there was no arc and hence extremely high dI/dt. Later versions alloyed a small amount of other metals with the tungsten to provide a small but controlled arc to dissipate some of the stored energy in the bottle. We tried other techniques, like using a small amount of inert gas to provide an arc path but the special contact alloy was the most successful. To put this in perspective, the final design, which is used to this day, interrupts 5kV

360 amp loads with a contact separation of less than 1/2 inch. Vacuum contactors also have a specification called "chop current," which is the current level at which the intentionally developed arc extinguishes, typically in the neighborhood of 15 amps.
Reply to
BFoelsch
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It is always determined by the supply. Fault current is the voltage at the generator (the "voltage applied to it") divided by the total impedance from the generator to the fault. This includes all of the transmission and distribution impedances. You MUST know the available short-circuit current from the supply at a defined location, generally the transformer primary or secondary or the service connection, in order to determine the current at the fault. When you do a short-circuit analysis in a facility, you get that information from the utility.

Ben Miller

Reply to
Ben Miller

There isn't one.

In the US at least, the "AC only" switch rating, with its slow break properties, came about in the early 1950's as a side effect of customer demand for an audibly quieter switch. The AC only rating of the "quiet switch" was necessary to prevent their use on the DC systems which were still in use in downtown areas of some large cities. A further improvement was the "silent switch," which was a tilting mercury tube. If you look through issues of the Popular Mechanics type magazines of the era, a frequent article was on putting the new "quiet" switches in the baby's room.

The slow break of the AC only switch exists because it can be tolerated, not because it has any redeeming qualities.

Reply to
BFoelsch

only true if your fault has zero impedance. IRL they normally dont, thus IRL the current through a fault is usually determined by the fault. Possibly a terminology issue.

NT

Reply to
meow2222

Yes. The issue is that a carbonised plastic fault is much more likely to develop into a high current arc on 240 than 110, which trips the overcurrent protection. On 110, there is more chance of just resistance heating, which is much less likely to trip anythng, and thus more likely to start a fire.

NT

Reply to
meow2222

YAHOO! You have hit it right on the head!

From other comments you have made, it appears that you are quite knowledgeable in the area of circuit breakers.

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Don Kelly @shawcross.ca remove the X to answer

Reply to
Don Kelly

It is always true for any fault impedance. If the supply impedance is much lower than the fault impedance, the current will primarily depend on the fault, but you can't make a general statement about this. The type of fault (bolted, arc, etc), location within the electrical system, internal wiring details, and supply system characteristics are all factors.

Perhaps. Can you describe what you are talking about?

Ben Miller

Reply to
Ben Miller

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