Simple eledtrical testing (C) Daniel Friedman Electrical System Inspect, Install, Repair

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Building electrical system installation, troubleshooting & repairs & electrical hazards.

Guide to articles describing residential & light commercial electrical systems: installation, inspection, diagnosis & repair.

How to install, inspect, & troubleshoot building electrical systems, appliances, components. How to detect & report electrical hazards, defects in residential and commercial electrical panels, switches, fixtures, electrical wiring & grounding systems. Proper electrical repair methods for unsafe electrical conditions.

Safety for the electrical inspector, aluminum electrical wiring hazards, how to determine service voltage and ampacity, how to inspect electrical panels, and significant electrical hazards of Federal Pacific Electric Stab-Lok breakers and panels, Federal Pioneer product warnings, certain Square-D product concerns, Zinsco and Sylvania circuit breakers and panels, multi-wire branch circuit protection, inspection and repair of low voltage wiring systems, and proper installation of lightning protection systems on buildings are addressed.

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Electrical Hazards in Buildings, Electrical Inspection Procedures, Electrical Repair Procedures

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Backup electrical generator hookup © D Friedman at Reader Question: Can I Hook Up Two Power Sources to One Electrical Panel?

Is it possible to connect two sources of Electricity to one Sub-Panel? - Muhammad K., Jordan

Reply: Use an isolation switch to avoid simultaneous (and unsafe) electrical power feeds to an electrical panel or sub panel - use backup electrical generator hookups as an example

By "possible to connect two sources of Electricity to one Sub-Panel" I presume you don't mean is it physically possible to connect two power sources to a single electrical panel since that would be trivial to accomplish; Rather I presume you mean is it acceptable practice or is it "safe" to connect multiple power sources into a single electrical panel or sub-panel.

The basic answer is no. In general it is very dangerous to have multiple sources of power into a single panel or subpanel because of the possibility of backfeeding and shocking someone who thinks power is off from a given source. So we don't hookup simultaneous live electrical power sources to a single electrical sub panel or main panel.

We do not hook up multiple electrical power sources to a single electrical sub panel without using an isolation switch.

In our photograph of an isolation switch hook-up (above left), the main electrical panel (1) is at left. In the photo center is an isolation switch (2) that allows the homeowner to switch individual circuits from being powered either by the main electrical panel's service entry mains or by an electrical generator (located outdoors) that is connected to the isolation switch by a removable plug shown hanging on the wall (3). You can see the receiving plug receptacle at the bottom center of the isolation switch (2).

The sub panel shown at right (next to our client) was an addition to the original electrical system to support central air conditioning and is not part of this discussion.

Watch out: Emergency backup electrical generators produce 120V and 240V which may cause fatal electrical shock if precautions are not followed. DO NOT under any circumstances connect your electrical generator to any circuit or receptacle that is receiving electrical power (home, office, etc) from any other sources as this is likely to result in a fire and damage all electrical systems and could also shock someone working on the system.

Our auxiliary electric generator photo at left shows a backup "home" generator that the owner had set up to keep a basement sump pump working. Some owners connect an extension cord to the generator's electrical supply receptacle and connect the other cord end to a tool or appliance - which may be safe. But using that same extension cord to "back-feed" a home electrical circuit without an isolation switch is unsafe and should not be done.

Other key safety warnings include the warning that the backup generator should not be operated indoors nor in an enclosed area - there are fire and also potentially fatal carbon monoxide hazard risks. Note that other safety precautions also apply - be sure to see the instructions provided with the generator.

See BACKUP ELECTRICAL GENERATORS for complete details.

Reader Question: History of Electrical Wiring in the U.S. - Split Receptacles or Electrical Outlets?

When were houses commonly built with split outlets/receptacles? I am doing research to find out how many houses in the US may not have them. Thanks! - Maria S.

Reply: A Nano-History of Electrical Wiring Devices in North America - Guessing: 1950 - 1965

You are referring to the practice of providing separate power from separate electrical circuits individually to the upper and lower receptacle openings of a duplex electrical receptacle.

That feature has been technically possible and therefore almost surely was done in some homes from around the time that duplex receptacles were first installed. (See Split Wired Receptacles under MULTI-WIRE CIRCUITS for details.)

The two-pin electrical receptacle was invented by Hubbell in 1904 as a device that screwed into light bulb sockets (electrical power for lighting was sold at a lower rate! - Wiki.)

Grounded electrical receptacles date to around 1915, though they were by no means in widespread use until much later.

Because sources (Wikipedia and others) note that the dominant way to plug in electrical appliances was by connecting them (using the screw-in adapter) to light bulb sockets into the 1920's (in North America) it is reasonable to argue that it was not until the mid or later 1920's that duplex electrical receptacles began appearing in homes.

Labre patented the grounded plug in 1928. Ten years later, twist-on locking electrical receptacle connectors date to Harvey Hubbell way back in 1938.

Polarized plugs (one blade wider than the other) were not introduced until 1948 and were not widespread before the 1950's. (The neutral wire is connected to the larger slot on the electrical receptacle)

Ring circuits (adopted only in the U.K.) first saw use in the U.K. around 1947 - provided the functional equivalent of our split wired receptacle approach and suggest the history of origin of the idea.

The sum of this history to date (subject to revision as our research continues) is that I'd place duplex receptacles in widespread use by 1935-1940 and it would be fair to assume (barring a code restriction yet to be found) that split wired receptacles, being physically possible, appeared in some uses as early as 1940.

IN sum it is more likely that split wired electrical receptacles were in use by the late 1940's, expanded during the post-Korean war boom, and began seeing widespread use in North America after 1965-1970.

That last OPINION derives from guessing at the onset of surge in use of multiple electrical appliances at once in home kitchens.


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