transfer circuit

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Jon 16 years ago

view in courier

SERVICE

LINE1 LINE2 120v 120v o o | | | | *-------(A)-------* | | | | * * \ DISCONNECT \ \ \ * * | | | | | | | | | | o o

LOAD

o o | | | | --- C --- C --- --- | | | | | | | *---------*---------* | | | | | | | | | | --- A --- B | | --- -/- | | | | | | | | | | (B) (C) | | | | | | | | | | --- --- | | - - | | | | | | | | | | o GENERATOR o

A is a mercury relay with a 240vac coil. B is a N.C. 120vac coil with 30amp contact. C is a dpst 120vac coil with 90amp contacts

When the disconnect is ON and the service is either ON or OFF,

the generator is isolated from service feed
the generator cannot backfeed into the grid

When the disconnect is OFF and the service is ON

the generator isolates from the load

When the disconnect is OFF and the service is OFF

the generator is connected to the load

The generator can't work or be damaged unless the disconnect is OFF.

I designed this circuit around some parts I had. The disconnect is in the breaker box.

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IanM 16 years ago

*WRONG* Consider the case where you start with the service off and generator off. (A and C contacts open, B contacts closed) Start the generator. C closes, and you back feed the grid, probably overloading the generator. A now closes IF you reach its pull-in voltage, opening B which in turn releases C. This removes the voltage from A's coil (and stops back-feeding the grid), which in turn allows B to close. The result is buzzing relays and C's contacts arcing till they weld closed.

If you mean you have already installed this abomination, you are culpably negligent. Leaving aside all legal issues and compliance with your local electrical code, using anything except MECHANICALLY INTERLOCKED changeover or paired N/O and N/C contacts for this with SEPARATE isolators for the service and generator feeds is *CRAZY*. The changeover contactor or switch *MUST* have far greater contact separation than a normal relay ever has and it *MUST* be incapable of switching if one contact is welded. Multiple devices electrically interlocked cannot be made to fail safe so cannot be used.

If you really want to cheapskate this, not to code, install a wall mounted socket after the service disconnect and a suitable breaker and add a sub-panel fed from a wall mounted plug. Use a plug to socket fly lead to either connect the sub-panel to the service or the generator.

Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED) ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:

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IanM 16 years ago

The cycle then repeats.

....

that are not designed to be used with an electrical interlock

If there are suitable auxiliary contacts it may be possible to use an electrical interlock, but it must fail safe for any single or double fault (e.g. O/C contact, welded contact, O/C or shorted to ground coil circuit, jammed armature, broken return spring etc.) and indicate that a faulty condition exists. Its *MUCH* simpler to prove that a Mechanical interlock will fail safe.

Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED) ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:

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Michael A. Terrell 16 years ago

A single set of contacts is illegal in the US. You need interlocked contacts with the load connected to t he center points to meet the NEC. This guy has been trolling his illegal crap for months.

Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.

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transfer circuit

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