The switches I've installed in homes have only switched the two hot wires even the automatic variety. I don't remember if the Onan three phase units I installed switched the neutral. Even the Kohler 40kw unit I installed didn't switch the neutral. I've installed Generac generators in 8 to 20kw sizes and none of them switched the neutral.
TDD
I have the Square D interlock on my panel, and it was not $150. I seem to recall it cost about $35 when I ordered it from my electrical supply place.
It is by far the least expensive transfer device available, particularly considering it can handle up to a 125A / 30KW generator connection (125A max branch circuit breaker size, ~$90 breaker).
Ho! I didn't check the price at an electrical supply house; I just looked at the SRP on the Square-D web site and what the things were being offered for on Ebay.
Thanks for the tip.
No it is not a transfer switch. In that much you are correct. What it is a part of a transfer switch assembly consisting of the panels main breaker, a breaker for the generator installed in a particular set of slots, a tie down for the generator breaker to keep it from being removed or dislodged from the buss bar accidentally, and an interlock mechanism that assures that the two breakers cannot be closed at the same time. Unless it is tampered with it is just as effective as a stand alone transfer switch in preventing the cross connection of the generator and utility power. So no there is no likelihood of an explosion or other difficulty when the utility power is restored.
-- Tom Horne
On Jun 22, 10:05=A0pm, JIMMIE wrote:
Jimmie The answer lies in the level of training and expertize of the project manager and the electrician. On small jobs like most homes these are one and the same person. Since a three pole transfer switch would cost more then a two pole switch many project managers will order the two pole switch without further thought. What also needs to be ordered and installed is the kit that is available from the generator manufacturer that will allow the installing electrician to wire the generators neutral connection completely insulated from ground. Those kits are available for all generators that are intended to be permanently installed. If the transfer system is to be supplied with generator power from a portable or mobile generator then the transfer switch should transfer the neutral unless it is integrated into the Service Equipment enclosure. Even were the transfer mechanism is integrated into the Service Disconnecting Means (SDM) enclosure it is a bad idea to leave the portable set plugged in when operating from utility power. Some inlet boxes, installed on some military installations, are equipped with an automatic cord ejector that is on whenever public power is present. The source of the concern is that the generators main bonding jumper is a cross connect between neutral and ground that will exist on the load side of the SDM under utility operation. This can cause a lot of stray current to flow on Equipment Grounding Conductors (EGCs) when operating from the utility. It can also result in arcing and heating in the event of any failure of the neutral conductor on the line side of the transfer connections. That is because EGCs are only sized to carry momentary fault currents sufficient to allow the Over Current Protective Device (OCPD) protecting the circuit to open rapidly enough to promptly deprive the fault of current. The EGCs are not sized to carry normal operating current back to the Main Bonding Jumper (MBJ) and via the MBJ to the utility's neutral conductor and back to the transformer winding. That is why the code specifically forbids any connection between the EGCs and the Grounded Current Carrying Conductor on the load side of the SDM. Viz "250.24 Grounding Service-Supplied Alternating-Current Systems. (A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5). (5) Load-Side Grounding Connections. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means except as otherwise permitted in this article."
-- Tom Horne
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