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
I don't know if anyone else mentioned manually operated
double pole double throw safety switches. I've installed
several of them over the years as an economical way to
transfer power from the power company feed to a generator.
Does a transfer switch switch the neutral or just the two hots? In the
commercail 3 phase units we uses at work the nuetral is switched. I
was wondering if this was true for residential units also.
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.
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
(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
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