Come to think of it, if it is the RV type it could indeed switch the
neutral by using the contactors together and utilizing the NC and NO
sets of contacts with no provision for an "off" state. NC to connect to
one source until the other source is available, cycles the time delay
relay and then energizes both contactors. Of course this type of setup
has the potential for a nasty failure mode if one contactor fails.
What happens if a coil burns out on one of the contactors? One side
switches and the other does not, you could end up with a situation where
you have lost your neutral, i.e. hots from one source and neutral from
the other source. Could be ugly.
On the transfer switch I built I had a solid neutral, but only used the
NO main contacts on the contactors. If a contactor coil were to fail I
would not have a situation where I lost a neutral or a phase. The NC
auxiliary contact blocks on the contactors were used to provide the
interlock to prevent any possibility that both contactors could be
energized at the same time. It would require two failures for that to
occur, both the upstream control to try to energize both contactors and
the failure of one of the aux contacts to allow it to actually happen.
If the coil fails, most contactors will be unenergized and that makes
the contacts OPEN in most cases. This is standard for MOST electrical
systems design. You design the system so that if therer is a failure,
It is in the unenergized way and all contactors are OPEN.....
Right and that was my point and also the way I designed my electrically
interlocked transfer switch using only the NO contacts on the contactors
and one contactor per source.
In the RV style auto transfer switches I've seen that use DPDT relays,
they use a single DPDT relay and switch either a 120V source with
neutral, or a 240V source with a solid neutral. A relay coil failure
will leave you stuck on one source but will not create a hazardous
The transfer switch as described by Steve appears to be using two DPDT
relays to emulate a 4PDT relay with one source on the NO contacts and
the other on the NC contacts, the common feeding the load. With one
relay you are safe, but with two relays a failure of one will put you in
a half switched state which could be hazardous.
Steve stated that the relay on the left was switching the neutral and
the relay on the right was switching the two hot legs of the 240V feeds.
If one of these relays were to fail you would get the two hot legs from
one source and the neutral for the other source which could certainly
cause significant problems.
My recommendation is that Steve review carefully the "what if" scenarios
for the cases of the failure of either relay. I think a safer route
would be to locate a suitable 3PDT or 4PDT contactor to replace the two
relays or to go with a solid neutral.
The short answer is if the generator has the neutral bonded to the
frame (equipment ground) the transfer equipment will have to switch
the neutral. You can't have 2 bonding jumpers in one system, switching
the neutral isolates one of them.
Look at "separately derived systems" in the NEC art 250
Yes, but that's not relevant to the apparent situation. Essentially he
has portions of a single circuit switched by what is effectively two
separate transfer switches that are supposed to operate in parallel
(neutral on one transfer switch and two hots on another transfer
That is right. When you switch the neutral it has to also switch all
ungrounded poles simultaniously.
I have to admit this thread has wandered around so much I am not sure
I responded to the right post. I was just referring to the idea that
transfer equipment does/does not switch the neutral and when that was
Many areas operate with primary voltages higher than 7200 volts.
It's the same current that it takes to kill you at 120/240 volts. The
difference in not in the current requirement, it is in the amount of
insulation required to protect you.
Your limited knowledge is forming incorrect assertions.
The amount of current flowing across the heart required to kill you is
small and a constant. The voltage necessary to achieve this current
varies depending on the contact conditions. Once you exceed the voltage
necessary to produce that current across the resistance of the human
body over the distance between the contact points, you're toast.
On Thursday, October 27, 2005 11:49:15 AM UTC-5, email@example.com wrote:
doing it (I have a new generator and new uninstalled transfer switch) so sa
ve your flames. I plan on installing my transfer switch when I get some tim
e in the next month or two.I read a post suggesting that in a power outage,
you could flip your main breaker off to prevent anything going to the line
s and killing a line worker, and use a suicide cord from your generator to
you clothes dryer outlet (240 vac) and then your main panel would serve as
your switch for what circuits are using the generator--and that this would
power both 240 and 120 outlets.As dangerous as this is, is this even possib
Yes, it can and has been done (by me) in an emergency. Whether you feed th
e 240V back thru the dryer cord outlet or directly tap in at the breaker bo
x, either way you must open the main circuit breaker to disconnect your hou
se from the mains to avoid electrocuting a worker, or heavily overloading y
our generator trying to feed your entire neighborhood.
When finished my wiring will have a breaker in the panel for just this
purpose . Out here in the woods we can be left without power for days if we
get another bad ice storm . Also part of the reason we heat with wood ...
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