They are actually quite large, the photo does not do them justice.
My plan is to place interlocks so that either the generator's
disconnect is closed, or the main panel breaker is closed, but never
That's what they were designed for, if it is unclear to anyone, I can
explain it in more detail, or see:
I would lock up another key (they are keyed alike) in my safe so
that its use cannot defeat this system.
I believe that this arrangement fully satisfies the rule that a
mechanical interlock device should prevent both sources of power from
coming in contact.
I don't think your local inspector is likely to approve of such a setup.
You would be better off with an approved interlock system produced by
the manufacturer of the electrical panel such as the Square D setup I
AT&T had this arrangement on a battery plant that was in two cabinets.
depended on one key that had to turned to release the key so it could be
used in the
So some idiot had a spare key made so he didn't have to move the key.
Some time later
he disconnected both battery strings and failed the service.
Not much is idiot proof
Currently approved mechanical interlock devices physically do not permit
both breakers to be on.
With this, a second key already exists, and if it did not, a hardware
store could make one - nothing to prevent both from being turned on at
once, other than the assumption that there's only one key. A key in a
safe can be removed from a safe, a key in place can be copied. Your
taste for weird old surplus has got the better of you - there's a reason
this stuff is surplus.
Mark, thanks. To answer other people's concerns, I am not going to
duplicate the keys. I think that to prevent use of the second key, I
may simply put both keys on one steel "connecting link" (a shackle
used to connect chains) and will weld the link shut so that they
cannot be separated. Or else I will bolt it to a wall in some secret
location or save it in the safe.
These interlock keys are indeed used relatively widely in industry for
all sorts of purposes. See the kirkkey.com website for examples.
A power station which is presumed to be accessible only to qualified
persons. A residential setting which is presumed to be accessible to
unqualified persons. Your interlock may well be acceptable in a power
station, it will not however be acceptable in a residence. Inspection
requirements and codes applicable are also very different between
residential (NEC) and power station (NESC).
I suppose that being a somewhat ethical and caring person, and like
the KISS principle (Keep it Simple Stupid), I would have to go with
the tradition 3-pole, 3-position, transfer switch. On these, one
position puts your home on grid power, in the center postion it is
connected to nothing, and in the thrid position it is connected to
Only a week or two ago I was running my home on generator power, with
the generator connected to the house with a "suicide cord", and the
master breakers to the house pulled. I went outside and spent some
time talking with the line crew that was replairing the down line in
front of my home, in the rain. Having noticed that I was running a
generator, one of the guys asked me if I was sure that I was
disconnected from the grid. I replied that I knew I was, because the
breaker connecting me to the grid was not only turned off an pulled,
so I could see about 2-3 inches of physical separation.
No problem, he replied to me, and also informed me that the top like
supplying the transformers on my street was an 8,000 volt line. I'm
not a lineman but am an electrical engineer, and would not sleep at
night if I were risking backfeeding a distribution transformer that
stepped the voltage up to 8,000-volts, or even 2,200, no matter how
clever the interlocking arrangement. I am not that desperate to power
my home, and as an engineer I recognize the too frequent failure of
such arrangements, let alone to build one myself. It's not so much the
design of such interlocks, but the failure modes that amateur
designers often fail to take into account, with tragic consequences.
Especially the retarded lineman that doesn't use equi-potential
technique grounding devices to protect himself from induction and
static charges that exist on lines running accross the countryside
just by laws of nature. Your little generator is not a problem for any
lineman that isn't a lazy and lucky jerk.
Interlocks are still necessary to help prevent frying the lazy and
lucky jerk though. You are right. You don't want to run the child
over, green light or not.
Before you invest allot of time and effort check with your local
inspector as in the end he's the final authority. If he doesn't like it
go with whatever he recommends, he's the one you need to keep happy. If
he's OK with it you should be good. Kirk keys are a common interlocking
means in power distribution schemes, and in settings other than the
power company. You'd have a tough time duplicating the key as they are
only available from Kirk, I don't know what they require before they
sell you another. The only problems apparent are the presence of the
second key, remove it from the premises to a safe deposit box or such,
and the ultimate design of the locking mechanism that the locks are
Yeah, I don't think an inspector would go for it. The inspector would
say "but if someone had the other key..." and you'd say "but it's in
the safe" and the inspector would say "but if someone had the other
key..." and out would come the [REJECTED] stamp.
That kind of stuff, the safety is in physical impossibility of linking
sources, not the reliance on a meat puppet to do the right thing and
not throw both switches at once.
In a restricted industrial setting, there can be different rules.
Residential is another matter...
Our situation is a little different, were too far from the grid to be
connected. But have the same problem
So we have our own power making setup.
ie generators. that means 2 off .
One 6KW for daily use and a 15KW when I need lots of power
@240Volts.(Were in the UK where all normal domestic supplies are at this
How do we seperate the 2 ? to make sure there never connected at the
same time when either one or the other is running?
we have a board to which the 2 generator outputs come into 2 identical
but seperate sockets. one labeled BIG and the other SMALL
the domestic connection is wired into 1 plug.
So you can only plug this into 1 or the other socket. Which we do before
we power up .
Both gemerators also have big cast iron breaker boxes on their own
panels. these have knife switches inside a well as fuses.
I know it doesnt have the convenience of a change over switch but does
the same thing with the same safety in my opinion. Also a lot cheaper
that a 20KW change over switch.
The code inspector is likely to treat this with the same respect he treats
a Wilma interlock.
What's a Wilma interlock? A piece of electrical tape across the main
breaker handle. What every household was using with a portable generator
and suicide cord for a disconnect after Hurricane Wilma here in Palm Beach
Sure, it works, but they won't trust your discipline and brains. Nor
Locking things in a safe? C'mon.
Certainly not wishing anything but how could you know that? Anything can
Part of the reason for doing a safe/standard installation is so that no
"hocus pocus" is required so it will be safe if you aren't there or are
Cross posts deleted
In a word, NO!
If your system requires instructions or training to make it function, it
isn't safe for residential use.
I'm all for keeping the cost down, this is not one of those places.