Making a generator interlock kit

But even a three pole transfer switch at each branch circuit won't fill your bill. The bond at the panelboard means that even once the neutral is interrupted you could still see a fault to *ground* at a piece of equipment. That makes the messenger wire to the pole hot by your reconning.

So it would seem you would advocate some way to open the ground wire out to the pole as well.

Jim

I did.

The hole in my panel is not paint filled.

Solid #8 copper wire at least

made of copper

there are no gaskets, these are NPT pipes screwed together.

Yep, I see nothing wrong. Remember that likely, the same utulity neutral is also bonded to the ground in several other homes.

The chance of my neutral to ground connection failing is negligible because it is a good connection. Besides, the same utility neutral is grounded in numerous places.

I believe that I am making an informed decision. The real issue is whether a faulty main breaker might, hypothetically, to fail to disengage the hots. I have not seen any real cases where that happened, and that's probably why interlocks are legal.

i

One always hopes, but in my previous residence, it appeared that None of the houses on my transformer had a good N-G bond. when I measured it I had a variable 55-65 volts Phase or neutral to ground [ single phase 120 V service, yuck]

I have seen cases where that has happened. It is quite rare. Usually happens only from abuse of long duration.

However, the case where you neutral neutral becomes unbonded, does not present a significant hazard, unless you see this breaker failure.

If the generator neutral is tied to the building neutral (as it is required to be) and the N-G bond is bad, there is STILL no current path to the lineman. You have to connect the other side of the generator to him to have a current flow, and you can do that unless both breakers are closed.

Or viewed another way, there is no more risk than if you just have a generator, sitting there running, connected to NOTHING, but its neutral is connected to the service entrance bond point.

The remote chance that your equipment may fail is why in many locations, you are required to apply for an interconnection permit if you are installing a transfer switch.

jk

It is a "Kirk key", and are unsed in many different types of interlocks. They can and do get used in the sort of interlock you were describing. The kirk key is a little like jello. Trade marked but used generically.

jk

That's weird.

As will be hte case, unless I want to make my generator a "separately derived system".

The other path can be ground, as I think the other poster was trying to say.

i

Well, they sell all kinds of interlocks, it was not quite obvious which one would apply to my situation.

i

What I was saying (should have been "cant do that unless..." was that you have to have two paths, the ground can be one, but unless you complete a circuit it is ok.

Or in other words what you want to do is fine. jk

Let me try to rephrase my understanding of what the OP said.

My understanding is that, in some unusual circumstances, a path for the current that might kill a lineman could be the neutral wire as one side, and ground as another side of the path.

Figuratively (and incorrectly for AC), the current would arrive to the lineman from the generator through the neutral wire, and leave the lineman through the ground to return back to my generator.

This, obviously, requires that the neutral to ground bond on my panel fails open. It also requires that the hot of my generator fails to ground, that is, hot of the generator is condicted to ground. It also requires that the neutral wire is not grounded at other homes in the vicinity, on its way to the utility transformer.

These three conditions are rather unlikely to occur together.

I agree with that. Thank you for your opinion.

i

Apparently peter did not see my question, so I am posting the exchange again, as I am truly interested in his take on this issue. According to him, one would need to completely isolate a residence from teh pole to make it safe for linemen - meaning a three pole interrupt at the service entrance, or a four pole interrupt at each branch circuit, which isolates every conductor including the grounding conductor.

Jim

I think your comments agree with his scenario.

My question is, how do the code-approved transfer switches that one buys on the market today prevent the above series of events from happening?

It would seem to me that the center-tap wire from the house to the pole would have to be interrupted to prevent this.

Jim

I simply repeated the scenario, which does not contradict any known laws of physics. Where I disagree with the OP is that in my opinion, chances of this happening given conditions of MY house (quality of ground) are nearly nil.

Come to think about it, the neutral to ground potential probably is not in fact "transformed" into high voltage. What is transformed is hots to neutral. Think about it.

They do not.

I agree.

My understanding is that there are only three wires between the service entrance and the transformer: two hots (L1 and L2) and a ground/support cable from the center tap. The first place where the ground and neutral are separate is at the main service disconnect, right after the meter. This is also supposed to be the only place where they are bonded or connected together. The ground should be hooked to a good earth ground (two 16' copper rods, a few feet apart with thermite-welded connections to the braided ground wire, at my house); it should also be connected to the house side of the plumbing and to any structural steel in the building. BTW, the plumbing is grounded **BY** the electrical ground here; it doesn't provide the ground. I have a buried plastic pipe out to the well with a 12 ga. tracer following it. No trustworthy ground there.

My transfer switch is maybe a little more elaborate than most. It is a Zenith in its own 7'h, 3.5'w,

1.5' deep box. On loss of power, it waits a second or two, fires up an 80kW Koehler natural gas gen set, lets it stabilize, tells the house computer to shut down large loads like air-conditioners, transfers to the Koehler, and then restarts the heavy loads one-by-one. If the heavy loads are more than the generator can handle, the house computer runs them in rotation. When power returns, it monitors for about ten minutes to be sure the power is back for good, and then transfers back to line power. It also test runs the genset once a week and phones the service company if it finds a problem.

BTW, it transfers the neutral as well as the two hots. All grounds stay connected, so the generator frame is grounded to ground, but the coils are connected to the house neutral and the two hot leads. Power flows through hots and neutral but the neutral voltage is held to ground potential via the neutral- ground bond in the main disconnect.

-- --Pete "Peter W. Meek"

I am curious, are your large loads like A/C so incredibly energy consuming tat an 80 kilowatt generator cannot handle them? I have hard times believing that. How big is your house? We have a almost 5,000 sq ft house (including a finished basement), and a 28 amp A/C, which works out to about 7kW.

i

Peter W. Meek wrote: ... The ground ... should also

Why would you want to ground the plumbing, if it's not grounded by plumbing connections? It seems to me that by doing so you're just providing lots of places for ground faults to occur. Possibly through humans.

Why not just let the plumbing be independent of the wiring?

Just asking, Bob

This is the way it's wired at my house (Alberta Canada) i.e. no ground wire goes to the pole although the neutral is bare and wound around the two hots and is in effect grounded as it is direct burial to the pole. In my case, the in-house ground wire is connected to the metal well casing.

In regards to lineman hazard, do they not typically isolate an under repair line fault from the upstream source *and* short the hot(s) to ground or neutral whilst they are working on it (to avoid said shock hazard)?

I'd also be interested in comments on my gen hookup method. My gen only puts out 120VAC so, to supply both halves of the breaker box, I plug the gen (in parallel) into both sides of a split receptacle located in the garage, this providing 120V to all 120V circuits in the house (the 240V circuits are not functional as the voltage is the same on both sides). Be gentle please............:)

Laurie Forbes

Commercial transfer switches only switch the two hot lines - the Neutral and Ground are all bonded and stay connected. They stay bonded to ground rod(s) and cold water grounds at the house, and they stay bonded through the ground riser and butt plate ground on the pole. (Or a ground rod system at the manhole for an underground feed.)

I'm not a power lineman, but that's how I see them doing it all the time - it's either dead and grounded for safety, or hot and they've got line hose and blankets over everything. And they're wearing rubber gloves and/or working with hot sticks either way.

And "A Lineman's Pencil Has No Eraser" Their first big screw-up is likely to be their last, so they are really careful... That's a job I don't really want, bad enough I work with 120V hot.

You have to be /really/ careful about your phasing if you feed your house from a 120V source like that, Laurie! If you have two circuits in one conduit sharing a neutral (3-wire circuit) and you feed them both with 120V from the same phase, you can overload the neutral wire easily. Load down both circuits and run 40 amps through that 12-gauge white wire back to the panel for very long, and you'll have both a power outage /and/ a house fire to worry about.

In your case, I'd install a separate transfer switch & sub-panel for the emergency-fed loads. And from there out to the loads, rewire those 3-wire circuits that share a neutral with separate ones for each line, to eliminate that hazard. If your house is wired in Romex you won't be repulling anything like you could in flex, you will be better off to run separate new lines to those critical receptacles.

Where did you get this beastie? Sounds like it'd be perfect for people like me so I don't have to way over-buy the generator plant (100KW for a residence when I have a 10KW available) just to be sure it can take up the whole load at once without stalling.

I'd want to see the wiring diagrams on this one - the NEC might not take kindly to switching the Neutral, and I'd want to research the codebook before hooking it up. Seems rather counter-intuitive to me - the LAST thing you want to risk is an open neutral on a 120/240 or

120/208 system.

Open that neutral and make an unbalanced load, and all sorts of really nasty things can happen... ("Snap!" "Crackle!" "Pop!")

-->--

NEC calls systems with their own separate neutral a "separately derived system". Separately derived systems must have their own ground (ground rod), and they must bond their neutral to their ground.

i

Thanks Bruce - I appreciate your comments about overloading the neutral. I should have however included in my first post some additional info. In my case, I have #10 wire running from the garage to the breaker panel (which should be good for 30A). The breaker is also 30 A. My gen is a 3500W unit which limits its output (under normal conditions at least) to about 30A. So, I think I'm OK in that regard although the split receptacle in the garage and the short run of #14 going to the junction box would be overloaded under max load. I've examined it when the gen is working hard however and have not noticed anything getting particularly warm - I probably should however at least upgrade both receptacle and wire to 30A capacity.

You're correct of course - that would certainly be a better way of doing it.

One other thing I've been wondering about if you could comment. It seems to me that the powering both sides of the breaker box with in-phase voltage might result in voltage cancellation at the pole transformer such that a feedback shock hazard might not then exist (not that I would consider dispensing with a disconnect in any case).

Laurie Forbes

You don't want to know. For the cost of the smart-house system, you could buy as many 80kW gensets as you need for any likely load. I'm pretty sure I could get 20 or 30 gensets of that size for the cost of the smart-house components and installation. (The Zenith transfer switch just tells the smart-house what it is doing; the smart-house system does all the load calculations and switching.)

-- --Pete "Peter W. Meek"