Do It Yourself: 240v from two 120v sources

[nsayer]

LTLFTcomposite posted the following comments back in 2011:

This might be overkill but if you had extra relays could you protect against hot-neutral reversed wiring on the receptacles? If the relay is double throw it could open the neutral wired across the normally closed terminals if it detects current between (what should be) the neutral and ground.


Then
garygid wrote:

If Hot-Neutral is reversed on one input, you typically just get 120v out, not 240v.

If the phases of the two inputs is the same, you get HOT zero-voltage out.

This suggest that we do not need to add that third relay? I don't believe the box I used will accommodate the third relay. I will take it apart and get another box if this is the consensus.
Thanks
Manny

The third relay is there to insure you get 208/240 or you get nothing at all. If you have a hot-neutral reversal, that means that one hot has a 120 volt potential to ground, the other has 0 to ground. This means that the downstream equipment won't work, which means you may think it is malfunctioning and start poking at it, neglecting the fact that it's plugged in and one of the hots has 120v to ground on it.

The third relay is not supposed to be able to be "brought in" with 120v, so it will prevent hot-neutral reversal activation.

The first two relays are slightly overkill, perhaps, but they're there to insure that if anything goes hinky with one of the inputs that both are disconnected. This prevents any potential for a disconnected plug from becoming a virtual neutral conductor just waiting to find a damp meat path to ground. Yes, the third relay would likely prevent this as well, but on the off chance it was stuck closed...

 

[nsayer]

Can you provide us with the part numbers that you used?

DigiKey part numbers:

2 x PB489-ND
1 x PB738-ND
3 x 1091-1122-ND
1 x 377-1245-ND
1 x 377-1939-ND

I got the two 5-15P, the L6-30R, the ground bar and about 6 feet of 3x12 SOOW from Home Depot to complete it. I had on hand all of the QD terminals - they're the expensive ones I use to build Hydras, and it took 24 of them. I also had handy two CG-16 cable glands from PolyCase for the SOOW cable. The holes for the indicator lights and the receptacle ruin the NEMA-4 rating on the box, but the cable glands are at least good strain reliefs.

In the end, I came in at around half the cost of the commercial box, but that assumes my time is free, of course. It took, 3, maybe 4 hours to do, but it took longer because it's the first time I've ever done it. Having done it once, I could probably make another in only a couple hours (but why?).

 

[nsayer]

Here it is.

I didn't mount the lights yet. It's late. But I did test it, and it works.

As always, good work. Has anybody taken apart an actual "Quick 200" (the patented one) to see how they do it?

Patents are public. If you can find out the patent number, you can look it up in Google.

 

[nsayer]

One additional interesting discovery...

The 3rd relay coil and indicator light are enough to trip any GFI outlets that you plug this gizmo into. In a way, that's a feature, as it means that you discover them up front...

 

[srl99]

http://www.google.com/patents/US5977658" onclick="window.open(this.href);return false;

Here it is.

I didn't mount the lights yet. It's late. But I did test it, and it works.

As always, good work. Has anybody taken apart an actual "Quick 200" (the patented one) to see how they do it?

Patents are public. If you can find out the patent number, you can look it up in Google.

 

[nsayer]

So they're only using the last one?

I dunno. I'm not convinced that's sufficient.

Although neutral isn't used in the output, the 3 relay solution insures that each hot is good relative to its respective neutral conductor. A miswired switched outlet might switch off the neutral, leaving the hot energized. If you were counting on that switch to kill the equipment, you'd be surprised.

That's a weak rationalization, I'll grant you, but it is one.

Another one is that if the relay gets stuck closed, then if you pull a 120v plug, its hot becomes a virtual neutral and a shock hazard. With 3 relays, you'd need two (and the correct two) to fail to make that possible.

As long as we're constructing nightmare scenarios, a ground fault in the target equipment might make it possible for the 240 volt relay to remain closed even if you yank one of the 120v cables out - again, making its hot prong a shock hazard. With 3 relays, if you yank either one, then both hots go dead, positively, because there's no path from that relay's neutral line to anything.

The third relay I'm using has a pull-in spec of 192 volts, so a hot-neutral reversal won't activate it. That would also be true with only a single relay.

The patent office will grant patents to perpetual motion machines, so just because it's patented doesn't mean it's correct.

 

[Ingineer]

The "safety" argument for the third relay is not valid. There is no safety implication using my original design. A simple addition of the indicator will verify power.

The relays I spec'd are rated to break the full load current, so it's almost impossible they would weld shut. If whatever circumstances exist that could weld one relay, it could definitely weld 2.

Nobody should ever assume an outlet is dead and start touching potentially live conductors just because the equipment plugged in isn't operating.

If there was a ground fault, something would have tripped long before you pull the cables. Keep in mind the EVSE is upstream and it has GFCI and power fail fault detection.

What if a tree falls on your car and splits the battery pack wide open?

There are many more fault scenarios in your home wiring that present orders of magnitude more danger, so for best safety, you should have an electrician with you at all times to check everything before you plug anything in.

In fact, there is probably many many orders of magnitude that you'll perish in a fatal car accident on your way to wherever you intended to plug in anyway. There's your safety factor! =)

-Phil

 

[nsayer]

The "safety" argument for the third relay is not valid. There is no safety implication using my original design. A simple addition of the indicator will verify power.

The relays I spec'd are rated to break the full load current, so it's almost impossible they would weld shut. If whatever circumstances exist that could weld one relay, it could definitely weld 2.

Nobody should ever assume an outlet is dead and start touching potentially live conductors just because the equipment plugged in isn't operating.

If there was a ground fault, something would have tripped long before you pull the cables. Keep in mind the EVSE is upstream and it has GFCI and power fail fault detection.

What if a tree falls on your car and splits the battery pack wide open?

There are many more fault scenarios in your home wiring that present orders of magnitude more danger, so for best safety, you should have an electrician with you at all times to check everything before you plug anything in.

In fact, there is probably many many orders of magnitude that you'll perish in a fatal car accident on your way to wherever you intended to plug in anyway. There's your safety factor! =)

-Phil

So, in short, your argument is that things that are unlikely are not safety concerns.

I invite you to remove all of the GFI outlets from your bathroom and replace them with standard ones.

I'm glad you're not my electrician.

 

[Ingineer]

So, in short, your argument is that things that are unlikely are not safety concerns.

I invite you to remove all of the GFI outlets from your bathroom and replace them with standard ones.

I'm glad you're not my electrician.

What? You are comparing apples to oranges. What does removing GFCI protection have to do with this? Nothing.

I'm glad you didn't design the National Electrical Code. We'd have unreliable inefficient expensive systems.

-Phil

 

[nsayer]

So, in short, your argument is that things that are unlikely are not safety concerns.

I invite you to remove all of the GFI outlets from your bathroom and replace them with standard ones.

I'm glad you're not my electrician.

What? You are comparing apples to oranges. What does removing GFCI protection have to do with this? Nothing.

Actual GFI faults in bathrooms are quite rare, but quite unsafe. That doesn't seem to bother you any, so you ought to just get rid of them to avoid the nuisance trips.

I'm glad you didn't design the National Electrical Code. We'd have unreliable inefficient expensive systems.

-Phil

Maybe, but they'd probably kill fewer people.

 

[drmanny3]

I appreciate the discussion and want to learn as much as possible. I have the original design with the two relays. I guess my question would be if I plug each of the extension plugs into live receptacles and then one trips the circuit because it is say an older home (45 years) and perhaps has 15 amp circuits. This would stop the charging from taking place and the light (on the top of the box) would go out. In this particular situation if I pull out the plug that no longer gets current could it be live from the other plug that is still in a live circuit? I hope I said that correctly.
Thanks,
Manny

 

[fooljoe]

I appreciate the discussion and want to learn as much as possible. I have the original design with the two relays. I guess my question would be if I plug each of the extension plugs into live receptacles and then one trips the circuit because it is say an older home (45 years) and perhaps has 15 amp circuits. This would stop the charging from taking place and the light (on the top of the box) would go out. In this particular situation if I pull out the plug that no longer gets current could it be live from the other plug that is still in a live circuit? I hope I said that correctly.
Thanks,
Manny

No, as long as the relays in your box haven't failed closed, you'll never have any hot plugs. That's the whole idea. I'm not sure what the idea of adding a 3rd or 4th relay is, unless for some reason you'd be concerned about having one of the slots in the receptacle live (in which case why aren't you concerned about having live slots in every receptacle in your house? :? )

 

[nsayer]

No. The relay opening will keep the plug from being connected to anything.

What making it a double-pole relay does is disconnect both hots from the receptacle instead of just one.

What the third relay does is prevent connecting the hots if they're not on opposite phases or if there is a hot-neutral swap.

IMHO your device is not UNsafe, but I think the 3 relay design prevents more incorrect situations (which can become hazardous) automatically, which I believe has value.

 

[fooljoe]

IMHO your device is not UNsafe, but I think the 3 relay design prevents more incorrect situations (which can become hazardous) automatically, which I believe has value.

I'm struggling to think of what type of situations there are that could cause a problem with the two-relay design.

disconnect both hots from the receptacle instead of just one

What's the problem with having a live slot in a receptacle? Again, this is EVERY receptacle in your house.

prevent connecting the hots if they're not on opposite phases or if there is a hot-neutral swap

If they're on the same phase, you'll have live slots in a receptacle again, but again I fail to see why that's a problem. If there's a hot-neutral swap, then you'll just have one less live slot in a receptacle.

 

[nsayer]

Why are the breakers for 240v circuits ganged?

That's why I designed it that way. If you don't think it's important, then take a Dremmel and cut the breakers apart on your fuse box.

At the end of the day my design prevents every incorrect presentation I can envision. I believe that has value. Opinions will vary, but opinions are like assholes: everyone has one and they usually stink.

 

[fooljoe]

Why are the breakers for 240v circuits ganged?

That's why I designed it that way. If you don't think it's important, then take a Dremmel and cut the breakers apart on your fuse box.

At the end of the day my design prevents every incorrect presentation I can envision. I believe that has value. Opinions will vary, but opinions are like assholes: everyone has one and they usually stink.

:lol: Well, I don't think there's anything wrong with your design, but unfortunately the implication that the existing design is somehow unsafe has led to some unnecessary FUD around here, and that's no fun.

 

[TonyWilliams]

Why are the breakers for 240v circuits ganged?

That's why I designed it that way. If you don't think it's important, then take a Dremmel and cut the breakers apart on your fuse box.

At the end of the day my design prevents every incorrect presentation I can envision. I believe that has value. Opinions will vary, but opinions are like assholes: everyone has one and they usually stink.

:lol: Well, I don't think there's anything wrong with your design, but unfortunately the implication that the existing design is somehow unsafe has led to some unnecessary FUD around here, and that's no fun.

That's a good point. Is it "unsafe" to have 120 volts on one inlet and an open on the other?

 

[nsayer]

That's a good point. Is it "unsafe" to have 120 volts on one inlet and an open on the other?

I'm going to assume that you mean on, say, an L6-30 you've got one live hot and one disconnected hot.

It means that you have equipment that appears to be dead, but is still hazardous.

No matter how you slice it, that is simply less safe than equipment that can only possibly be in one of two states: completely and successfully powered or completely dead.

Imagine that you need to work on the device, so you unplug one of the 120 plugs, then your phone rings. You answer it, have a conversation, then hang up and get back to work. You open the device.... forgetting that your phone call interrupted your flow and you still have one of the hot lines hot.

Corner case? sure. Is it something that can be mitigated with better processes or behavior? sure. But dammit, **** happens, and that's what engineering for safety is all about.

 

[TonyWilliams]

That's a good point. Is it "unsafe" to have 120 volts on one inlet and an open on the other?

I'm going to assume that you mean on, say, an L6-30 you've got one live hot and one disconnected hot.

It means that you have equipment that appears to be dead, but is still hazardous.

No matter how you slice it, that is simply less safe than equipment that can only possibly be completely and successfully powered or dead and safe.

Yes, we are starting to slice the problem pretty thin, and I'm merely addressing "safety".

The 240 volt (or even a 120 volt one) product shouldn't be energized with 120 volt on one pin and open on the other. It is "dead" that only completing the circuit will fix.

 

[nsayer]

Let's not forget, a hot-neutral swap will cause a two-relay design to present 120 volts to a 240 volt device. That may not be unsafe from a hazard to life perspective, but it may result in equipment damage.