2 x AC Inputs Possible Problem?

[Biggles]

Whats the worst that could happen if 2 AC sources are fed in together.

 

1 x shore power the other from a generator?

 

I made this to prevent it happening.

 

See the comment from one of my readers at the bottom of the post. This now has me thinking whats the worst that could happen, and what is the likely real world outcome?

 

Biggles

Edited March 4, 2011 by Biggles

[nb Innisfree]

Whats the worst that could happen if 2 AC sources are fed in together.

 

1 x shore power the other from a generator?

 

I made this to prevent it happening.

 

See the comment from one of my readers at the bottom of the post. This now has me thinking whats the worst that could happen, and what is the likely real world outcome?

 

Biggles

 

I think our Victron Multiplus can match the waveform from the the inverter to the the ac input, either the grid supply or generator, but not both simultaneously. I think something would go pop if grid and genny were connected together?

[Keeping Up]

Well of course the fuses or breakers in the shore supply and the generator output SHOULD just quietly blow or trip out and nothing else would happen. Both supplies ARE properly protected by fuses or breakers, aren't they?

 

In practice of course they take time to operate. I would think it is quite possible that all the relay contacts would be blown to pieces, possibly welding all the contacts together or alternatively presenting just enough resistance to not blow the fuses or breakers and maybe therefore causing a small fire. The small fire may choose to become a large fire given time. In addition if the generator has electronic controls (especially if it has an inverter output) then there is a fair chance that those electronics would be damaged, which could then cause the generator to self-destruct even after it has become disconnected by its own breaker. The general mains wiring of the boat would probably survive, but the resultant surges on the line could destroy any item of electronic equipment that was plugged into any socket.

 

I'm not sure if that's the worst case scenario but it's bad enough. Probability? Don't know, but I wouldn't want to try it.

[nb Innisfree]

Well of course the fuses or breakers in the shore supply and the generator output SHOULD just quietly blow or trip out and nothing else would happen. Both supplies ARE properly protected by fuses or breakers, aren't they?

 

In practice of course they take time to operate. I would think it is quite possible that all the relay contacts would be blown to pieces, possibly welding all the contacts together or alternatively presenting just enough resistance to not blow the fuses or breakers and maybe therefore causing a small fire. The small fire may choose to become a large fire given time. In addition if the generator has electronic controls (especially if it has an inverter output) then there is a fair chance that those electronics would be damaged, which could then cause the generator to self-destruct even after it has become disconnected by its own breaker. The general mains wiring of the boat would probably survive, but the resultant surges on the line could destroy any item of electronic equipment that was plugged into any socket.

 

I'm not sure if that's the worst case scenario but it's bad enough. Probability? Don't know, but I wouldn't want to try it.

 

A big pop then?

[Guest]

Whats the worst that could happen if 2 AC sources are fed in together.

 

1 x shore power the other from a generator?

 

I made this to prevent it happening.

 

See the comment from one of my readers at the bottom of the post. This now has me thinking whats the worst that could happen, and what is the likely real world outcome?

 

Biggles

ONE COULD KILL SOMEBODY.

Edited March 5, 2011 by Guest

[smileypete]

Whats the worst that could happen if 2 AC sources are fed in together.

 

1 x shore power the other from a generator?

 

I made this to prevent it happening.

 

See the comment from one of my readers at the bottom of the post. This now has me thinking whats the worst that could happen, and what is the likely real world outcome?

Generator vs National Grid.

 

National Grid 1, Generator 0.

 

Possibly.

 

Out of interest what crimp tool did you use for the connections?

 

cheers,

Pete.

[Gibbo]

I wouldn't have used two relays in parallel for exactly that reason. I'd have used a bigger single relay.

 

National grid into a generator output is a very likely blown generator, possible fire. Of course it should just be some blown fuses.

[rjasmith]

Here's something else for you to consider which hasn't been mentioned so far (except perhaps this was what Catweasel's comment was suggesting).

 

Say you have a power cut on the shore supply, your relay box energises and your generator starts up. However one of the relays has got stuck in the unenergised position, the other operates normally. All appears to be well on the boat with the generator now feeding the loads. BUT it is also now feeding back up the "dead" shore supply right up to where the fault is. It may just cough and splutter because it can't cope with all the other boats and bits of marina it's now trying to feed!

 

However the fault may be nearer and no other load apart from your boat is left on the faulty shore line circuit.

 

Along comes a friendly maintenance man or Supply Co gang to fix the fault, having isolated their upstream connections they will start working on what they believe is a dead circuit - only it won't be - and they could be dead instead!!!

 

This scenario may seem unlikely perhaps but it isn't impossible and during the aftermath of 1987 storm round these parts there were a number of near misses where back feeds due to home grown generator connections actually occurred.

 

I would certainly echo the calls for you to use a single changeover relay or manual switch with a correctly rated Break Before Make contact set.

 

Richard

[Biggles]

I think our Victron Multiplus can match the waveform from the the inverter to the the ac input, either the grid supply or generator, but not both simultaneously. I think something would go pop if grid and genny were connected together?

 

Thanks guys once again for some informative views.

 

I too have A Victron Mutipluss. You must have the Quarto as this handles multi AC inputs.

 

Well of course the fuses or breakers in the shore supply and the generator output SHOULD just quietly blow or trip out and nothing else would happen. Both supplies ARE properly protected by fuses or breakers, aren't they?

 

 

 

 

Yes they are as would the shore power post.

 

ONE COULD KILL SOMEBODY.

 

Is that all? :blush:

 

Generator vs National Grid.

 

 

Out of interest what crimp tool did you use for the connections?

 

cheers,

Pete.

 

The crushing power of mole grips.

[Biggles]

Here's something else for you to consider which hasn't been mentioned so far (except perhaps this was what Catweasel's comment was suggesting).

 

Say you have a power cut on the shore supply, your relay box energises and your generator starts up. However one of the relays has got stuck in the unenergised position, the other operates normally. All appears to be well on the boat with the generator now feeding the loads. BUT it is also now feeding back up the "dead" shore supply right up to where the fault is. It may just cough and splutter because it can't cope with all the other boats and bits of marina it's now trying to feed!

 

However the fault may be nearer and no other load apart from your boat is left on the faulty shore line circuit.

 

Along comes a friendly maintenance man or Supply Co gang to fix the fault, having isolated their upstream connections they will start working on what they believe is a dead circuit - only it won't be - and they could be dead instead!!!

 

This scenario may seem unlikely perhaps but it isn't impossible and during the aftermath of 1987 storm round these parts there were a number of near misses where back feeds due to home grown generator connections actually occurred.

 

I would certainly echo the calls for you to use a single changeover relay or manual switch with a correctly rated Break Before Make contact set.

 

Richard

 

The generator would not automatically fire up. It needs me to start it via the fly panel inside the boat. If as you suggest the generator did try to back feed the marina it would trip out. It has its own MCB.

 

I am still minded to stay with the parallel relays as it has the extra redundancy. I can see how the contacts could become contra connected but in the real world how often does a relay fail (I know it only takes once) I will check the duty cycle and report back.

 

I suppose if I disconnect or isolate the shore power plug before the genny is started the problem can't happen.

 

Is there some sort of diode I could put in the shore power line to prevent back flow?

 

Biggles

[sociable_hermit]

The generator would not automatically fire up. It needs me to start it via the fly panel inside the boat. If as you suggest the generator did try to back feed the marina it would trip out. It has its own MCB.

 

I am still minded to stay with the parallel relays as it has the extra redundancy. I can see how the contacts could become contra connected but in the real world how often does a relay fail (I know it only takes once) I will check the duty cycle and report back.

 

I suppose if I disconnect or isolate the shore power plug before the genny is started the problem can't happen.

 

Is there some sort of diode I could put in the shore power line to prevent back flow?

 

Biggles

Having the two relays is a bad idea. Better to have one big relay and carry a spare which can be fitted if it fails.

 

If you don't want to listen to the advice then there's little point in asking the question, eh?

[Biggles]

Relay Spec.

 

Expected Mechanical Life: 5 million operations.

Expected Electrical Life: 100,000 operations.

 

Extended life –

>100,000 operations at 30A, 240VAC (AC coil).

 

Must Operate Voltage: AC Coil: 80% of nominal voltage or less.

DC Coil: 75% of nominal voltage or less.

Must Release Voltage: 10% of nominal voltage or more.

Initial Operate Time(2): 15 ms typical, (25 ms max. w/bounce).

Initial Release Time(2): 10 ms typical, (25 ms max. w/bounce).

Max Operating Frequency: 14 operations per minute.

 

Biggles

[ditchcrawler]

Relay Spec.

 

Expected Mechanical Life: 5 million operations.

Expected Electrical Life: 100,000 operations.

 

Extended life –

>100,000 operations at 30A, 240VAC (AC coil).

 

Must Operate Voltage: AC Coil: 80% of nominal voltage or less.

DC Coil: 75% of nominal voltage or less.

Must Release Voltage: 10% of nominal voltage or more.

Initial Operate Time(2): 15 ms typical, (25 ms max. w/bounce).

Initial Release Time(2): 10 ms typical, (25 ms max. w/bounce).

Max Operating Frequency: 14 operations per minute.

 

Biggles

So no one would ever need to change one.

[Guest]

Here's something else for you to consider which hasn't been mentioned so far (except perhaps this was what Catweasel's comment was suggesting).

 

Say you have a power cut on the shore supply, your relay box energises and your generator starts up. However one of the relays has got stuck in the unenergised position, the other operates normally. All appears to be well on the boat with the generator now feeding the loads. BUT it is also now feeding back up the "dead" shore supply right up to where the fault is. It may just cough and splutter because it can't cope with all the other boats and bits of marina it's now trying to feed!

 

However the fault may be nearer and no other load apart from your boat is left on the faulty shore line circuit.

 

Along comes a friendly maintenance man or Supply Co gang to fix the fault, having isolated their upstream connections they will start working on what they believe is a dead circuit - only it won't be - and they could be dead instead!!!

 

This scenario may seem unlikely perhaps but it isn't impossible and during the aftermath of 1987 storm round these parts there were a number of near misses where back feeds due to home grown generator connections actually occurred.

 

I would certainly echo the calls for you to use a single changeover relay or manual switch with a correctly rated Break Before Make contact set.

 

Richard

Exactly that. I have had more than one belt from gennies back feeding without proper isolating switches (farms were the worst for this.) Death is a very real possibility. Whatever method is used, it is vital that all precautions are taken against back feeding.

[sociable_hermit]

In theory you're unlikely to ever need to change one, though I'll bet the test conditions didn't include the same dampness, vibration and temperature changes as the average narrowboat engine room. I have one relay, but I'm reliant on the thing so I also carry a spare, just in case. I understand Murphy's Law: if I carry a spare, I'll never need it.

 

What ARE more likely to fail are all of those connections, if you have two relays. Twice the risk of something coming undone.

[ditchcrawler]

The generator would not automatically fire up. It needs me to start it via the fly panel inside the boat. If as you suggest the generator did try to back feed the marina it would trip out. It has its own MCB.

 

 

Biggles

Look at it the other way round, The mains power is on and you are working on the genny cable because the genny breaker dropped out for some unknown reason. Investigation later showed that the breaker dropped out due to a back feed, the coroners verdict was ..........

[Rojja]

Relay Spec.

 

Expected Mechanical Life: 5 million operations.

Expected Electrical Life: 100,000 operations.

 

Extended life –

>100,000 operations at 30A, 240VAC (AC coil).

 

Must Operate Voltage: AC Coil: 80% of nominal voltage or less.

DC Coil: 75% of nominal voltage or less.

Must Release Voltage: 10% of nominal voltage or more.

Initial Operate Time(2): 15 ms typical, (25 ms max. w/bounce).

Initial Release Time(2): 10 ms typical, (25 ms max. w/bounce).

Max Operating Frequency: 14 operations per minute.

 

Biggles

 

These relays are not suitable for the purpose.

 

When switching, when both (unsynchronised)ac supplies become available, the relay's contacts are subjected to ~490 volts ac (650V peak) and that's without any back emf spikes from the connected devices.

 

As it stands, be prepared to expect trips to trip, fuses to blow and the relay contacts to vaporise.

 

I've had to sort this simple 'solution' too many times.

 

Separate mechanically interlocked devices is one correct solution.

 

Roj.

[Gibbo]

These relays are not suitable for the purpose.

 

When switching, when both (unsynchronised)ac supplies become available, the relay's contacts are subjected to ~490 volts ac (650V peak) and that's without any back emf spikes from the connected devices.

 

 

The spec on this type of relay already takes that into account. That's why it's described as a changeover relay rated to 240V.

 

If you read the datasheet for that relay it is UL approved to switch 600VAC with a maximum of 1500VAC between contacts and 2000VAC between poles. 650 volts is miles within its spec.

 

However what might be an issue is the current rating:-

 

The NC contacts are only rated to 3 amps.

 

Separate mechanically interlocked devices is one correct solution.

 

They're great until the mechanical interlock breaks which happens with monotonous regularity (or just as bad someone pulls it out!).

[Keeping Up]

Relay Spec.

 

Expected Mechanical Life: 5 million operations.

Expected Electrical Life: 100,000 operations.

 

Extended life –

>100,000 operations at 30A, 240VAC (AC coil).

 

 

Quoting expected lifetime figures like these is like quoting the life of a car tyre as 30,000 miles without taking into account the fact that it might get a puncture.

 

 

Relays can and do fail regularly. So do the connections to them (which has the same effect)

[Biggles]

 

 

However what might be an issue is the current rating:-

 

The NC contacts are only rated to 3 amps.

 

 

 

 

This is the spec. sheet 3 amps? not 30 amps or have I got the wrong end of a stick?

 

Biggles

[blackrose]

I must be a bit slow as I haven't understood the basic premise of this thread. My boat has one AC input. If I'm on shore power I don't need the generator so it isn't connected. If I'm away from shore power I connect the generator to the AC input that is now free. It seems like a simple and safe system to me. Why would anyone need two AC imputs and what's the advantage?

Edited March 6, 2011 by blackrose

[Gibbo]

This is the spec. sheet 3 amps? not 30 amps or have I got the wrong end of a stick?

 

Biggles

 

Look closely at the specs:-

 

Normally Open Contacts:

40A @ 277VAC, resistive; 6K Ops. (Flange Mount);

30A @ 120/277VAC, resistive

 

<snip>

 

Normally Closed Contacts:

3A @ 28VDC or 277VAC, 2A @ 480VAC, 1A @ 600VAC.

 

It is very common to have the normally closed contacts rated lower than the normally open contacts but not usually by that much!

 

I must be a bit slow as I haven't understood the basic premise of this thread. My boat has one AC input. If I'm on shore power I don't need the generator so it isn't connected. If I'm away from shore power I connect the generator to the AC input that is now free. It seems like a simple and safe system to me. Why would anyone need two AC imputs and what's the advantage?

 

So they don't have to bother plugging and unplugging things.

[Guest]

I must be a bit slow as I haven't understood the basic premise of this thread. My boat has one AC input. If I'm on shore power I don't need the generator so it isn't connected. If I'm away from shore power I connect the generator to the AC input that is now free. It seems like a simple and safe system to me. Why would anyone need two AC imputs and what's the advantage?

Exactly what I do with a portable genny, but I am something of a minimalist. Some people,especially with permanent gennies installed, prefer an automated relay system, to save all the faff of unplugging stuff.

[blackrose]

So they don't have to bother plugging and unplugging things.

 

It sounds like a very minor adavantage to me when compared to the potentially fatal impact of an electrical malfunction should one occur. Presumably one still has to unplug the shore power cable before moving the boat from the mooring? Personally I think I'd rather take another minute to plug in the generator when I'm away and then do the reverse when I get back to the mooring. The simplest solution is often the best even if it means taking a couple of minutes out of one's day.

[nb Innisfree]

It sounds like a very minor adavantage to me when compared to the potentially fatal impact of an electrical malfunction should one occur. Presumably one still has to unplug the shore power cable before moving the boat from the mooring? Personally I think I'd rather take another minute to plug in the generator when I'm away and then do the reverse when I get back to the mooring. The simplest solution is often the best even if it means taking a couple of minutes out of one's day.

 

I agree, we use plugs for our SP and genny but with the advantage of swapping plugs undercover in the pramhood, wouldn't fancy mucking about with 230v in a downpour.