Contactors/Relays

Nope, it's what's needed.... Control power drops, relay contacts close.

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Reply to
David Lesher
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Now I'm confused. Above you said if control power drops the relay contacts should close. Here you say they should be NO. Those two things are not compatible.

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Rick C 

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Reply to
rickman

Do you have room for three separate relays each with a single NC contact ?

Reply to
upsidedown

Where does control power come from? I'd expect use a power fail signal to do it's business during a hold-up time period.

If there is only control power, then there's nothing to control, so the normal state, in that condition, should be your 'fail-safe' default.

RL

Reply to
legg

legg wrote on 10/19/2017 3:02 PM:

Are you suggesting the relay needs to be powered from the contacts and it needs to *remember* the state it was last in when control power fails? Seems simpler to just power the coil from the control side and use a NC relay. No small part of the use of relays comes from the isolation. Powering the relay from the load side means you have to start all over again with an isolation barrier.

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Rick C 

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Reply to
rickman

There's no memory involved. This is the designer's decision. If it is important that the contacts be in a certain configuration under certain conditions, then it's the designer's responsibility to provide circuitry that's smart enough to enforce it.

Intentionally configuring a lower power electronic control circuit that can't function, always, when the higher-powered controlled electrical quantity is present doesn't make sense.

RL

Reply to
legg

That is the usual function of the physical component of an electronically controlled transfer switch, partly due to the power requirement of the movement.. The auxilliary signal contacts can provide information on the operating state and may also be used directly to energize or disable the appropriate coil.

RL

Reply to
legg

So all designed equipment has to be 100% reliable? How about since the requirement is to have the contacts closed when the controlling circuitry isn't powered to be implemented by a relay with normally closed contacts?

I don't get what your problem with this is. The controlling system will be built to run from whatever power is deemed appropriate. Your assertion that it *must* be powered by the load is without basis. You are aware that the controlling circuit might be doing other things. YOU seem to be unaware that there may be a larger system involved.

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Rick C 

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Reply to
rickman

This isn't reliability, it's functionality.

About the only power control circuits I can think of that could use normally closed power contact is a circuit breaker (fuse function) or motor starter (who's on first function). Both of these are motivated by the controlled source of power or it's physical effect, and the latter isn't really affecting the source, so much as it is signalling the load.

For signal level applications, potential damage to the switch itself, or the effects of normal power flow on the switch's ability to function are not usually a consideration.

I've worked on the development of an ultrafast multiphase transfer switch, with hybrid semiconductor/mechanical brush contacts, and it's automatic/manual control interface. At a certain power levels, you have to enforce your own 'fail safe' environment. Gravity, leaf springs or permanent magnets just don't cut it.

Isn't the apparent rarity of the requested hardware sufficient indication that the OP might be barking up the wrong tree?

RL

Reply to
legg

Or maybe you just don't understand his application?

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Rick C 

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Reply to
rickman

Quite likely.....

But then, I didn't get to be where I am today by (insert trumpism here)!

RL

Reply to
legg

What happens when that 100A 3P NC contactor fails from an open coil, or a loose wire?

Reply to
Michael A Terrell

OP wants NC as result of loss of control, which an open coil represents.

RL

Reply to
legg

OK, but how does that help? The contactor closes. What is on each side of it, when that happens when it's not supposed to? Is a generator or inverter connected to a live power line, out of phase?

Reply to
Michael A Terrell

I meant your suggestion was NFG because it is NO; I need NC.

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A host is a host from coast to coast.................wb8foz@nrk.com 
& no one will talk to a host that's close.......................... 
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Reply to
David Lesher

The issue there is will the AHJ allow such....

But wait! I did not check any Aussie relay suppliers; they should be able to help me out...

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A host is a host from coast to coast.................wb8foz@nrk.com 
& no one will talk to a host that's close.......................... 
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Reply to
David Lesher

And rickman gets the prize....

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A host is a host from coast to coast.................wb8foz@nrk.com 
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Reply to
David Lesher

NFG? Not Functioning on Ground?

It wasn't my suggestion. I'm just having trouble following some of the conversation.

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Rick C 

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Reply to
rickman

I've followed this thread and its predecessor pretty closely, but have yet to see any app info save:

'240 VAC resistive; we are interupting both sides of a 240V circuit.' 'I need 2 or 3P.'

Of course the question is, what happens if you can not open one or either of these terminals, when required? That's the 'failed' condition, and one that is difficult to to comprehend as being 'safe'.

RL

Reply to
legg

That's because you only know the small part; not the big picture. You (& others...) assume too much. The bad thing is not when the downstream is powered, it's when it's not able to be powered.

Alas, I'm getting convinced that no one makes the hardware I seek. The closest I've found are several quite pricy 100A+ contactors with 2 poles NO and 2 NC. I guess I spec two of them for 3 ph. loads.

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A host is a host from coast to coast.................wb8foz@nrk.com 
& no one will talk to a host that's close.......................... 
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Reply to
David Lesher

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