# Contactors for Automatic Transfer Switch



## just the cowboy (Sep 4, 2013)

I would not roll my own with a transfer switch due to you don't know what can happen in the future. 
They make what you need PowerMax PMTS-30 Automatic Transfer Switch or something like it.
Someone just posted about feedback onto the grid almost killing an electrician. If you made your own and anything happens you are going to end up on the sue list

Cowboy


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## John Valdes (May 17, 2007)

Welcome to the forum BreakYoSelf!


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## BreakYoSelf (Apr 12, 2021)

just the cowboy said:


> I would not roll my own with a transfer switch due to you don't know what can happen in the future.
> They make what you need PowerMax PMTS-30 Automatic Transfer Switch or something like it.
> Someone just posted about feedback onto the grid almost killing an electrician. If you made your own and anything happens you are going to end up on the sue list
> 
> Cowboy


I appreciate the concern… That product appears to be less robust than what I am building… appears to be just a relay with a basic logic control.

What I am building is mechanically and electrically interlocked. It is physically and electrically impossible to allow both contactors to close or energize at the same time. I also mentioned the load side of the system isn’t connected to the panel so there is no risk of back feed.

But to your point... this is why I am building this, because I am well aware of the concern to the linemen. Protection of life and saftey is my primary concern. I have no interested in just slamming dual male ended plug into my wall to power some appliances.

Also... I do have my answer. I was also advised by an electrical contractor at work switch both hot and neutral through the contactor to prevent the possibility of current moving onto to neutral conductor from either side.


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## just the cowboy (Sep 4, 2013)

BreakYoSelf said:


> I appreciate the concern… That product appears to be less robust than what I am building… appears to be just a relay with a basic logic control.
> 
> What I am building is mechanically and electrically interlocked. It is physically and electrically impossible to allow both contactors to close or energize at the same time. I also mentioned the load side of the system isn’t connected to the panel so there is no risk of back feed.
> 
> ...


Yes but prebuilt takes away the liability. Go with better if you want mine was only an example.


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## splatz (May 23, 2015)

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## Southeast Power (Jan 18, 2009)

BreakYoSelf said:


> I appreciate the concern… That product appears to be less robust than what I am building… appears to be just a relay with a basic logic control.
> 
> What I am building is mechanically and electrically interlocked. It is physically and electrically impossible to allow both contactors to close or energize at the same time. I also mentioned the load side of the system isn’t connected to the panel so there is no risk of back feed.
> 
> ...


That's all fine and dandy until something goes wrong.
Dead people are very expensive, I wouldn't go for less than a 20 million dollar umbrella policy.
Good luck with your project.


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## Easy (Oct 18, 2017)

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## SWDweller (Dec 9, 2020)

I commend you on thinking this through. I have always like switching the neutral. When you are designing the system you are not intimately involved with the electrical distribution. So you do not know what will happen if you tied the neutrals together. Maybe nothing. Also you may want to put a timer on both sides of the switch like 2 minutes. If the motor is running and you change power sources, it might be better to have a delay for the motor to come back on.


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## just the cowboy (Sep 4, 2013)

BreakYoSelf said:


> What I am building is mechanically and electrically interlocked. It is physically and electrically impossible to allow both contactors to close or energize at the same time. I also mentioned the load side of the system isn’t connected to the panel so there is no risk of back feed.


Sorry to say you are wrong. If the contacts weld closed on the line side contactor, you then back feed and dead electrician. You are on the hook.

Cowboy


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## BreakYoSelf (Apr 12, 2021)

just the cowboy said:


> Sorry to say you are wrong. If the contacts weld closed on the line side contactor, you then back feed and dead electrician. You are on the hook.
> 
> Cowboy


This concern can be protected with a good controls design. If the contacts are welded closed on the line side, then the mechanical and electrical interlock won’t allow the power to flow to the backup-side contactor to allow it to close because its depended on the AUX contact being open, which just so happens to be welded closed now. 

In addition, the inverter won’t be allowed to power on either adding an additional layer of protection as that also had a remote cable which requires the contacts to be closed in order to turn on.

Additional fail saves can be include to monitor the status. If any one fault condition is met, the DC battery bank shunt trip can fire and bring it all down requiring manual intervention to bring the engine system back online.


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## BreakYoSelf (Apr 12, 2021)

SWDweller said:


> I commend you on thinking this through. I have always like switching the neutral. When you are designing the system you are not intimately involved with the electrical distribution. So you do not know what will happen if you tied the neutrals together. Maybe nothing. Also you may want to put a timer on both sides of the switch like 2 minutes. If the motor is running and you change power sources, it might be better to have a delay for the motor to come back on.


I appreciate your input. As of right now this is a completely isolated standalone off-grid solar/battery system with about 12 kWh of usable lithium battery capacity. It’s heavily over provisioned for “garage loads”. Only recently I have wanted to have a more convenient way of providing a fail-safe to switch to grid power should there be a critical battery event to prevent fool spoilage.

While it is very unlikely this would happen, the battery management system is capable of monitoring various performance metrics including battery and system health. If there was ever a scenario it would have to switch to grid, it would be because a user defined fault condition has been met. The Batrium BMS has the ability to monitor and control a variety of DC side concerns from over temp, under/over volage conditions, load (by current shunt), battery balance, and more.

I have a large shunt trip breaker on my battery mains, and the Barium BMS is setup to trip if any one of these thresholds are met. If this happens the entire off-grid system and inverter would be shut down. All that is required now is an automatic transfer to grid power. 

At the end of the day, there would be no power to back feed even if the ATS had some catastrophic fault which has been expressed by other members. The entire battery system would be disconnected from the inverter in this scenario.

Either way... I get and appreciate the concerns.


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## just the cowboy (Sep 4, 2013)

BreakYoSelf said:


> This concern can be protected with a good controls design. If the contacts are welded closed on the line side, then the mechanical and electrical interlock won’t allow the power to flow to the backup-side contactor to allow it to close because its depended on the AUX contact being open, which just so happens to be welded closed now.
> 
> In addition, the inverter won’t be allowed to power on either adding an additional layer of protection as that also had a remote cable which requires the contacts to be closed in order to turn on.
> 
> Additional fail saves can be include to monitor the status. If any one fault condition is met, the DC battery bank shunt trip can fire and bring it all down requiring manual intervention to bring the engine system back online.


You must never of seen a welded set of contacts then. A welded set of contacts will not stop operation or effect the aux contacts in any way. You are thinking of a stuck contactor. I have seen many reversing contactors weld closed on a single set of contacts and when told to reverse cause a short, and in your case voltage will go back out the line side at all times. If incoming power is down there is no way a control interlock will prevent back feed if a contact is stuck.

You are determined to do this and not care what a half dozen people said Good luck.
Cowboy


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## BreakYoSelf (Apr 12, 2021)

just the cowboy said:


> You must never of seen a welded set of contacts then. A welded set of contacts will not stop operation or effect the aux contacts in any way. You are thinking of a stuck contactor. I have seen many reversing contactors weld closed on a single set of contacts and when told to reverse cause a short, and in your case voltage will go back out the line side at all times. If incoming power is down there is no way a control interlock will prevent back feed if a contact is stuck.
> 
> You are determined to do this and not care what a half dozen people said Good luck.
> Cowboy


The off-grid power would be completely down in my system… there is nothing to send back to the grid. 

The components are already oversized for the load… if my fridge can weld the contactors together id be impressed. My industrial controls rep would be getting a phone call to discuss this one for sure.


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## EJPHI (May 7, 2008)

BreakYoSelf said:


> The off-grid power would be completely down in my system… there is nothing to send back to the grid.
> 
> The components are already oversized for the load… if my fridge can weld the contactors together id be impressed. My industrial controls rep would be getting a phone call to discuss this one for sure.


It sounds you are designing this on the fly. You talk to an electrician and get input about the neutral so you change your design. After you to talk to the industrial controls rep to learn about welded contacts I wonder what you will change. I have another tangent for you to consider; single point of failure. By that I mean what happens if any one component in your system fails? This a bare minimum to consider in designing such a safety critical device.

And hey as long as you are into getting the opinions of others I suggest you call your POCO, AHJ, and any of your lawyer friends. After you here from them please post their recommendations so we may all learn together.


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