Question on Contactor Based Design for switching

[OCPD]

Hello All,

I am looking for any ideas and a sample electrical design if available. A design to feed I want to connect and feed a separate load contactor-based design to switch between different loads when from a source. As shown in the attached picture (it is high level) the generator feeds the normal loads during utility outages or emergencies but when the generator is not being used it feeds the load bank. The PLC will control the contactor operation (open and close), generator signal (start and stop), and sense voltage. I have seen something similar done with motorized/electrically operated breakers with PLC control. I was wondering if something similar can be done with relay-based contactors as shown (like how in 83 throw-over for control power switching). The reason to do this is cost and footprint. If all this can be accommodated in a small enclosure rather than a big electrical panel. Please do provide thoughts and if possible share similar designs. I am concerned with the contactor breaking capacity and are there lighting contactors for 600Vac rating small enough to be wired in an enclosure and wired for automation?

 

[ptonsparky]

The small residential grade transfer switches have control options designed for load shedding. The last one we were involved in had four normally closed contacts. We used only one. I see no reason they could not be an input to a PLC and then to a mechanically held latching contactor rated for your load.

Are you designing your own transfer switch?

 

[petersonra]

Looks like a transfer switch to me. Pretty sure they're required to be listed. I don't think you can make your own.

 

[gar]

220503-0921 EDT

OCPD:

I find your drawing and description incomphrehencable.

Are switch contacts A and B one SPDT switch ( or a relay contact set ), or two independent switches?
Why is the bottom horizontal line drawn as a straight line with one very small gap at the middle?
What are normal loads?
What is a testing load bank?
What is the purpose of this circuit?
Are there any other power sources than the generator?
How are A and B controlled?

.

 

[petersonra]

220503-0921 EDT


What is a testing load bank?


.

Most gensets have some kind of connection so they can be tested with a load bank. Sometimes the load bank is brought in and hooked up just for the test, other times it is permanent.

 

[jim dungar]

This type of switching is pretty common.
For single phase circuits it is easy to find 'definite purpose' contactors with both NC and NO contacts in a single device. For most three-phase circuits you probably need to look at an interlocked reversing contactor. The switching duty is probably not severe so IEC style devices should work.

I had one customer that used a single contactor design to automatically switch fire trucks from shore power to on board generators.

 

[gar]

220503-1657 EDT

So a "Load Bank" is a dummy load, and only serves a test purpose.

Apparently the relay contacts are functionally a SPDT relay contact. So the generator is not connected to the dummy load unless the relay is energized. Therefore, the generator is always connected to the normal load. This produces a conflict with whatever else is connected to the normal load to power the normal load, unless there is another contractor to prevent this.

Another question is whether one wants to start the generator under load.

The question as presented by OPCD does not provide adequate information.

.

 

[texie]

The OP seems to be indicating something fairly low power and not Art. 700 related. This can be built fairly simply. But if he is referring to a load banking system for an Art. 700 system this get's a whole lot more complicated. There are vendors out there that make equipment to make Art. 700.3 compliance fairly simple but costly.

 

[don_resqcapt19]

You also have to be careful that there is no over lap in the operation of the two relays..

If this is related to an Article 700, you need listed transfer equipment.

 

[Jraef]

From your asking about 600V, I deduce this is 3 phase and probably Canada, eh? If so I’m not sure about the rules up there, but they are generally the same. So assuming from the description that the generator is an emergency power source, there needs to be a listed automatic transfer switch (ATS) to transfer between utility and generator power for the facility loads. I don’t believe you are allowed to “roll your own” ATS.

If that listed ATS is already there, then it’s simpler than you might think. All you need for the load bank is a single contactor and a “test” or “exercise” command, which usually comes from the ATS for safety reasons. If the utility power happens to fail DURING a load bank operation, the ATS must be capable of disconnecting the load bank before connecting the generator to the facility loads. But most ATS will have an option to control the load bank already. If not and you need to add the contactor, load banks are resistive loads, so all you need to know is the current rating of the load bank, then any industrial contactor will be fine.

For some of the others who might not be familiar with larger diese powered generators, load banks are most often used when a diesel generator must be periodically “exercised” under full load so that the diesel engine does not suffer from “wet stacking”; the buildup of carbon in the system that can happen when run only partially loaded. That’s not something that is typically needed in small residential generators.

 

[OCPD]

The small residential grade transfer switches have control options designed for load shedding. The last one we were involved in had four normally closed contacts. We used only one. I see no reason they could not be an input to a PLC and then to a mechanically held latching contactor rated for your load.

Are you designing your own transfer switch?

Thank you. Do you have a part number of the transfer switch with for NC contacts. For my application it is a lab opposite or reverted reverse transfer switch ( if you will) instead of switch load between sources, it is to feed one load at any time (Use gen to feed either load-1 or Load-2). Instead of using breakers with motorized options and PLC controllled for automation. I am hoping to use contact it’s

 

[paulengr]

1. I have seen multiple Caterpillar branded systems that use a stock standard PLC and stock switchboards with shunt trip and close functions to switch one or more generators in either open or closed transition.
2. ATS is generally a custom embedded controller. It could very well be an off the shelf PLC but most look like a panelized custom embedded controller just like generator controls. The power side is one of two designs. Some are effectively an electrically controlled double throw switch. The other design uses two mechanically and electrically interlocked contactors.
3. What inherent advantage is there in building your own? ATSs are pretty cheap for what they do. Frankly I like the switchgear system much better. It is much more rugged, reliable, and easier to maintain. You don’t need an extra layer of breakers (ATS is not designed to open under fault).

An alternative would be a miniaturized fully breaker design. This company makes them in a fully motor operated/shunt trip with six contacts and electronic trip unit down to 150 A:

https://www.lselectricamerica.com/wp-content/uploads/2021/11/Susol-UL-MCCB_Catalog_EN_202110.pdf

That puts all your electronics into the breaker itself except for some kind of capacitive trip or battery power supply to operate during open transitions or trips. It is UL489 listed. Two or three of these breakers gives you a simple combination ATS/breaker panel. You can add more to do a load shedding scheme or fully double ended operation. Adding a low cost synchronizing relay. Crompton makes a very inexpensive one and once again I’ve seen these in not so cheap Cat branded gear.

This puts you into something that can be constructed in an NEC or 508A panel with fully UL Listed assemblies. Again the question would be why. Plus you are going to get a lot of questions asked and rightly so.

Don’t forget you still need a rectifier/diode bridge and battery charger and a “cheap” battery to power everything. This could potentially fit into a typical Generac ATS sized box. Using a cheap PLC and display jump starts a lot of embedded design if that’s just not your thing. Just not sure where the advantages are.