Protection of ATS

[ElecDar]

Hi everyone,

I would appreciate your comment on the following case,

We have an essential load connected to one transformer 2000kVA where the whole load is backed up by a generator through Automatic Transfer switch (ATS).

Q1: Could we have the ATS installed between the transformer and the Main distribution board main circuit breaker, in this case the ATS will be protected from the medium voltage side (circuit breaker or fuse).

Of course we could have a separate circuit breaker between the ATS and the transformer, but we will have to have another circuit breaker or switch of the same rating in the main distribution board just few meters away form the first one.

Q2: instead of the ATS could we have two interlocked circuit breakers.

 

[waross]

Electrically, I have used interlocks between the main breaker and the generator breaker. This was a 280 KW set. A new box was constructed and both breakers were installed side by side with a mechanical interlock. It works fine. It is manual.
For automatic operation you can instal motor operators on the breakers but the breakers must be adjacent so that a mechanical interlock may be installed.
It is sometimes possible to install the main breaker and the generator (or a second breaker for the generator) in the main panel and instal a walking beam interlock. Check with your breaker supplier to see if a walking beam kit is available or if it is a factory installed option for your brand of breaker.
If you are asking about code issues, "It depends". Where are you. What codes are you bound by? What are the sizes of your primary protection, transformer, main breaker, gen set, and is this a generator added to an existing installation, a new installation, in the planning stages?
The more information you supply the better answer you will get.
yours

 

[ElecDar]

Waross

Thanks for your reply,

Actually it is NEC code issue, this is a new installation, the transformer and generator rating is 2000kVA the main breaker size is 3000 Amp 380/220 V.
The question is it acceptable to have the ATS installed dirctlly after the transformer before the main breaker.
Also is it acceptable to have two interlocked circuit breakers insted of the ATS.
The main issue is the protection of the ATS.

 

[waross]

I suggest that you may want to reconsider installing the ATS ahead of the main breaker unless you are able to easily open the transformer primary for isolation. Check with the inspector.
There should be no problem with two mechanically interlocked breakers in the main panel. Depending on the distance to the gen set, you may be able to use one breaker for the generator breaker.
You should have a main breaker and a generator breaker in any event. Using interlocked breakers in the main distribution panel for main and gen set breakers results in an economical installation. At this point you have a manual transfer switch. You can add motorized breaker operators and some phase/voltage monitors and automate the operation.
I don't have my NEC code with me.
There may be added requirements for bypass breakers if this installation has to meet life safety standards. Hopefully someone with recent experience will post a reply.
respectfully

 

[dpc]

A typical "ASCO" type ATS cannot be place ahead of the main service overcurrent protective device. You must provide overcurrent protection fot the ATS to meet NEC. Been there, tried that. You can put a separate fused disconnect or CB ahead of the ATS.

A "Westinghouse" (Cutler-Hammer) style transfer switch consisting of two molded-case circuit breakers can be used as a service entrance device, with no additional upstream protection.

 

[ElecDar]

The additional breaker will be 3000A (air circuit breaker) which is expensive especially I’ll have another one in series after the ATS for the main distribution board.

Actually I was trying to avoid the use of Cutler-Hammer service entrance ATS (i.e. ATS with two circuit breaker), I believe it is manufactured by Cutler-Hammer only!!.

Also I thought specifying the famous ASCO/Zenith. ATS will be cheaper, but I want to be sure that the ATS is protected, so I’ll go for Cutler-Hammer service entrance ATS unless there is any other alternative/suggestions.

 

[waross]

If your main distribution panel has not yet been constructed, the panel builder may be able to incorporate two mechanically interlocked breakers in the construction. It's worth a phone call to the panel builder.

 

[dpc]

A fused disconnect will probably be less expensive than a circuit breaker.

The Cutler-Hammer ATS is a good option for this particular predicament.

 

[ElecDar]

waross,

We are still in the design stage, but the load is critical and I don’t think that two mechanical interlocked breakers, made by an assembler, will comply with the requirements of UL 1008 and IEC 60947-6-1, unless it has been tested to UL 1008 and IEC 60947-6-1as a complete assembly and is labeled as such. That is why I prefer Cutler-Hammer ATS.

dpc,

Are you aware of any other manufacture that can provide listed service entrance, ATS rather than Cutler-Hammer.

 

[powerwagon75]

ElecDar,

"Are you aware of any other manufacture that can provide listed service entrance, ATS rather than Cutler-Hammer."

Yes. ASCO Series 300SE and Cummins Power Generation(Onan) PLT series to name two.

 

[dpc]

As powerwagon says, ASCO does offer a service-entramce rated ATS - basically their standard blade-type switch with a circuit breaker on the utility incoming leg ahead of the switch.

 

[davidbeach]

I believe that Thomson Technology can also provide service entrance listed ATSs.

 

[ElecDar]

ASCO Series 300SE ATS service entrance is a typical ATS with additional breaker for the utility power supply as dpc said, while Cutler-Hammer service entrance ATS uses two circuit breaker, instead of switches, to transfer the power, so there is no need for the extra ASCO Series 300SE breaker. I believe the same applied to Cummins ATS.

davidbeach is correct, Thomson Technology can provide service entrance with two breaker but my understanding it is up to 1200A only, I’m looking for 3000A.

 

[powerwagon75]

ElecDar,

The Cummins PLT-series are dual-breaker applications--up to 3000 amps. The PLT series comes in open-transition, hard-closed and soft-closed transitions--depending on the available options of your genset. Check out the link for the open trans. (PLTO):

http://www.cumminspower.com/Commercial1/Switchgear/S-1363.pdf

There are several places to find this type of switching, to include small companies who will custom build for you--my experience with this route has been that it's not a money saver, and usually operates with unique devices and proprietary PLC software.

 

[ElecDar]

powerwagon75,

I agree, it is expensive and has a microprocessor-based control system, but the alternative solution is more expensive (ATS + breakers.).